Radiation Protection Review Course





Hygienic requirements to planning, sanitary and technical, radiation protection equipment and operating regime of X-ray and radiological departments of patient care institutions


Radiation protection of personnel and radiation safety of patients during X-ray procedures.

Amongst sources of ionizing radiations, used in medical departments, the most common are X-ray diagnostic apparatuses. X-ray radiation, generated by those apparatuses, is characterized by significant penetrating power and, as a result, may pose hazard for personnel of X-ray subdivisions, patients, undergoing radiological procedures, persons that are in adjacent premises and on adjacent territory. That is why their allocation, planning and exploitation must satisfy the requirements of radiation safety.

Requirements to allocation, planning, arrangement, sanitary and technical equipment of radiological subdivisions of hospitals, radiation protection of their personnel and radiation safety of patients are stated in «Building rules and norms», «Sanitary rules and norms – X-ray departments (rooms)» (SRandN 42–129–11–4090–86), «Sanitary rules of work during medical X-ray procedures» (№ 2780–80).

Sanitary legislation does not permit allocation of radiological departments (rooms) in residential houses and child's institutions. No particular requirements to their allocation in patient care institutions are in place. But with the purpose of decrease of amount of adjacent premises for permanent sojourn of personnel and patients, advantage has been given to block-type arrangement in separate outhouse or on the ground or on the last floor of buildings.

Main premises of X-ray room is the treatment room – it is the premises, where X-ray apparatuses are located and all kinds of X-ray procedures are conducted. Existing legislation forbids their allocation under (over) wards for pregnant or children or in adjacent with them premises.

Radiation protection of adjacent territory (in case of location of X-ray room on the ground floor) and adjacent premises is provided by shielding by building structures (walls, overlappings, partitions), material and thickness of which must decrease radiation intensity to allowable level.

Weak spots in radiation protection by using building structures are doors and windows. Elimination of this defect is achieved by covering of doors by iron, leaden or lead-impregnated rubber plates; equipping windows with iron shutters (wooden with covering of iron or lead-impregnated rubber) or by raising of window-sill to 1.6 m height above floor-level.

Area of treatment room is regulated with purpose of protection of adjacent premises and it must be no less than 34 m2 for every X-ray apparatus, which must be located such way, that the distance between focus of X-ray tube and walls would not be less than 2 m and its radiation would be directed mostly in direction of main wall. Area of treatment room is enlarged by 15 m2 for every extra X-ray apparatus. X-ray tube is located in flask with collimator that forms work beam.


Protection of radiologist is provided by:

-         lead-impregnated glass that covers fluorescent screen;

-         multiple-stripe apron of lead impregnated rubber, that is hung onto screening device;

-         small protective screen;use of individual safety means (gauntlet, apron from lead impregnated rubber (in textile cover for protection from diffusion of lead)) in special cases.

Protection of laboratory assistant of X-ray room is provided by allocation of his work place in separate adjacent premise that is called control room (panel room). This work place is provided with window of lead-impregnated glass to treatment room and with means of direct communication with doctor.

In addition to treatment room and panel room, planning of X-ray room or X-ray department must have:

-         consulting room – 10 m2;

-         photographic laboratory – 6 m2;

-         booth for preparation of barium solutions – 4 m2;

-         cloakroom – 2.5m2;

-         toilet;

-         waiting room (in polyclinic).

Sojourn of paramedical personnel in treatment room or panel room during radiological procedures is not allowed.

During radiological examinations persons that take part in them – personnel of other departments of hospital, alliance of patient, attendants that have to support child or infirm may stay in treatment room in conditions that dose received by them does not exceed level of irradiation of category B.

Radiation safety of patients is based on decrease of radiation exposure during X-ray examination of population especially pregnant, children and adolescent that can be achieved by complex of organizational, medical and technical measures. Organizational measures provide regulation of X-ray examination of population, restriction of annual dose of irradiation for different categories of patients, raising the level of personnel's skill and responsibility for performance of procedures.

They are given in order, sanitary regulations, methodical instructions by Ministry of Public Health of Ukraine. All patients that are subject to X-ray examination according to their destination are divided into four categories.

Category Ad - patients with diagnosed or suspected oncological diseases, patients, examinations of which is conducted with purpose of differential diagnosis of congenital cardiovascular pathology, patients that get radiotherapy, patients that are examined on living indications in urgent practice. Recommended limit of annual irradiation for persons of this category is 100 mSv.

Category Bd – patients, examination of which is conducted on clinical indications at non-oncological diseases with purpose of specification of diagnose and (or) selection of treatment tactics. Recommended limit of annual irradiation for persons of that category is 20 mSv.

Category Cd – persons from risk groups including workers of enterprises with harmful conditions of work and those that pass through occupational selection for work at such enterprises, patients that are taken off the books after curative treatment of oncological diseases. Recommended limit of annual irradiation for persons of that category is 2 mSv.

Category Dd – persons that pass through all kinds of prophylactic examinations except those referred to category Cd. Recommended limit level of annual irradiation for persons of that category is 1 mSv.

Medical measures include: selection of method of examination, restriction of irradiation area to minimum values necessary for arrangement of diagnose of disease, protection of surrounding tissues by shields of lead-impregnated rubber, right selection of pose at roentgenography. Such shields (and apron of radiologist) are to be in textile covers for protection from diffusion of lead.

For decrease of gonadal dose during X-ray examination of organs of abdominal cavity, lumbosacral part of vertebral column and other organs shielding of gonads is foreseen.

Different ways of improving of X-ray image: production and use of fast X-ray films, right selection of operating mode of X-ray apparatus (conducting of examinations at minimum values of anode current and voltage on X-ray tube), use of electro-optical image amplifier that permit to get more sharp and brilliant image at dose-sparing regimen of work of apparatus, use of wide-screen Roentgenofluorography during prophylactic examinations are referred to technical measures that provide decrease of radiation exposure.

Maintenance of dark adaptation of sight of radiologist during X-ray examinations has high profile.

Channels of exhaust ventilation in treatment room must be located in upper part of premises – for removal of ionized by high voltage air and in lower part (under floor) – for removal of leaden dust.


Radiation protection of personnel and radiation safety of patients in radiological departments of hospitals

Different quantum and corpuscular irradiations are used for radiotherapy. Their sources are:

-         β-, γ- radiating radioactive nuclides in a form of bare and sealed sources;

-         X-ray apparatuses that are generators of quantum radiation of low and middle energies;

-         betatrons and linear accelerators that generate inhibitory and corpuscular irradiations of high energies.

Existent ways of radiotherapy are divided into two basic parts: 1) ways of teleirradiation; 2) ways of contact irradiation.

In case of teleirradiation source is located at considerable distance from patient (long-distance irradiation) or at insignificant distance (short-distance irradiation). In both cases, beam of radiation is given necessary width and shape and directed onto region that is subject to irradiation.

Contact irradiation includes: application way, when sealed sources are located on body surface that is irradiated by special devices – masks, applicators; intracavitary – when source of radiation is introduced into one of body cavities and intraorganic – when source of radiation is introduced directly into tissue of tumour.

Variety of ways and methods of radiotherapy is determined by necessity of fulfillment of basic principle of radiotherapy – concentration of radiation energy in abnormally changed tissues, combined with maximum decrease of dose in surrounding tissues and the whole body.

Radiation hazard for personnel of radiological departments, patients that receive radiotherapy, persons that can be in different premises and on territory that is adjacent to building depends on the way of radiotherapy and technical ways for its conduction.

Because of that, a number of requirements, stated in «Building rules and norms» and «Rules on work with radionuclides in establishments of Ministry of Public Health» are made for allocation of radiological departments of hospitals, organization of radiation protection of personnel and radiation safety of patients and population.

Radiological departments of hospitals are usually located in one-storey buildings with asymmetric-block planning that provides isolated location of every organization department:

- department of teletherapy;

- department for treatment by sealed sources;

- department for treatment by bare sources;

- department (laboratory) of radioactive nuclide diagnostic.


Department of teletherapy

Basic organization units of this department are treatment rooms with control rooms.

The following devices are used for teletherapy:

-         roentgenotherapy units that generate radiation with energy 0.1 – 0.3 MeV;

-         betatrons that generate electronic radiation with energy 15 – 30 MeV;

-         γ-therapeutic unit with activity of radioactive nuclide (cobalt-60) from 1 200 to 6 000 Curie and energy of γ-radiation 1.17 and 1.33 MeV.

Teleirradiation can be static and mobile. In case of static irradiation, source of radiation during session of irradiation is in fixed position relative to patient, mobile irradiation is characterized by removal of source in relation to patient in the process of irradiation, that can be rotary, sectored and tangent.

Radiation hazard in department of teletherapy is characterized by possibility of only external irradiation of personnel and patients.

Radiation protection of adjacent premises and territory that is adjacent to block of teletherapy is provided by:

-         building structures of lead with wall thickness more than 1 m;

-         organization of treatment rooms without daylight;

-         rational formation of beam of radiation, generated by source with help of different devices – apertures, filters, collimators to put it into certain measures and shapes for maximum decrease of penetrability in adjacent premises;

-         equipping of unavailability zone on adjacent territory.

Radiation protection of personnel is provided by:

-         sojourn of personnel in control room (protection by shielding);

-         application of technical ways of observation and language contact with patients during procedures;

-         equipping the labyrinth-like entrance into the treatment room;

-         regulations of continuance of working day (protection by time).

Radiation safety of patients is provided by:

-         rational selection of way of irradiation;

-         rational formation of beam of radiation in order to decrease possibility of deleterious effects on healthy tissues.


Department for treatment by sealed sources

Contact methods of irradiation (application, intracavitary, interstitial) when sources of irradiation in form of radionuclide preparations are located in direct contact with surface of pathologic process or are introduced right into tumour, are used in this department.

Sealed sources are radioactive nuclides, physical state of which (metal), or envelope they are in prevent the possibility of pollution of environment with them (including tissues of patient). In most cases sealed sources have shape of cylinders with noses or needles with rounded and sharpened ends, short shanks, small balls that contain γ- radiating radioactive nuclides – cobalt-60, cesium-137, tantalum-182, iridium-192 or β-radiating radioactive nuclides – phosphorus-32, strontium-90, yttrium-90, promethium-147, thallium-204.

