DESIGN CALCULATION OF

INDUSTRIAL RADIOGRAPHIC EXPOSURE ROOM

The use of X-ray generators and radioactive sources is no doubt danger to the operator, therefore

precautions must be taken to prevent exposure to direct and scattered radiation and to high voltages. The

operator is normally protected from electrical shocks by the design of the X-ray equipment, but protection

against the insidious affects of radiation has to be provided partly by the construction of shielded

accommodation for the source of the radiation and partly by the operator's appreciation of the dangers

involved. Whenever practicable a permanent exposure room should be provided for industrial

radiography. Such facility must necessarily be designed. The following points should be considered in the

design or selection of a radiographic exposure room.

(i) It should have electrical and water connections within especially for X-ray room.

(ii) There should be a separate place outside the exposure room to house the control unit so that the

operator is not exposed.

(iii) The thickness and the material of the walls or doors should be sufficient to reduce the dose below

the maximum permissible level. Dense concrete or lead of calculated thickness is usually used.

(iv) Audible and/or visual warning signs shall be provided within the exposure room. These signs shall

be actuated before irradiation begins and remain actuated until completion of the irradiation.

(v) Reliable locks or interlocks shall be provided to prevent any person from entering a radiation room

during irradiation. In the event of an exposure being terminated by interlock, it shall only be

possible to initiate the irradiation from the control panel.

(vi) Suitable means of exit shall be provided, so that any person who is accidentally shut in the

irradiation room can leave the enclosure without delay.

(vii) There should be a shielded apartment within the room preferably underground where gamma

containers can be stored while not in use. The key for this enclosure should be kept with care and

responsibility,

(viii) A survey meter should be available in the laboratory. This is required for checking the dose level

before entering the room.

(ix) The flooring and the ceiling of the room should be such as to give minimum backscatter. Lead

lining of the walls, floor and the ceiling would be an ideal situation.

The design of X-ray or gamma ray radiographic exposure room requires some calculations on shielding to

provide safe operation of the facility and minimum exposure to radiation workers.

Careful design can lead to economical installations with minimal barriers.

The design depends on the following factors:

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(i) Maximum tube voltage or gamma energy,

(ii) The maximum tube current or source strength,

(iii) The permitted full-body dosage at the point of interest. For radiation workers this would be 100

millirads per-week (10 Gy per-week), and for other persons, 10 milli-rads per week (10 Gy per

week).

(iv) The workload (W). For X-ray equipment this is given in mA-min. per-week and in rad in air per-

week at lm for gamma source.

(v) The use factor (U). This represents the fraction of the work time that the beam is turned towards

the point under consideration. In the absence-of information obtained by monitoring. Table I gives

the values of U recommended by ICRP.

(vi) The-occupancy factor (T). This is the fraction of the work time spent in the area in question. The

recommended values laid down by the ICRP are given in Table II.

(vii) Maximum dose output from the tube or RHM factor of radioactive source.

(viii) Shielding materials. Choice of material for a barrier depends on convenience and cost. The

radiographic exposure room is usually made of concrete with lead lining.

Primary Protective Barriers

Primary protective barriers are those "sufficient to attenuate the useful beam to the required degree" (1).

The thickness required may be obtained after calculating B, the maximum allowable transmission given by

[2],

(1)

where P = maximum permissible exposure for design purpose (0.1 rem/week or 0.01 rem/week)

d = distance in metres from source to position occupied.

W = weekly workload in mA-min/week or R/week at l m.

U = use factor (Table I)

T = occupancy factor (Table II).

For X-ray up to 3MV, equation (1) yields B in units of R/mA-min at 1 m: for gamma rays B is transmission.

The shield thickness corresponding to the calculated value of B is read from the appropriate transmission

curve. Figures 5 to 8 show transmission curves for a range of X-ray energies and gamma rays with lead

and concrete.

Lead linings for walls and floors are useful particularly when converting existing buildings or providing

enclosures for X-ray work.

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Secondary Protective Barriers

Barriers for scatter radiation

For scatter radiation, the maximum allowable transmission B by [2],

(2)

P and T are the same as in equation (1). W is also the same, but if the source to scatterer distance is not 1

m, equation (2) must be modified according to the inverse square law; thus if the source to scatterer

distance is 50 cm the denominator is multiplied by 4. S is the percent of the incident absorbed dose rate or

exposure rate scattered to 1 m for the irradiated area of interest; values of S may be derived from Figs. 9

and 10. It is useful to note that a change in the source to scatterer distance is balanced by the resulting

change in irradiated area. For high energy X-rays, S must be multiplied by the ratio of the output at the

potential of interest to that of 0.5 MV namely 20 at 1 MV, 300 at 2 MV and 850 at 3 MV. ds is the distance in

metres from the scatterer to the location of interest. The shield thickness corresponding to the calculated

values of B is read from the transmission chart that is used for calculating primary-barriers.