In case of application method of irradiation, special fixing device (colpostat, endostat) must be introduced into cavity and the source of irradiation only after that. Then the source of irradiation can be installed without participation of medical personnel by programmed automatic systems or by unmanned manipulators.

Basic structural units of department for treatment by sealed sources are block of radionuclide security, that contains: depository of sources of irradiation, treatment room, manipulation room, radiological wards, domestic and other premises.

Radiation hazard in this department is characterized by possibility of external irradiation only.

Radiation protection of adjacent premises and territory is provided by:

-         usual building structure, thickness of which must correspond to requirements of existing legislation;

-         regulation of summary activity of radionuclide sources in radiological wards;

-         equipping of zone of unavailability on adjacent territory.

Radiation protection of personnel is provided by:

-         use of all ways of radiation protection (protection by distance, by time, by amount, by shields (all manipulations with sources must be conducted only in protective housing and behind shields, entrance in manipulation room must have protective wall of concrete from inside));

-         maintenance of regulations on radiation safety and sanitary regulations during work with sources of irradiation.

Radiation safety of patients is provided by:

- rational selection of way of irradiation;

- maintenance of existing rules of conduction of radiotherapy.


Department for treatment by bare sources

Bare sources are radioactive nuclides, during work with which the pollution of environment – air, hands, clothes, other surfaces is possible. Open sources are β-, γ- radiating substances in powder–like form and in form of true solutions, colloidal solutions, suspensions that are introduced in tumours through injection needles. Radioactive nuclides of iodine are introduced into organism by alimentary tract.

Department of treatment by bare sources contains:

-         block of radionuclide security that contains: depository of sources of irradiation, filling room, treatment room, washing room, rooms of temporarily storing of radioactive waste, settling bowls of collecting system;

-         radiological wards;

-         sanitary and domestic premises.

Radiation hazard in department for treatment by bare sources is characterized by possibility of external and internal irradiation of personnel, possibility of ejection of radioactive nuclides behind the borders of department.

In this connection, special requirements are made to equipping of premises of block of radionuclide provision, radiological wards, water-supply, sewerage, sanitary and domestic premises, operating mode, rules of personal hygiene, working clothes, special air discharge purification systems, filtering of air.

Character of those requirements depends on class of work with radioactive nuclides.

According to MASRU-01 all works with bare sources are divided into 3 classes. Class of work depends on two conditions:

-         groups of radiation hazard, to which radioactive nuclide belongs (MSRU-01 all radioactive nuclides depending on possible radiation hazard, made by them are divided into 4 groups: group A – radioactive nuclides with particularly high radiation hazard; group B – radioactive nuclides with high radiation hazard; group C – radioactive nuclides with moderate radiation hazard; group D – radioactive nuclides with small radiation hazard);

-         activity of radioactive nuclide at the workplace.

Radiation protection of personnel is provided by:

-         use of all ways of protection from external irradiation;

-         maintenance of requirements of radiation asepsis, that prevent possibility of internal irradiation;

-         maintenance of rules of personal hygiene. Radiation safety of patients is provided by maintenance of requirements of radiation asepsis inside the department.

Finally it has to be marked that all methods of protection from ionizing radiation (by amount, by distance, by time, by shield) can be divided into legislative (normative) and organizational and technical.

Protection by amount is legislatively regulated by NRSU-97(dose limit, allowable levels of entrance of radioactive nuclides into organism by inhalation, alimentary track, allowable concentrations of radioactive nuclides in the air, drinking water, allowable levels of pollution by radioactive nuclides of working surfaces, clothes, hands of personnel, regulated activities of radioactive nuclides at workplaces and other).

Protection by time is legislatively regulated by decrease of working time of personnel (category A), increase of continuance of leave and more earlier retiring on a pension.

Protection by distance and shielding is provided legislatively by construction regulations; rules that provide for proper standards of area, capacity of corresponding premises, their technical equipment and others.



Scheme of sanitary inspection of radiological department of hospital

1.     Name and address of hospital or polyclinic, allocation of rooms (building, floor, adjacent premises).

2.     Presence and condition of paper maintenance (journal of dosimetry, instructions etc.).

3.     Planning of rooms (list of rooms, their area).

4.     Type of X-ray apparatus, voltage and current strength in tube.

5.     Destination of X-ray apparatus (diagnostic, therapeutical, photofluorographic, defectographic). Immovable, unidirectional, various directional working beam.

6.     Presence and type of ventilation in treatment room, upper and lower exhaust ducts. Natural and artificial lighting.

7.     Protection from X-ray radiation of work places of radiologist, X-ray technician and adjacent premises (protective screens, lead-impregnated glass, walls, windows, individual safety methods). Calculations of effectiveness of their protection.

8.     Presence and type of Ionometers, personal dosimeters, their logbooks, dates of examinations.

9.     Degree of preparation of personnel (special education, improvement).


Scheme of hygienic estimation of radiation protection in radiological department of hospital

1.     General characteristic of radiological department of hospital.

              Name of patient care institution, its departmental submission, address.

              Characteristic and assessment of allocation of building of radiological department on the area, type of building, presence of zone of unavailability, presence of control area, its measures.

              Structure of department, peculiarities of allocation and planning of its subdivisions, functional connection between them.

              Assessment of radiation environment on territory of control zone and outside of it by determination of absorbed dose rate in the air of γ-radiation and radioactive pollution of soil.


2.     Department of teletherapy.

                        Allocation and planning of department, basic premises, characteristic of devices used for radiotherapy.

                        Radiation protection of control room, adjacent premises and territory from γ-radiation (presence of protective shroud on radiating device, materials and thickness of walls in treatment room, presence of protective labyrinth at entrance, protective doors, their freeze, presence of attentive light alarm).

                        Observing system for irradiation of patients.

                        Characteristic and assessment of ways of protection of patients from accessory irradiation.

                        Assessment of effectiveness of radiation protection in control room and other adjacent premises by calculation method and measurement of absorbed dose rate in the air.


3.     Departments for treatment by sealed sources.

                        Allocation and planning of department.

                        Sources of irradiation that used in department, their activity, methods of application of sources to patients (manual-linear and consistent).

                        Characteristic of radiation dangerous premises (depository for sources of irradiation, radiomanipulation room, radiotreatment room), their accordance to hygienic requirements.

                        Conditions of storage and transportation of sources of irradiation.

                        Ways of radiation protection of personnel in depository for sources of irradiation, radiomanipulation room, radiotreatment room.

                        Radiation protection of adjacent premises and territory.

                        Assessment of effectiveness of radiation protection by necessary calculations and measurement of absorbed dose rate in the air of workplace, behind shields, in adjacent premises, on adjacent territory.


4.     Department for treatment by open sources.

                        Allocation of department, characteristic of use of radioactive nuclides, class of radiation hazard it belongs to.

                        Characteristic of radiation dangerous premises (depository of radioactive nuclides, filling room, treatment room, washing room, radiological wards) their accordance to permitted class of works, sanitary improvement (covering of walls, floor, exhaust hoods, ventilation, collection, removal and sterilization of solid and liquid radioactive waste).

                        Presence of means of radiation protection: protective shields, boxes, remote instruments.

                        Presence of individual radiation protection devices for personnel: working clothes, overalls, aprons, arm-bands, breathing masks and others.

                        Sanitary and domestic premises for personnel.

                        Results of measurements and assessment of level of radioactive pollution of workrooms and other premises.


5.     Acquaintance with documentations of radiological department, its types.

Analysis and assessment of materials of radiological and medical control during previous year and current year.


6. Conclusions.

Training instruction on calculation of parameters of protection from external γ-radiation based on weekly doses of radiation, expressed in roentgens


For assessment of labour conditions during work with sources of γ-radiation and for calculation of protection from external radiation, formulas (1), (2), are used, which indicate dependence of radiation dose (D) from amount of radioactive nuclide (activity of source), time of radiation and distance between source of radiation and exposed object:




where: Q - activity of the source in milliCurie;

            M – activity of source in mg/equv. of Radium;

            Kγ – γ-coefficient of radioactive nuclide (table 1);

            8.4 - γ-coefficient of Radium;

            t – time of radiation during workweek – in hours(30 hours at roentgenologist and radiologist at work with sealed sources; 27 hours – at work with bare sources);

            R – distance between source and exposed object in centimeter.


Assessment of labour conditions is carried out by comparing calculated dose with allowable level for category A – 20 mSv per 50 workweeks = 0.4 mSv/week that is equal to 0.04 roentgens per week for γ- radiation.

Transforming the above-mentioned formula regarding Q or M, t or R, activity, time or distance, that provides safety for the personnel, can be determined. In transformed formula dose of radiation is indicated as D0 and it corresponds to allowable dose during workweek – 0.04 roentgens (0.4 mSv).

In case of protection by amount, by distance or by time, does not provide radiation safety, shields are used.

For determination of thickness of shield, damping has to be found – the number that shows how many times with shield’s help, radiation must be decreased in order to receive dose of radiation that would not exceed allowable limit. Damping is found by formula (3)


where: D – is calculated real dose of radiation for certain labour conditions;

           D0 – allowable dose of radiation.

On basement of damping and energy of γ- radiation of given radioactive nuclide (that is given in table 1) in special tables (look at table 3, 4, 5) thickness of shield, made of corresponding material – lead, iron, concrete is found.


Training instruction on calculation of parameters of protection from external γ-radiation based on determination of absorbed dose rates in the air, expressed in microGray per hour


For assessment of effectiveness of radiation protection during work with sources of gamma-radiation and calculation of its parameters, you are to have such data on radiation conditions:

-         activity of source of gamma-radiation in Becquerel (Bq);

-         energy of gamma-radiation in mega-electronvolt (MeV);

-         distance between source of radiation and object of irradiation in meters (m);

-         time of radiation in hours (hour);

-         kerma of radioactive nuclide;

-         absorbed dose rate in the air in microGray per hour, (μGy/hour);

-         material for protection (its name and density).

Assessment of accordance of parameters of radiation protection to requirements of existing legislation is based on comparison of calculated absorbed dose rate in the air (AD) with permissible absorbed dose rate in the air (PAD).

Amount of absorbed dose of external radiation rate in the air is calculated according to the following formula:

Р = ,         (4)

where:  P – is absorbed dose rate in the air Gy/hour (calculated by this formula absorbed dose rate in the air is expressed in Gy/hour, for recalculation it in μGy/hour it has to be multiplied by 10-6);

   A – activity of the source of γ-radiation in Becquerel (Bq);

   G – kerma of radioactive nuclide – summary initial kinetic energy of all charged particles, framed by influence of secondary ionizing radiation. Systemic unit of kerma is Gray, off-system unit is rad. Value of kerma is found in special table or is counted by multiplying gamma coefficient of radioactive nuclide by coefficient – 6,.5 and γ-coefficient is found in table 1 («Physical characteristics of radioactive nuclides»);

   t – time of irradiation in seconds (if that time is given in hours than it has to be multiplied by 3600 for recalculation of time expressed in seconds);

   R – distance between source of radiation and exposed object in meters (m).

Similarly to calculations by formulas (1) and (2), transforming formula (4) relatively to A, t or R, or parameters of protection by amount (activity), by distance or by time can be detected at necessity.

At the same time dose rate in transformed formulas is indicated as P0 has to be suited to amount of allowable absorbed dose rate in the air (see table 6).

Calculation of protection from external γ-radiation by shields is carried out similarly to indicated above sample.

First stage of calculation of protection by shield is calculation of absorbed dose rate in the air from certain source according to given above formula.

Second stage of calculation is determination of necessary damping of absorbed dose rate in the air. Formula (5) is used for that:

К =         (5)

where: K – damping factor;

            P – calculated actual absorbed dose rate in the air;

            P0 – permissible absorbed dose rate in the air (see table 6).

Third stage is determination of thickness of shield from suitable material (lead, iron, concrete) by quantity of necessary damping of γ-radiation and its energy. The same tables 3, 4, 5 are used for that purpose.


Table 6

Permissible absorbed dose of gamma-radiation rates in the air that are used for planning of protection from external irradiation


Categories of

exposed person

Destination of premises and territories

Continuance of irradiation hour/year

Permissible absorbed dose rate in the air mcSv/hour


Persons of category A

Premises of permanent stay of the personnel

1 700


Premises of sojournment stay of the personnel



Persons of category B

Premises and territory of object where persons that refer to category B are

2 000


Persons of category C

Other premises and territory

8 800


Comment: Values of AD are given with double safety factor that is caused by peculiarities of planning of protection.

Training instruction on method of calculation of thickness of protector from X-ray radiation


Calculation of thickness of walls, floor, and ceilings of premises of X-ray room, protective screens and shields consists of three operations:

-         determination of necessary damping factor of X-ray radiation (K) that shows how many times dose rate has to be decreased to permissible;

-         determination of thickness of protection by lead that is necessary for decrease of absorbed dose rate in the air that was produced by source of X-ray radiation to allowable level;

-         recalculation of founded thickness of protection by lead to material which planned or existing constructions are made of.

Formula (6) is used for calculation of damping of X-ray radiation during determination of dose rate in the air in roentgens per hour:


where : Ist – standard anode current of X-ray tube (1-3 mA);

             R – distance between X-ray tube and place of protection, m;

             PDR – permissible absorbed dose rate in the air (exposure dose) of radiation, mR/hour (see table 7).


Table 7


Permissible dose rate in roentgenologic departments and X-ray room, mR/hour


Kind of premises



Premises for permanent stay of the personnel (treatment room, panel room)



Premises of sojournment of the personnel and adjacent premises



Wards for patients




Necessary thickness of protection by lead depending on damping and voltage on X-ray tube are to be found in special table (see table 8).

Thickness of protection by building materials are to be found by their leaden equivalents in table 9.


Training instruction on calculation of protection from X-ray radiation at determination dose rates in μGy/hour


Calculation of protection from X-ray radiation by shielding is based on determination of damping of absorbed dose of X-ray radiation rate in the air (DR) at absence of protection to permissible level of absorbed dose rate in the air (PDR) at the same spot of the premises due to shield. These calculations are in roentgens per hour at expression of dose rates in μGy/hour.

Stationary ways of radiation protection of treatment room and X-ray room (walls, ceiling, floor, doors, watch window between treatment room and panel room) are to provide damping of X-ray radiation to such level when absorbed dose rate in the air at work places of the personnel, in adjacent rooms and on the territory that borders upon treatment room at location of X-ray room on the ground floor, will not exceed absorbed dose rate.

Damping of X-ray radiation (K) is calculated by formula (7):


where: DR – is calculated actual absorbed dose of X-ray radiation rate in controlled spot, mGy/hour;

            PDR – is permissible absorbed dose rate in the air by means of permanent protection, μGy/hour. (see table 10);

           103 – is coefficient for recalculation of absorbed dose rate in the air, expressed in mGy on dose rate expressed in μGy;

            H – radiation outlet is absorbed dose rate in the air in initial beam of X-ray radiation at 1 meter distance from focal spot of X-ray apparatus mGy×m2/mA×min. Value of this radiation outlet is found in log of X-ray apparatus or in table (see table 11);

            W – working load (anode current) of X-ray apparatus (mA×min) per week. It is calculated based on regulated continuance of carrying out of radiological data at standard values of anode current. These data are given according to type and destination of X-ray apparatus in table 12.

            N – coefficient of directivity of radiation. In X-ray apparatus this coefficient is equal to 1, in apparatus with mobile source of radiation (X-ray computed tomographic scanner, panoramic tomographic scanner) coefficient of directivity is 0.1 and in directions where only scattered radiation hits – 0.05.

            30 – value of regulated time of work of X-ray apparatus during week (hour/week);

            r – distance between focus of X-ray tube and place of measurement of radiation level in meters is determined by project documentation of X-ray room.

Table 10


Permissible absorbed dose rate of X-ray radiation (PDR) at permanent protection of treatment room of X-ray room


Premises, territory






Premises of permanent stay of the personnel of category A (treatment room, panel room, room for preparation of barium meal, photographic darkroom, consulting room)




Adjacent premises to treatment room of X-ray room in horizontal and vertical directions that have places of permanent stay of personnel of category B




Adjacent premises to treatment room of X-ray room in horizontal and vertical directions without permanent work places (entrance hall, checkroom, footsteps, corridor, rest room, toilet, stockroom and others)




Premises of occasional stay of the personnel of category B (technical floor, cellar, attic etc.)




Wards of hospital adjacent in horizontal and vertical directions with treatment room of X-ray room




Adjacent territory to external walls of treatment room




Adjacent quarter to treatment room of radiodontics




Table 11

Value of radioactive outlet H at 1 m distance from focus of X-ray tube (anode current is constant, anode current rate is 1 mA, additional filter 2 mm AI for 250 kV-0.5mm Cu)


Anode current, kV








Radioactive outlet,

mGy×m2 (mA×min)








Table 12


Standard values of working load W and anode current U during calculation of permanent protection


X-ray equipment

Working load, (mA×min)/week


current,  kV


Roentgenofluorography apparatus without shielded box

4 000



Roentgenofluorography apparatus with shielded box, numerical photoroentgenograph, X-ray examination apparatus with digital image processing

2 000



X-ray examination complex with full set of tripods

1 000



X-ray apparatus for radioscopy

(first workplace - rotary table-tripod – RTT)

- in horizontal position of RTT

- in vertical position of RTT










X-ray apparatus for radiography

(2 and 3 workplaces – table of roentgenogram)

1 000



Angiography complex

1 000







Surgical moving apparatus with X-ray image amplifier




Ward X-ray apparatus




Roentgenourology table




X-ray apparatus for lithotripsy




Mammographic X-ray apparatus




X-ray apparatus for programming of radiotherapy (simulator)




Apparatus for short-focus roentgenotherapy

5 000



Apparatus for long-focus roentgenotherapy

12 000



Osteodensitometer for whole body




Osteodensitometer for limbs




Calculation of protection is normally carried out at following locations:

-         right up to internal surface of walls of the premises that border upon treatment room of X-ray room or external walls;

-         at 0.5m distance from floor level if treatment room is located under premises that has protection;

-         at 2 m distance from floor level if treatment room is located over premises that has protection.

By using calculated values of damping (K) from table 8, taking into account anode current on X-ray tube, leaden equivalents of protection are to be found and used for following calculation of thickness of protection from other materials (see table 9).


Radiation Safety

Once someone decides to include radioactive materials in his/her research, he/she must apply for a radioisotope permit. During the process of obtaining the permit, the radionuclide work procedures will be examined together with other aspects such as the applicant's training, previous work experience with radioactive materials, adequacy of workplace facilities and preparation, dosimeters used, protective equipment, etc.

As explained earlier, it is better to order radioactive materials only when they are needed or as close as possible to the date of the experiment from both an economic and ALARA perspective. This will also reduce the risks associated with long-term storage, source leakage, external irradiation, etc.

There are three essential methods used to minimize external exposure to radiation in radiation safety: time, distance, and shielding


Reduce the time spend working with radioactive materials as much as possible. A good work practice is to perform the experiment without radioactive material first, to get used to the procedures, and perform the first experiment (if possible) with the smallest amount of radioactive material that will give a readable result. After becoming familiar with the procedures and safe handling of these materials, the quantities used can be increased.


The second method involves increasing the distance between the body and radioactive materials. Always store radioactive materials and radioactive waste far from other working areas and/or offices. What if the procedure requires working with radioactive materials close to the body? Whenever possible, especially for strong beta and gamma emitters, use tools. Don't touch the materials with hands unless strictly necessary. However, if hand contact cannot be avoided, manipulation of the materials with gloved hands is required.


Most work with radioactive materials at the University will require that the user be quite close to the material. Therefore, working behind shielding is recommended. As explained earlier, different kinds of shielding must be used for different radionuclides. No shielding is required for pure alpha or pure low energy beta emitters. Plexiglass shielding is required for beta emitters, metal for gamma or X-rays, water, and wax or concrete for neutrons. Large enough layers of air, water, or concrete can protect the human body from all types of radiation.

Always check the effectiveness of the shielding before starting an experiment.

Radiation Safety for X-ray Units

Analytical X-ray machines produce intense beams of ionizing radiation that are used for diffraction and fluorescence studies. As stated earlier, X-rays can be characteristic (especially from K shell emission of the target material) or continuous (also called bremsstrahlung). X-ray machines may be hazardous because of their potential for producing high radiation fields. Special attention should be paid to the central beam but secondary emissions from samples, shielding materials and fluorescent screens should also be considered.