Barriers for Leakage Radiation

Leakage radiation is defined as "all radiation except the useful beam, coming from the tube or source

housing [1]). It must be below certain limits of exposure rate. Shielding required for leakage radiation may

be calculated from the number of tenth value thickness NTVT corresponding to the maximum allowable

transmission [2],

T, d and P are the same as in equation (1). WL is the weekly leakage exposure rate, or absorbed dose rate

at 1 m from the source. The number of half-value thickness NHVT is 3.3 NTVT.

The shield thickness is obtained by multiplying NTVT or NHVT by the values given in Tables IV and V.

If the shield thickness for scatter and leakage radiation differ by 1 TVT or more, the thicker shield should be

adopted for the secondary barrier thickness. However, if they differ by less than 1 TVT, the thicker shield

should be adopted and 1 HVT added.

Example 1

Examples of X-ray shielding requirements are given in Table VII for primary barrier and Table VIII for

secondary barrier [2]. Table VIII is based on typical irradiation characteristics:

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50 cm source to scatterer distance;

900 angle of scatter;

400 cm2 irradiated area implying 0.1% of the incident exposure rate scattered to 1m;

200 mA.min/h maximum continuous tube rating at 100 and 150kV and 1000 mA.min/h at 200-400kV;

leakage radiation 0.1R/h at 1m from the target for 100 and 150kV and 1R/h at 1m for 200-400 kV, at the

maximum continuous tube ratings.

Example 2

Let us consider a design of exposure room for a 50Ci Ir-192 uncollimated source. Labyrinth door design is

preferred. Consider the design is as in Fig. 4.

For primary barrier (wall)

P = 0.01 rem/week (for non-radiation worker)

d = 1 m

W = 0.48 X 50 X 40 R/week (40 working hours/week) = 960

U = 1

T = ¼ = 0.25

= 4.17*10-5

Concrete wall thickness = 650 mm.

Primary barrier for control area

P = 0.1 rem/week (for radiation worker)

d = 1 m

W = 0.48 X 50 X 40 R/week (40 working hours/week) = 960

U = 1

T = 1

= 1.04*10-4

Concrete wall thickness = 600 mm.

Secondary barrier for lead door

P = 0.01 rem/week (for non-radiation worker)

ds = 3 m

W = 0.48 X 50 X 40 R/week (40 working hours/week) = 960

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T = ¼

S = 0.1% incident absorbed dose rate scattered to 1m per 400cm2 irradiated area. Scattering angle = 900

(Fig. 9).

= 0.375

Lead door thickness = 6 mm (Fig. 11).

Shielding Construction

To provide protection against radiation, the construction should be leak proof to radiation and this can be

achieved by over-lapping the lead sheets or concrete blocks. Further, nails or screws, which are used to

unite plies of entry door, must be covered with extra lead. In addition, the conduits, pipes and air ducts,

passing through the walls of the shielded area must be completely shielded. The entry door must overlap

with the boundary of concrete wall to avoid leakage of radiation. Figs. 1 to 4 show typical protective

constructions practice.

If the exposure room is on the lowest floor of a building, the floor of the room need not be completely

protected. However, if the wall is lined with lead, the lead lining should not stop at the floor level. It should

be extended inward from all four walls. This is to prevent radiation from escaping from the room by

penetrating the floor and then scattering upward outside the protective barriers. An alternative is to extend

the lead protection in the walls downward from some distance below floor level. The same considerations

apply to the ceiling if the room is located on the top floor of a building. Of course, if there is occupied space

above or below the exposure room, the ceiling or floor of the exposure room must have a full radiation

protection over its whole area.

Although lead is the most common material for x-ray protection, other materials may be used. In particular,

structural walls of concrete or brick may afford considerable protection and may reduce the thickness, and

therefore the cost of the lead required. Above 500 kV, concrete is most used as protective material. The

thickness of lead required at these higher energies are so great, where fastening the lead to the walls

becomes a serious problem. Therefore, concrete is often used because of the ease of construction. In

new construction, the use of concrete may have economic advantages even for protection against

radiation generated at low energies (well below 500 kV). Applicable codes should be examined and any

installations checked for compliance with their requirements.

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References

1- ICRP Publication 3, Report of Committee III on Protection against X-rays, Oxford, Pergamon, 1960.

2- ICRP Publication 15 and 21, 1976 edition.

3- LPTA Safety Code of Practise for Industrial Radiography

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Table I: Use Factors [1]

Full Use(U=1)

Floors of radiation rooms except dental installations, doors, wall and ceiling areas of radiation rooms routinely exposed to useful beam.