The shielding, safety equipment, and safety procedures prescribed for X-ray diffraction equipment are applicable only for up to 75 kV-peak X-rays. Additional precautions are necessary for machines operating at higher voltage (such as interlock systems). The supervisor is responsible for providing a safe working environment by ensuring that all equipment is operationally safe and that users understand safety and operating procedures.

Do not put any part of the body into the X-ray beam. Use safety glasses or prescription glasses to protect eyes from secondary exposure (glasses can not protect eyes from direct exposure).

Biological Effects of Radiation


Can lead to: OR


There are two types of biological effects of radiation. One is acute, where the amount of damage is proportional to the value of the dose equivalent received by the person. These effects typically relate to high dose levels. This type of biological damage is called a non-stochastic effect of radiation. Sometimes, when controlled, this type of effect may be beneficial to our health. For instance, some forms of cancer therapy utilise high doses of radiation to kill cancerous cells. In our university, large doses causing acute effects are not commonly encountered.


The second types of effects are delayed and statistical (or stochastic) effects. These effects are related to intermediate and low-level doses received by a person. There is no dose-response relationship. The dose relates to a statistical probability of developing a certain effect. The best example is cancer. Exposure to a certain dose can increase the risk of developing cancer. With respect to the foetus, if the dose was received in the first two months of gestation, mental retardation may occur in the offspring.

Radiation is one of the best known carcinogens. Since the last half of the 20th century, our knowledge of this type of cancer has increased dramatically. A statistical proportionality between the level of dose received by a large number of people and the expected effects was proven at a high-to-intermediate level of dose equivalent. Only at much higher doses than those encountered at the University is there a statistical proportionality between cause and effect. A linear extrapolation of this data has been made to low and very low levels of dose equivalent. However, this linear extrapolation method has not been proven scientifically.

Conversely, some studies show that low levels of irradiation are in fact beneficial to our health. However, in the absence of scientific evidence, the regulators adopted a conservative approach and consider all levels of radiation as being damaging to the human body. Because of this, any procedure that involves radioactive materials must abide by a principle called ALARA, keeping all doses 'As Low As Reasonably Achievable'.

Radiation Detection and Measurements

Two types of methods are used to measure external dose. One consists of using instruments such as survey meters, surface contamination meters, and neutron detectors. These instruments comprise the gas filled detectors (Geiger-Müller or proportional detectors), the scintillation detectors, and some special detectors for neutrons.
When performing monitoring the following actions are required:

1.     Check first if the right method (direct or indirect monitoring) is required for the type of radionuclide used.

2.     Check if the instrument has been calibrated less that a year ago (check the sticker on your instrument) ensure that the battery and HV (when there are available) indicators are in the correct range.

3.     Measure the background before reading the values in the work area.

4.     Subtract the background after taking readings in the work area.

5.     With the remaining number and the instrument's efficiency for that particular radionuclide (see sticker), estimate the level of contamination.

6.     Take the necessary actions to reduce the contamination below the set limits.

The second method for measuring external dose is personal dosimetry.


This consists of whole body Thermoluminescent Dosimeters (TLDs), extremity TLDs, and neutron dosimeters. The TLDs are contracted with a licensed company and the dose is measured every three months. The results are reported to the UTRPS and to the Health Canada. After checking each report and comparing it to the administrative action levels, the UTRPS sends the results to the workers. These dose measurement results are strictly confidential.

With respect to internal dosimetry, U of T performs two types of bioassays. A baseline measurement for each person starting the usage of sufficiently large quantities of radioactive Iodine, Tritium, or other radionuclides is performed before starting the work. This measurement is important as a reference, but will not be used as a substitute for background of our measurements.

The first type of bioassay is dedicated to Iodine users (I-131 and I-125) and consists of measuring the concentration of radioactive Iodine in the thyroid. This measurement must be performed shortly after the time of use, but not later than 4 days post-usage.

The second type of bioassay is urinalysis. Because most radionuclides used at U of T can be detected in body fluids, this is a very good method to check for possible ingestion or inhalation of various radionuclides. Urinalysis is performed shortly after usage of radionuclides, but no later than 4 days post-usage. If radionuclides are discovered in someone's body, an estimate of the inhaled or ingested quantity is performed, and compared with the annual Allowable Limit on Intake (ALI) for that particular radionuclide. Immediate action is taken to stop the intake by analysing the work practices, changing the work procedures, etc.


Regulatory Requirements

As stated earlier, the possession, storage, use and disposal of radioactive materials is highly regulated at the international, federal, provincial, municipal, and local (U of T) levels.

U of T Policy, Standards and Procedures for Radiation Safety

A number of radiation safety policies have been developed and adopted by the UTRPA.

Training is considered the first important step in radiation safety. All radioisotope users must have either U of T radiation safety training or equivalent, which must be recognised by the RPS. One-hour administrative training is required for all users, including those with equivalent radiation training. When applying for a new user's TLD, the permit holder must sign the application and send it to the RPS. A Radiation Safety Officer will check if the new radioisotope user was trained or will contact the new user to arrange for radiation training. At the end of the course, the applicant must pass a written examination. Upon successful completion of the course and examination, a Radiation Safety Officer will administer a short oral quiz in the laboratory where the radioisotope work is being conducted, before providing a radiation course certificate.

A comprehensive set of procedures and standards for radiation safety has been developed and maintained by the RPS. There are procedures for ordering, receiving and transferring of radioactive materials , both from outside and within U of T. Special procedures have also been developed for the disposal of radioactive materials .

Procedures for working with radioactive materials are developed for each particular use by the permit holder. These are audited by the RPS. Special evaluation of the procedures will be performed:

  • When an external exposure above the administrative action limit is recorded

  • When an intake is discovered during bioassay

  • In the case of a self-declared pregnant worker

Procedures and checklists are established for commissioning a new radioactive laboratory. Laboratories are classified as basic, intermediate or high depending on the amount and type of radioactive materials to be used. During commissioning, radiation signs and labels are posted inside the laboratory. Depending on the type of laboratory, special security may be required. Radioactive materials should always be kept locked to prevent access by unauthorised persons. Strict record keeping of received, stored, used, and disposed radioactive materials , as well as contamination monitoring , is also required. These records must be available for the last three years of radioactive use in the laboratory.

To prevent any accidental ingestion of radioactive materials, food and beverages are strictly prohibited inside the radioactive laboratories. After performing experiments with radioactive materials, contamination monitoring must be performed by each user. Results of the monitoring must be recorded and made available to the RPS Radiation Safety Officer and/or CNSC inspectors.

A special procedure is required for checking sealed sources with activities greater than 1mCi (37 MBq). Also, decommissioning of radioactive labs or instruments with radioactive sources is performed according to special procedures.

U of T Radiation Emergency Procedures

During the course of normal operations with radioactive materials, a spill can occur resulting in contamination of personnel or lab equipment and areas. Also, external irradiation can be encountered from a strong gamma or neutron source left unshielded. Appropriate actions must be taken during such incidents to prevent unnecessary doses to personnel and further spread of contamination.

In case of serious injury, medical attention takes precedence over radiological or other concerns. Dial 416-978-2374 or the Campus Police immediately and inform U of T RPS or Campus Police about the incident, injury, and that radioactive materials are involved. Alert everyone in the area and take all reasonable precautions to limit the spread of radioactive contamination or further exposures. Never risk external or internal contamination to save equipment or an experiment.

In case of a radioactive spill, contact RPS immediately if internal irradiation (inhalation/ingestion of radioactive material) is suspected, if excessive external dose is suspected, if significant contamination of personnel is suspected, or in the case of contamination of large areas. In this case, besides the general emergency procedures listed above follow the radioactive spill instructions.

In any clean-up of a radioactive spill, always clean the area until measurements indicate that the contamination is below the contamination limits before removing radioactive warning signs. If there is any questions of the effectiveness of the spill clean up, contact the RPS or U of T Campus Police.

In case of an emergency involving strong gamma or neutron sources, evacuate the area and contact the U of T RPS or Campus Police. Determine the limits of the area with dose rates above 2.5 mSv/hr (0.25 mrem/hr). Restrict access to that area by using radioactive signs and/or tape, closing the doors, etc. Guard the area until RPS/U of T Police representatives arrive.

In case of emergencies involving radiation-producing machines (X-ray), turn off the machine and unplug or shut off the circuit breaker for the machine. In the case of injury to personnel, call 416-978-2374 or the U of T Campus Police, notify the laboratory supervisor, and record all information about the incident (eg. operating voltage and current, exposure time, distance from radiation source).

Radiation protection and patient dose

         Both ionizing and non-ionizing radiation as well as ultrasound are used in medical imaging methods. Somatic (occurs in own tissues) or genetic (in descendants) damage in patients or personnel are always a risk of examinations which employ ionizing radiation. Photons of non-ionization radiation (radio wave radiation in a strong magnetic field), as well as ultrasound, carry insufficient energy to cause injuries at diagnostic energy levels. Consequently, radiation protection is needed in practice only in X-ray and isotope examinations and in radiotherapy.

There are many factors which influence image quality. By increasing the amount of radiation (and patient dose) image quality can be increased to a certain level, but simultaneously several factors in the imaging chain can diminish quality. The quality control of imaging methods should be arranged such that high image quality with a dose as low as reasonably achievable (ALARA) is maintained.

The purpose of radiation protection is to eliminate the acute toxicity of radiation exposure and diminish the somatic and genetic risks to patients and personnel. It is useful to remember that the natural background radiation in the Nordic Countries varies between 3-6 mSv (300-600 mrem) per year. There is radiation coming from space, soil (radon gas is a very considerable source of radiation) and construction materials, as well as from our own tissues. Background radiation can vary depending on residential area, life style, etc. This value of 3-6 mSv is the same order as the skin dose from an X-ray image of the body.

Quantities and units of radiation dose

Interactions of X-ray and gamma photons always set electrons in motion with sufficient energy to ionize and excite atoms and molecules. An electron therefore deposits energy in its wake. Around 10-100 ionizations/ m caused by an electron are generated at diagnostic X-ray energies (approximately 33 eV/ion pair). The concept of linear energy transfer, LET (unit keV/ m) can be used to describe this phenomenon together with the concept quality factor Q, explained later. In addition, part of the energy of the electron is absorbed by secondary electrons, so-called delta particles; they in turn have sufficient energy to cause new ionizations.


Exposure implies that ions are generated in air as a consequence of the passage of radiation. Ions can be measured with an ionization chamber, which is an air space between two conducting plates coupled to the positive and negative poles of a voltage source. The exposure = the number of ions with negative (or positive) charges divided by the mass of air in the ionization chamber. The SI-unit is C/kg (C = coulomb). The older unit is roentgen R = 2,58 10-4 C/kg.

Absorbed dose

This quantity is the energy per unit mass, which matter has absorbed from radiation. The SI-unit is the gray Gy = J/kg (the old unit was rad = 0.01 Gy). At X-ray and isotope imaging energies (15-500 keV) one R exposure causes approximately 10 mGy (one rad) absorbed dose in all other tissues except in bone, where the absorbed dose at low energies (around 20 keV) reaches up to around 40 mGy.


The concept kerma comes from the words Kinetic Energy Released in Matter. It takes into account the dose generated by the aforementioned delta electrons. It is approximately equal to the absorbed dose in air at diagnostic X-ray energies.

Dose equivalent

When energy has been absorbed in tissue the biological effect varies depending on the organ in question, the type of radiation and energy, dose rate, exposure time etc. These are incorporated in the concept quality factor Q, by which the absorbed dose must be multiplied to get the equivalent dose. Its unit is sievert Sv = J/kg (= 100 rem, the old unit).

In X-ray and isotope imaging, Q is approximately 1, because X and gamma radiation deposit relatively small amounts of energy in tissue. Another concept, effective dose, describes the probability of damage to different organs with a weighting coefficient, which is high for radiation sensitive organs such as gonads, bone marrow, lungs, colon, breast etc. and small for other tissues, e.g. muscle. The sum of the weighting factors equals to 1.

From the foregoing it is clear that in diagnostic imaging, the units Gy and Sv, as well as R, rad and rem, have about the same numeric values, although the concepts have different meanings.

Dose rate

One useful concept in dosimetry is the rate, with which a given amount of radiation strikes tissues, for instance kerma rate and exposure rate mR/min, R/h etc. Activity (see the chapter Radioisotopes and radio pharmaceuticals) is also a concept which incorporates the function of time. Whether X-rays from an X-ray device or gamma radiation from radionuclides are discussed, the same concepts can be used to describe radiation phenomena and the biological effects of radiation.

Radiation biology

Ionization and excitation result in fragmentation of molecular bonds with potentially harmful consequences to cell structure, metabolism and organ function. Injuries are divided into genetic and somatic ones. The former can appear in descendants after a long time has elapsed, and the latter may occur quickly (acute consequences) or after a considerable delay. In the peaceful usage of ionizing radiation acute toxicity does not occur.

A distinction is also made between stochastic and non-stochastic effects of radiation. Stochastic implies that even a single "hit" of radiation to one cell or to a small cell group can cause a biological consequence. Damage may be either hereditary (in gonads) or carcinogenic (in tissue). There is no threshold, i.e. the extent of the damage does not depend on absorbed dose (cancer is contracted or not), although the probability of an adverse event increases with dose. This stochastic nature of radiation is therefore the basis of conservative radiation protection.

The non-stochastic effect of radiation has a definite threshold (normally different for every tissue and organ). These have been found from past experience, e.g. in cancer treatment with radiotherapy during this century. Diagnostic radiation examinations (where skin dose varies between 0.1 mSv and 0.1 Sv / examination) expose the patient to very small doses so the consequences of non-stochastic effects do not evolve. One clear exception is the dose to a fetus, particularly during the sensitive period of organogenesis. Therefore, the indications for pediatric examinations involving ionizing radiation must be examined particularly closely.

It is estimated that if 200 000-2 000 000 people get a dose of l mSv (the same as the background dose per year without radon) it is probable that one person will develop cancer. It is, however, impossible to separate so few cases from cancers caused by other factors, such as environmental toxins and unknown reasons etc.

Many other factors such as the type of radiation and energy, LET value, dose rate, time between exposures or fractionation of dose, different sensitivity of tissues for radiation, biological variations etc. have a significant effect on the likelihood of injury.

Radiation protection

Because injuries from small doses can partly be stochastic the starting point of radiation protection is to avoid and reduce somatic and genetic doses to as low a level as possible (ALARA, As Low As Reasonably Achievable). The consequences of small doses given over long periods of time are partly unknown, and as the time for a carcinoma to appear can be decades, damages caused by low level radiation are often impossible to separate from diseases caused by other factors. On the other hand it is important to use sufficient radiation to achieve good quality images. These examinations, which are clinically indicated, must be performed with sufficient radiation to achieve an image of diagnostic value.


The dose can be measured or estimated at different depths in the patient, or in different parts of the environment. Terms like skin dose (or surface or entrance dose), depth dose, dose in patient's centre, exit dose (approximately the same as dose to the screen without a grid) and organ dose are fairly self-evident. Dose diminishes as the depth at which it is measured increases. In the diagnostic examination of the body only a 1/100-1/1000

Figure 7.
Patient thickness very strongly influences the entrance dose needed for an image. Measurement conditions are also shown.

part of the initial dose penetrates through. Dose decreases also without matter, even in air. Radiation intensity (as well as light intensity) decreases in inverse proportion to the square of the distance from the focus.

Fig. shows how skin dose and exit dose are changed with patient thickness when the exposure of film to a constant blackness (optical density) is made with an automatic exposure meter. In this case exit dose does not depend on thickness, because a screen-film combination always requires a certain amount of radiation.

Many features of X-ray devices and properties of patient tissues influence the dose needed for good image quality.

         There are big differences in the properties of different imaging methods and in radiation detectors. Screen-film-combinations are always used in practise instead of film alone. Screen sensitivities vary from speed value 20 to 1600 (that of the reference screen-film-combination being 100), when the speed of the film alone has a value of about l. Consequently corresponding alterations can be found in patient doses.


The first rule in the radiation protection of personnel is to go outside the X-ray laboratory when a patient exposure is made. In fluoroscopic examinations one must work l) quickly, 2) with sufficient protective clothing, and 3) at an appropriate distance from radiation sources. These three measures are of primary importance in both X-ray and isotope work. The staff who are most likely to be exposed to radiation are those who work with fluoroscopic devices (radiologists, surgeons etc.), nurses who hold small children or non-cooperative patients, as well as staff working with the nuclear medicine imaging of patients.

National and international radiation legislation and recommendations are universally in use. According to these regulations, for instance, examination rooms, devices and working conditions must be adapted so that doses are diminished to as low a level as possible and that the quality of images and examinations attains the highest possible level. The most recent ICRP recommendation (publication 60, 1991) puts the maximum dose level of 20 mSv per one year to the whole body of personnel. This value is 40% of the earlier maximum limit, which shows the increasingly conservative attitude in radiation protection.

One should remember that the dose to personnel from scattered radiation is 100-1000 times smaller than the dose in the entrance field on the patient's skin. Therefore it is essential for the radiation worker to avoid putting his hands in the primary radiation field (use lead gloves). The patient's body can also serve as good protection, if one can place oneself in such a position that one does not directly see the entrance field of radiation.

Biological Effects

The occurrence of particular health effects from exposure to ionizing radiation is a complicated function of numerous factors including:

  • Type of radiation involved. All kinds of ionizing radiation can produce health effects. The main difference in the ability of alpha and beta particles and Gamma and X-rays to cause health effects is the amount of energy they have. Their energy determines how far they can penetrate into tissue and how much energy they are able to transmit directly or indirectly to tissues.  

  • Size of dose received. The higher the dose of radiation received, the higher the likelihood of health effects.

  • Rate the dose is received. Tissue can receive larger dosages over a period of time. If the dosage occurs over a number of days or weeks, the results are often not as serious if a similar dose was received in a matter of minutes.

  • Part of the body exposed. Extremities such as the hands or feet are able to receive a greater amount of radiation with less resulting damage than blood forming organs housed in the torso. See radiosensitivity page for more information.

  • The age of the individual. As a person ages, cell division slows and the body is less sensitive to the effects of ionizing radiation. Once cell division has slowed, the effects of radiation are somewhat less damaging than when cells were rapidly dividing.

  • Biological differences. Some individuals are more sensitive to the effects of radiation than others. Studies have not been able to conclusively determine the differences.

The effects of ionizing radiation upon humans are often broadly classified as being either stochastic or nonstochastic. These two terms are discussed more in the next few pages.


Stochastic Effects

Stochastic effects are those that occur by chance and consist primarily of cancer and genetic effects. Stochastic effects often show up years after exposure. As the dose to an individual increases, the probability that cancer or a genetic effect will occur also increases. However, at no time, even for high doses, is it certain that cancer or genetic damage will result. Similarly, for stochastic effects, there is no threshold dose below which it is relatively certain that an adverse effect cannot occur. In addition, because stochastic effects can occur in individuals that have not been exposed to radiation above background levels, it can never be determined for certain that an occurrence of cancer or genetic damage was due to a specific exposure.

While it cannot be determined conclusively, it often possible to estimate the probability that radiation exposure will cause a stochastic effect. As mentioned previously, it is estimated that the probability of having a cancer in the US rises from 20% for non radiation workers to 21% for persons who work regularly with radiation. The probability for genetic defects is even less likely to increase for workers exposed to radiation. Studies conducted on Japanese atomic bomb survivors who were exposed to large doses of radiation found no more genetic defects than what would normally occur.

Radiation-induced hereditary effects have not been observed in human populations, yet they have been demonstrated in animals. If the germ cells that are present in the ovaries and testes and are responsible for reproduction were modified by radiation, hereditary effects could occur in the progeny of the individual. Exposure of the embryo or fetus to ionizing radiation could increase the risk of leukemia in infants and, during certain periods in early pregnancy, may lead to mental retardation and congenital malformations if the amount of radiation is sufficiently high


Cancer is any malignant growth or tumor caused by abnormal and uncontrolled cell division.  Cancer may spread to other parts of the body through the lymphatic system or the blood stream. The carcinogenic effects of doses of 100 rads (1 Gy) or more of gamma radiation delivered at high dose rates are well documented, consistent and definitive.

Although any organ or tissue may develop a tumor after overexposure to radiation, certain organs and tissues seem to be more sensitive in this respect than others. Radiation-induced cancer is observed most frequently in the hemopoietic system, in the thyroid, in the bone, and in the skin.  In all these cases, the tumor induction time in man is relatively long - on the order of 5 to 20 years after exposure.

Carcinoma of the skin was the first type of malignancy that was associated with exposure to x-rays. Early x-ray workers, including physicists and physicians, had a much higher incidence of skin cancer than could be expected from random occurrences of this disease. Well over 100 cases of radiation induced skin cancer are documented in the literature. As early as 1900, a physician who had been using x-rays in his practice described the irritating effects of x-rays. He recorded that erythema and itching progressed to hyper-pigmentation, ulceration, neoplasia, and finally death from metastatic carcinoma. The entire disease process spanned a period of 9 years. Cancer of the fingers was an occupational disease common among dentists before the carcinogenic properties of x-rays were well understood. Dentists would hold the dental x-ray film in the mouths of patients while x-raying their teeth.


Leukemia is a cancer of the early blood-forming cells. Usually, the leukemia is a cancer of the white blood cells, but leukemia can involve other blood cell types as well. Leukemia starts in the bone marrow and then spreads to the blood. From there it can go to the lymph nodes, spleen, liver, central nervous system (the brain and spinal cord), testes (testicles), or other organs. Leukemia is among the most likely forms of malignancy resulting from overexposure to total body radiation. Chronic lymphocytic leukemia does not appear to be related to radiation exposure.


Radiologists and other physicians who used x-rays in their practice before strict health physics practices were common showed a significantly higher rate of leukemia than did their colleagues who did not use radiation. Among American radiologists, the doses associated with the increased rate of leukemia were on the order of 100 rads (1 Gy) per year. With the increased practice of health physics, the difference in leukemia rate between radiologists and other physicians has been continually decreasing.

Among the survivors of the nuclear bombings of Japan, there was a significantly greater incidence of leukemia among those who had been within 1500 meters of the hypocenter than among those who had been more than 1500 meters from ground zero at the time of the bombing. An increase in leukemia among the survivors was first seen about three years after the bombings, and the leukemia rate continued to increase until it peaked about four years later. Since this time, the rate has been steadily decreasing.

The questions regarding the leukemogenicity of low radiation doses and of the existence of a non-zero threshold dose for leukemia induction remain unanswered, and are the subject of controversy. On the basis of a few limited studies, it was inferred that as little as 1-5 rads (10-50 mGy) of x-rays could lead to leukemia. Other studies imply that a threshold dose for radiogenic leukemia is significantly higher. However, it is reasonable to infer that low level radiation at doses associated with most diagnostic x-ray procedures, with occupational exposure within the recommended limits, and with natural radiation is a very weak leukemogen, and that the attributive risk of leukemia from low level radiation is probably very small.

Genetic Effects

Genetic information necessary for the production and functioning of a new organism is contained in the chromosomes of the germ cells - the sperm and the ovum. The normal human somatic cell contains 46 of these chromosomes; mature sperm and ovum each carry 23 chromosomes. When an ovum is fertilized by a sperm, the resulting cell, called a zygote, contains a full complement of 46 chromosomes. During the 9-month gestation period, the fertilized egg, by successive cellular division and differentiation, develops into a new individual. In the course of the cellular divisions, the chromosomes are exactly duplicated, so that cells in the body contain the same genetic information. The units of information in the chromosomes are called genes. Each gene is an enormously complex macromolecule called deoxyribonucleic acid (DNA), in which the genetic information is coded according to the sequence of certain molecular and sub-assemblies called bases. The DNA molecule consists of two long chains in a spiral double helix. The two long intertwined strands are held together by the bases, which form cross-links between the long strands in the same manner as the treads in a step-ladder.

The genetic information can be altered by many different chemical and physical agents called mutagens, which disrupt the sequence of bases in a DNA molecule. If this information content of a somatic cell is scrambled, then its descendants may show some sort of an abnormality. If the information that is jumbled is in a germ cell that subsequently is fertilized, then the new individual may carry a genetic defect, or a mutation. Such a mutation is often called a point mutation, since it results from damage to one point on a gene. Most geneticists believe that the majority of such mutations in man are undesirable or harmful.

In addition to point mutations, genetic damage can arise through chromosomal aberrations. Certain chemical and physical agents can cause chromosomes to break. In most of these breaks, the fragments reunite, and the only result may be a point mutation at the site of the original break. In a small fraction of breaks, however, the broken pieces do not reunite. When this happens, one of the broken fragments may be lost when the cell divides, and the daughter cell does not receive the genetic information contained in the lost fragment. The other possibility following chromosomal breakage, especially if two or more chromosomes are broken, is the interchange of the fragments among the broken chromosomes, and the production of aberrant chromosomes. Cells with such aberrant chromosomes usually have impaired reproductive capacity as well as other abnormalities.

Studies suggest that the existence of a threshold dose for the genetic effects of radiation is unlikely. However, they also show that the genetic effects of radiation are inversely dependent on dose rate over the range of 800 mrad/min (8 mGy/min) to 90 rads/min (0.9 Gy/min). The dose rate dependence clearly implies a repair mechanism that is overwhelmed at the high dose rate. Geneticists estimate that there are 320 chances per million of a "spontaneous" mutation in a dominant gene trait of a person. The radiation dose that would eventually lead to a doubling of the mutation rate is estimated to be in the range of 50-250 rads (0.5-2.5 Gy).


A cataract is a clouding of the normally clear lens of the eye. A much higher incidence of cataracts was reported among physicists in cyclotron laboratories whose eyes had been exposed intermittently for long periods of time to relatively low radiation fields, as well as among atomic bomb survivors whose eyes had been exposed to a single high radiation dose. This shows that both chronic and acute overexposure of the eyes can lead to cataracts. Radiation may injure the cornea, conjunctiva, iris, and the lens of the eye. In the case of the lens, the principal site of damage is the proliferating cells of the anterior epithelium. This results in abnormal lens fibers, which eventually disintegrate to form an opaque area, or cataract, that prevents light from reaching the retina.

The cataractogenic dose to the lens is on the order of 500 rad of beta or gamma radiation. No radiogenic cataracts resulting from occupational exposure to x-rays have been reported. From patients who suffered irradiation of the eye in the course of x-ray therapy and developed cataracts as a consequence, the cataractogenic threshold is estimated at about 200 rad. In cases either of occupationally or therapeutically induced radiation cataracts, a long latent period, on the order of several years, usually elapsed between the exposure and the appearance of the lens opacity. The cataractogenic dose has been found, in laboratory experiments with animals, to be a function of age; young animals are more sensitive than old animals.

Nonstochastic (Acute) Effects

Unlike stochastic effects, nonstochastic effects are characterized by a threshold dose below which they do not occur. In other words, nonstochastic effects have a clear relationship between the exposure and the effect. In addition, the magnitude of the effect is directly proportional to the size of the dose. Nonstochastic effects typically result when very large dosages of radiation are received in a short amount of time. These effects will often be evident within hours or days. Examples of nonstochastic effects include erythema (skin reddening), skin and tissue burns, cataract formation, sterility, radiation sickness and death. Each of these effects differs from the others in that both its threshold dose and the time over which the dose was received cause the effect (i.e. acute vs. chronic exposure).

There are a number of cases of radiation burns occurring to the hands or fingers. These cases occurred when a radiographer touched or came in close contact with a high intensity radiation emitter. Intensity on the surface of an 85 curie Ir-192 source capsule is approximately 1,768 R/s. Contact with the source for two seconds would expose the hand of an individual to 3,536 rems, and this does not consider any additional whole body dosage received when approaching the source.

More on Specific Nonstochastic Effects

Hemopoietic Syndrome

The hemopoietic syndrome encompasses the medical conditions that affect the blood. Hemopoietic syndrome conditions appear after a gamma dose of about 200 rads (2 Gy). This disease is characterized by depression or ablation of the bone marrow, and the physiological consequences of this damage. The onset of the disease is rather sudden, and is heralded by nausea and vomiting within several hours after the overexposure occurred. Malaise and fatigue are felt by the victim, but the degree of malaise does not seem to be correlated with the size of the dose. Loss of hair (epilation), which is almost always seen, appears between the second and third week after the exposure. Death may occur within one to two months after exposure. The chief effects to be noted, of course, are in the bone marrow and in the blood. Marrow depression is seen at 200 rads and at about 400 to 600 rads (4 to 6 Gy) complete ablation of the marrow occurs. In this case, however, spontaneous regrowth of the marrow is possible if the victim survives the physiological effects of the denuding of the marrow. An exposure of about 700 rads (7 Gy) or greater leads to irreversible ablation of the bone marrow.

Gastrointestinal Syndrome

The gastrointestinal syndrome encompasses the medical conditions that affect the stomach and the intestines. This medical condition follows a total body gamma dose of about 1000 rads (10 Gy) or greater, and is a consequence of the desquamation of the intestinal epithelium. All the signs and symptoms of hemopoietic syndrome are seen, with the addition of severe nausea, vomiting, and diarrhea which begin very soon after exposure. Death within one to two weeks after exposure is the most likely outcome.

Central Nervous System

A total body gamma dose in excess of about 2000 rads (20 Gy) damages the central nervous system, as well as all the other organ systems in the body. Unconsciousness follows within minutes after exposure and death can result in a matter of hours to several days. The rapidity of the onset of unconsciousness is directly related to the dose received. In one instance in which a 200 msec burst of mixed neutrons and gamma rays delivered a mean total body dose of about 4400 rads (44 Gy), the victim was ataxic and disoriented within 30 seconds. In 10 minutes, he was unconscious and in shock. Vigorous symptomatic treatment kept the patient alive for 34 hours after the accident.

Other Acute Effects

Several other immediate effects of acute overexposure should be noted. Because of its physical location, the skin is subject to more radiation exposure, especially in the case of low energy x-rays and beta rays, than most other tissues. An exposure of about 300 R (77 mC/kg) of low energy (in the diagnostic range) x-rays results in erythema. Higher doses may cause changes in pigmentation, loss of hair, blistering, cell death, and ulceration. Radiation dermatitis of the hands and face was a relatively common occupational disease among radiologists who practiced during the early years of the twentieth century.

The reproductive organs are particularly radiosensitive. A single dose of only 30 rads (300 mGy) to the testes results in temporary sterility among men. For women, a 300 rad (3 Gy) dose to the ovaries produces temporary sterility. Higher doses increase the period of temporary sterility. In women, temporary sterility is evidenced by a cessation of menstruation for a period of one month or more, depending on the dose. Irregularities in the menstrual cycle, which suggest functional changes in the reproductive organs, may result from local irradiation of the ovaries with doses smaller than that required for temporary sterilization.

The eyes too, are relatively radiosensitive. A local dose of several hundred rads can result in acute conjunctivitis.

Exposure Limits

As discussed in the introduction, concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895. Over the years, numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups. In general, the guidelines established for radiation exposure have had two principle objectives: 1) to prevent acute exposure; and 2) to limit chronic exposure to "acceptable" levels.

Current guidelines are based on the conservative assumption that there is no safe level of exposure. In other words, even the smallest exposure has some probability of causing a stochastic effect, such as cancer. This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure "as low as reasonable achievable" (ALARA). ALARA is a basic requirement of current radiation safety practices. It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible.

Regulatory Limits for Occupational Exposure

Many of the recommendations from the ICRP and other groups have been incorporated into the regulatory requirements of countries around the world. In the United States, annual radiation exposure limits are found in Title 10, part 20 of the Code of Federal Regulations, and in equivalent state regulations. For industrial radiographers who generally are not concerned with an intake of radioactive material, the Code sets the annual limit of exposure at the following:

1) the more limiting of:

  • A total effective dose equivalent of 5 rems (0.05 Sv) or The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (0.5 Sv).

2) The annual limits to the lens of the eye, to the skin, and to the extremities, which are:

  • A lens dose equivalent of 15 rems (0.15 Sv)

  • A shallow-dose equivalent of 50 rems (0.50 Sv) to the skin or to any extremity.

The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation. This value is the dose equivalent at a tissue depth of 0.007 cm averaged over and area of 10 cm2.
The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation. This value is the dose equivalent at a tissue depth of 0.3 cm.

The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation. This value is the dose equivalent at a tissue depth of 1 cm.
The total effective dose equivalent is the dose equivalent to the whole-body.

Declared Pregnant Workers and Minors

Because of the increased health risks to the rapidly developing embryo and fetus, pregnant women can receive no more than 0.5 rem during the entire gestation period. This is 10% of the dose limit that normally applies to radiation workers. Persons under the age of 18 years are also limited to 0.5rem/year.

Non-radiation Workers and the Public

The dose limit to non-radiation workers and members of the public are two percent of the annual occupational dose limit. Therefore, a non-radiation worker can receive a whole body dose of no more that 0.1 rem/year from industrial ionizing radiation. This exposure would be in addition to the 0.3 rem/year from natural background radiation and the 0.05 rem/year from man-made sources such as medical x-rays.

Controlling Radiation Exposure

When working with radiation, there is a concern for two types of exposure: acute and chronic. An acute exposure is a single accidental exposure to a high dose of radiation during a short period of time. An acute exposure has the potential for producing both nonstochastic and stochastic effects. Chronic exposure, which is also sometimes called "continuous exposure," is long-term, low level overexposure. Chronic exposure may result in stochastic health effects and is likely to be the result of improper or inadequate protective measures.

The three basic ways of controlling exposure to harmful radiation are: 1) limiting the time spent near a source of radiation, 2) increasing the distance away from the source, 3) and using shielding to stop or reduce the level of radiation.


The radiation dose is directly proportional to the time spent in the radiation. Therefore, a person should not stay near a source of radiation any longer than necessary. If a survey meter reads 4 mR/h at a particular location, a total dose of 4mr will be received if a person remains at that location for one hour. In a two hour span of time, a dose of 8 mR would be received. The following equation can be used to make a simple calculation to determine the dose that will be or has been received in a radiation area.

Dose = Dose Rate x Time

(click here for more information on using this formula)

When using a gamma camera, it is important to get the source from the shielded camera to the collimator as quickly as possible to limit the time of exposure to the unshielded source. Devices that shield radiation in some directions but allow it pass in one or more other directions are known as collimators. This is illustrated in the images at the bottom of this page.

Increasing distance from the source of radiation will reduce the amount of radiation received. As radiation travels from the source, it spreads out becoming less intense. This is analogous to standing near a fire. The closer a person stands to the fire, the more intense the heat feels from the fire. This phenomenon can be expressed by an equation known as the inverse square law, which states that as the radiation travels out from the source, the dosage decreases inversely with the square of the distance.

Inverse Square Law:    I1/ I2 = D22/ D12

(click here for more information on using this formula)

The third way to reduce exposure to radiation is to place something between the radiographer and the source of radiation. In general, the denser the material the more shielding it will provide. The most effective shielding is provided by depleted uranium metal. It is used primarily in gamma ray cameras like the one shown below. The circle of dark material in the plastic see-through camera (below right) would actually be a sphere of depleted uranium in a real gamma ray camera. Depleted uranium and other heavy metals, like tungsten, are very effective in shielding radiation because their tightly packed atoms make it hard for radiation to move through the material without interacting with the atoms. Lead and concrete are the most commonly used radiation shielding materials primarily because they are easy to work with and are readily available materials. Concrete is commonly used in the construction of radiation vaults. Some vaults will also be lined with lead sheeting to help reduce the radiation to acceptable levels on the outside.


Half-Value Layer (Shielding)

As was discussed in the radiation theory section, the depth of penetration for a given photon energy is dependent upon the material density (atomic structure). The more subatomic particles in a material (higher Z number), the greater the likelihood that interactions will occur and the radiation will lose its energy. Therefore, the denser a material is the smaller the depth of radiation penetration will be. Materials such as depleted uranium, tungsten and lead have high Z numbers, and are therefore very effective in shielding radiation. Concrete is not as effective in shielding radiation but it is a very common building material and so it is commonly used in the construction of radiation vaults.

Since different materials attenuate radiation to different degrees, a convenient method of comparing the shielding performance of materials was needed. The half-value layer (HVL) is commonly used for this purpose and to determine what thickness of a given material is necessary to reduce the exposure rate from a source to some level. At some point in the material, there is a level at which the radiation intensity becomes one half that at the surface of the material. This depth is known as the half-value layer for that material. Another way of looking at this is that the HVL is the amount of material necessary to the reduce the exposure rate from a source to one-half its unshielded value.

Sometimes shielding is specified as some number of HVL. For example, if a Gamma source is producing 369 R/h at one foot and a four HVL shield is placed around it, the intensity would be reduced to 23.0 R/h.

Each material has its own specific HVL thickness. Not only is the HVL material dependent, but it is also radiation energy dependent. This means that for a given material, if the radiation energy changes, the point at which the intensity decreases to half its original value will also change. Below are some HVL values for various materials commonly used in industrial radiography. As can be seen from reviewing the values, as the energy of the radiation increases the HVL value also increases.

Approximate HVL for Various Materials when Radiation is from a Gamma Source


Half-Value Layer, mm (inch)








44.5 (1.75)

12.7 (0.5)

4.8 (0.19)

3.3 (0.13)

2.8 (0.11)


60.5 (2.38)

21.6 (0.85)

12.5 (0.49)

7.9 (0.31)

6.9 (0.27)

Approximate Half-Value Layer for Various Materials when Radiation is from an X-ray Source


Half-Value Layer, mm (inch)

Peak Voltage (kVp)




0.06 (0.002)

4.32 (0.170)


0.27 (0.010)

15.10 (0.595)


0.30 (0.012)

22.32 (0.879)


0.52 (0.021)

25.0 (0.984)


0.88 (0.035)

28.0 (1.102)


1.47 (0.055)

31.21 (1.229)


2.5 (0.098)

33.0 (1.299)


7.9 (0.311)

44.45 (1.75)

Note: The values presented on this page are intended for educational purposes. Other sources of information should be consulted when designing shielding for radiation sources

Safety Controls

Since X-ray and gamma radiation are not detectable by the human senses and the resulting damage to the body is not immediately apparent, a variety of safety controls are used to limit exposure. The two basic types of radiation safety controls used to provide a safe working environment are engineered and administrative controls. Engineered controls include shielding, interlocks, alarms, warning signals, and material containment. Administrative controls include postings, procedures, dosimetry, and training.

Engineered Controls

Engineered controls such as shielding and door interlocks are used to contain the radiation in a cabinet or a "radiation vault." Fixed shielding materials are commonly high density concrete and/or lead. Door interlocks are used to immediately cut the power to X-ray generating equipment if a door is accidentally opened when X-rays are being produced. Warning lights are used to alert workers and the public that radiation is being used.  Sensors and warning alarms are often used to signal that a predetermined amount of radiation is present. Safety controls should never be tampered with or bypassed.

When portable radiography is performed, it is most often not practical to place alarms or warning lights in the exposure area. Ropes and signs are used to block the entrance to radiation areas and to alert the public to the presence of radiation. Occasionally, radiographers will use battery operated flashing lights to alert the public to the presence of radiation. Portable or temporary shielding devices may be fabricated from materials or equipment located in the area of the inspection. Sheets of steel, steel beams, or other equipment may be used for temporary shielding. It is the responsibility of the radiographer to know and understand the absorption value of various materials. More information on absorption values and material properties can be found in the radiography section of this site.

Administrative Controls

As mentioned above, administrative controls supplement the engineered controls. These controls include postings, procedures, dosimetry, and training. It is commonly

required that all areas containing X-ray producing equipment or radioactive materials have signs posted bearing the radiation symbol and a notice explaining the dangers of radiation. Normal operating procedures and emergency procedures must also be prepared and followed. In the US, federal law requires that any individual who is likely to receive more than 10% of any annual occupational dose limit be monitored for radiation exposure. This monitoring is accomplished with the use of dosimeters, which are discussed in the radiation safety equipment section of this material. Proper training with accompanying documentation is also a very important administrative control.

Radiation Detectors

Instruments used for radiation measurement fall into two broad categories:
   - rate measuring instruments and   - personal dose measuring instruments.

Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity). Survey meters, audible alarms and area


monitors fall into this category. These instruments present a radiation intensity reading relative to time, such as R/hr or mR/hr. An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time.

Dose measuring instruments are those that measure the total amount of exposure received during a measuring period. The dose measuring instruments, or dosimeters, that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual. An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units.

The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages.

Survey Meters

The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation. A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter.

There are many different models of survey meters available to measure radiation in the field. They all basically consist of a detector and a readout display. Analog and digital displays are available. Most of the survey meters used for industrial radiography use a gas filled detector.

Gas filled detectors consists of consists of a gas filled cylinder with two electrodes. Sometimes, the cylinder itself acts as one electrode, and a needle or thin taut wire along the axis of the cylinder acts as the other electrode. A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge). The gas becomes ionized whenever the counter is brought near radioactive substances. The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode. This results in an electrical signal that is amplified, correlated to exposure and displayed as a value.

Depending on the voltage applied between the anode and the cathode, the detector may be considered an ion chamber, a proportional counter, or a Geiger-Müller (GM) detector. Each of these types of detectors have their advantages and disadvantages. A brief summary of each of these detectors follows.

Ion Chamber Counter

Ion chambers have a relatively low voltage between the anode and cathode, which results in a collection of only the charges produced in the initial ionization event. This type of detector produces a weak output signal that corresponds to the number of ionization events. Higher energies and intensities of radiation will produce more ionization, which will result in a stronger output voltage.

Collection of only primary ions provides information on true radiation exposure (energy and intensity). However, the meters require sensitive electronics to amplify the signal, which makes them fairly expensive and delicate. The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies. This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator. An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used.

Proportional Counter

Proportional counter detectors use a slightly higher voltage between the anode and cathode. Due to the strong electrical field, the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas. The electrons produced in these secondary ion pairs, along with the primary electrons, continue to gain energy as they move towards the anode, and as they do, they produce more and more ionizations. The result is that each electron from a primary ion pair produces a cascade of ion pairs. This effect is known as gas multiplication or amplification. In this voltage regime, the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle. Hence, these gas ionization detectors are called proportional counters.

Like ion chamber detectors, proportional detectors discriminate between types of radiation. However, they require very stable electronics which are expensive and fragile. Proportional detectors are usually only used in a laboratory setting.

Geiger-Müller (GM) Counter

Geiger-Müller counters operate under even higher voltages between the anode and the cathode, usually in the 800 to 1200 volt range. Like the proportional counter, the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas. However, this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions. This all happens in a fraction of a second and results in an electrical current pulse of constant voltage. The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse. In other words, the size of the current pulse is independent of the size of the ionization event that produced it.

The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute. If the instrument has a speaker, the pulses can also produce an audible click. When the volume of gas in the chamber is completely ionized, ion collection stops until the electrical pulse discharges. Again, this only takes a fraction of a second, but this process slightly limits the rate at which individual events can be detected.

Because they can display individual ionizing events, GM counters are generally more sensitive to low levels of radiation than ion chamber instruments. By means of calibration, the count rate can be displayed as the exposure rate over a specified energy range. When used for gamma radiography, GM meters are typically calibrated for the energy of the gamma radiation being used. Most often, gamma radiation from Cs-137 at 0.662 MeV provides the calibration. Only small


errors occur when the radiographer uses Ir-192 (average energy about 0.34 MeV) or Co-60 (average energy about 1.25 MeV).

Since the Geiger-Müller counter produces many more electrons than a ion chamber counter or a proportional counter, it does not require the same level of electronic sophistication as other survey meters. This results in a meter that is relatively low cost and rugged. The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge).

Comparison of Gas Filled Detectors

The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage. In the ion chamber region, the voltage between the anode and cathode is relatively low and only primary ions are collected. In the proportional region ,the voltage is higher, and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected. In the GM region, a maximum number of secondary ions are collected when the gas around the anode is completely ionized. Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and proportional regions. Radiation at different energy levels forms different numbers of primary ions in the detector. However in the GM region, the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated the event. The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse. This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters.

Pocket Dosimeter

Pocket dosimeters are used to provide the wearer with an immediate reading of his or her exposure to x-rays and gamma rays. As the name implies, they are commonly worn in the pocket. The two types commonly used in industrial radiography are the Direct Read Pocket Dosimeter and the Digital Electronic Dosimeter.

Direct Read Pocket Dosimeter

A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen. The dosimeter contains a small ionization chamber with a volume of approximately two milliliters. Inside the ionization chamber is a central wire anode, and attached to this wire anode is a metal coated quartz fiber. When the anode is charged to a positive potential, the charge is distributed between the wire anode and quartz fiber. Electrostatic repulsion deflects the quartz fiber, and the greater the charge, the greater the deflection of the quartz fiber. Radiation incident on the chamber produces ionization inside the active volume of the chamber. The electrons produced by ionization are attracted to, and collected by, the positively charged central anode. This collection of electrons reduces the net positive charge and allows the quartz fiber to return in the direction of the original position. The amount of movement is directly proportional to the amount of ionization which occurs.

By pointing the instrument at a light source, the position of the fiber may be observed through a system of built-in lenses. The fiber is viewed on a translucent scale which is graduated in units of exposure. Typical industrial radiography pocket dosimeters have a full scale reading of 200 milliroentgens but there are designs that will record higher amounts. During the shift, the dosimeter reading should be checked frequently. The measured exposure should be recorded at the end of each shift.

The principal advantage of a pocket dosimeter is its ability to provide the wearer an immediate reading of his or her radiation exposure. It also has the advantage of being reusable. The limited range, inability to provide a permanent record, and the potential for discharging and reading loss due to dropping or bumping are a few of the main disadvantages of a pocket dosimeter. The dosimeters must be recharged and recorded at the start of each working shift. Charge leakage, or drift, can also affect the reading of a dosimeter. Leakage should be no greater than 2 percent of full scale in a 24 hour period.

Digital Electronic Dosimeter


Another type of pocket dosimeter is the Digital Electronic Dosimeter. These dosimeters record dose information and dose rate. These dosimeters most often use Geiger-Müller counters. The output of the radiation detector is collected and, when a predetermined exposure has been reached, the collected charge is discharged to trigger an electronic counter. The counter then displays the accumulated exposure and dose rate in digital form.

Some Digital Electronic Dosimeters include an audible alarm feature which emits an audible signal or chirp with each recorded increment of exposure. Some models can also be set to provide a continuous audible signal when a preset exposure has been reached. This format helps to minimize the reading errors associated with direct reading pocket ionization chamber dosimeters and allows the instrument to achieve a higher maximum readout before resetting is necessary.

Audible Alarm Rate Meters and Digital Electronic Dosimeters

Audible alarms are devices that emit a short "beep" or "chirp" when a predetermined exposure has been received. It is required that these electronic devices be worn by an individual working with gamma emitters. These devices reduce the likelihood of accidental exposures in industrial radiography by alerting the radiographer to dosages of radiation above a preset amount. Typical alarm rate meters will begin sounding in areas of 450-500 mR/h. It is important to note that audible alarms are not intended to be and should not be used as replacements for survey meters.

Most audible alarms use a Geiger-Müller detector. The output of the detector is

collected, and when a predetermined exposure has been reached, this collected charge is discharged through a speaker. Hence, an audible "chirp" is emitted. Consequently, the frequency or chirp rate of the alarm is proportional to the radiation intensity. The chirp rate varies among different alarms from one chirp per milliroentgen to more than 100 chirps per milliroentgen

Film Badges

Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays, X-rays and beta particles. The detector is, as the name implies, a piece of radiation sensitive film. The film is packaged in a light proof, vapor proof envelope preventing light, moisture or chemical vapors from affecting the film.

A special film is used which is coated with two different emulsions. One side is coated with a large grain, fast emulsion that is sensitive to low levels of exposure. The other side of the film is coated with a fine grain, slow emulsion that is less sensitive to exposure. If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted, the fast emulsion is removed and the dose is computed using the slow emulsion.

The film is contained inside a film holder or badge. The badge incorporates a series of filters to determine the quality of the radiation. Radiation of a given energy is attenuated to a different extent by various types of absorbers. Therefore, the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter. By comparing these results, the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy. The badge holder also contains an open window to determine radiation exposure due to beta particles. Beta particles are effectively shielded by a thin amount of material.

The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record, it is able to distinguish between different energies of photons, and it can measure doses due to different types of radiation. It is quite accurate for exposures greater than 100 millirem. The major disadvantages are that it must be developed and read by a processor (which is time consuming), prolonged heat exposure can affect the film, and exposures of less than 20 millirem of gamma radiation cannot be accurately measured.

Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives. Whole body badges are worn on the body between the neck and the waist, often on the belt or a shirt pocket. The clip-on badge is worn most often when performing X-ray or gamma radiography. The film badge may also be worn when working around a low curie source. Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation. A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body.

Thermoluminescent Dosimeter

Thermoluminescent dosimeters (TLD) are often used instead of the film badge. Like a film badge, it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received, if any. Thermoluminescent dosimeters can measure doses as low as 1 millirem, but under routine conditions their low-dose capability is approximately the same as for film badges. TLDs have a precision of approximately 15% for low doses. This precision improves to approximately 3% for high doses. The advantages of a TLD over other personnel monitors is its linearity of response to dose, its relative energy independence, and its sensitivity to low doses. It is also reusable, which is an advantage over film badges. However, no permanent record or re-readability is provided and an immediate, on the job readout is not possible.

How it works

A TLD is a phosphor, such as lithium fluoride (LiF) or calcium fluoride (CaF), in a solid crystal structure. When a TLD is exposed to ionizing radiation at ambient temperatures, the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material. Some of the atoms in the material that absorb that energy become ionized, producing free electrons and areas lacking one or more electrons, called holes. Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place.

Heating the crystal causes the crystal lattice to vibrate, releasing the trapped electrons in the process. Released electrons return to the original ground state, releasing the captured energy from ionization as light, hence the name thermoluminescent. Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor.

Instead of reading the optical density (blackness) of a film, as is done with film badges, the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured. The "glow curve" produced by this process is then related to the radiation exposure. The process can be repeated many times.


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