Partial Use(U=1/4)

Doors and wall areas of radiation rooms not routinely exposed to the useful beam, floors of dental installations.

Occasional Use(U=1/16)

Ceiling areas of radiation rooms not routinely exposed to the useful beam

Table II: Occupancy Factors [1]

Full Occupancy(T = 1)

Control space, offices, corridors and waiting space large enough to hold desks, darkrooms, workrooms and shops, nurse stations, rest and lounge rooms routinely used by occupationally exposed personnel, living quarters, children's play areas, occupied space in adjoining buildings.

Partial Occupancy(T = ¼)

Corridors too narrow for desks, utility rooms, rest and lounge rooms not used routinely by occupationally exposed personnel, wards and patients rooms, elevators using operators, unattended parking lots

Occasional Occupancy(T = 1/16)

Closets too small for future occupancy, toilets not used routinely by occupationally exposed personnel, stairways, automatic: elevators, sidewalks, and streets.

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Table III: Outputs of Gamma-ray Sources [2]

Nuclides Half-lifePrincipal -ray energies (MeV), and % photon per disintegration

Exposure rate, R/h at 1m from 1Ci

60Co 5.24y 1.17 (100%);1.33 (100%)

1.30

137Cs 30y 0.66 (85%) 0.32192Ir 74d 0.3 ~ 0.6 0.48

226Raand daughter

1620y 0.074 to 2.4 0.825

Table IV: Approximate Half-Value-Thickness and Tenth-Value-Thickness for Heavily Attenuated Broad Beams of X-Rays

X-ray Half-value-thickness, cm Tenth-value-thickness, cmsource Lead Concrete Lead Concrete50 kV 0.005 0.4 0.018 1.3

70 - 1.0 - 3.675 0.015 - 0.050 -

100 0.025 1.6 0.084 5.5125 - 1.9 - 6.4150 0.029 2.2 0.096 7.0200 0.042 2.6 0.14 8.6250 0.086 2.8 0.29 9.0300 0.17 3.0 0.57 10.000 0.25 3.0 0.82 10.0

500 0.31 3.6 1.03 11.9

Table IV: Approximate Half-Value-Thickness and Tenth-Value-Thickness for Heavily Attenuated Broad Beams of Gamma Rays

MaterialNuclide Uranium, cm Lead,cm Steel, cm Concrete, cm

HVT TVT HVT TVT HVT TVT HVT TVT60Co 0.7 2.2 1.2 4.0 2.0 6.7 6.1 20.3137Cs 0.3 1.1 0.7 2.2 1.5 5.0 4.9 16.3192Ir 0.4 1.2 0.6 1.9 1.3 4.3 4.1 13.5

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Table VI: Lead Equivalent of Various Materials for Low Energy X-rays

Material Material cm lead equivalent at applied kV of

Material Density

gm/cm3

Thickness,

cm50 75 100 150 200 250 300 400

Clay

brick

1.6 10

20

30

40

50

0.06

0.14

0.22

-

-

0.08

0.17

0.27

0.38

-

0.09

0.19

0.31

0.45

-

0.08

0.17

0.26

0.37

0.48

0.08

0.17

0.26

0.37

0.48

0.10

0.23

0.40

0.60

0.81

0.11

0.30

0.55

0.83

1.13

0.13

0.45

0.85

1.27

1.71

Barytes

Plaster or

concrete

3.2 1.0

2.0

2.5

5.0

7.5

10.0

12.5

0.09

0.18

0.23

-

-

-

-

0.15

0.27

0.33

-

-

-

-

0.18

0.33

0.40

-

-

-

-

0.09

0.18

0.22

0.43

0.59

-

-

0.07

0.14

0.17

0.34

0.50

0.68

-

0.06

0.13

0.17

0.36

0.56

0.77

-

0.06

0.14

0.18

0.39

0.61

0.84

1.08

0.08

0.16

0.20

0.43

0.68

0.95

1.21

Steel 7.8 0.1

0.2

0.3

0.4

0.5

1.0

2.0

3.0

4.0

5.0

-

-

-

-

-

-

-

-

-

-

0.01

0.03

0.05

0.07

0.09

-

-

-

-

-

0.02

0.03

0.05

0.07

0.09

-

-

-

-

-

0.01

0.02

0.03

0.04

0.05

0.09

0.17

0.25

0.33

0.40

0.01

0.02

0.03

0.04

0.04

0.08

0.16

0.23

0.30

0.37

-

-

-

-

0.03

0.08

0.17

0.28

0.38

0.49

-

-

-

-

0.03

0.08

0.19

0.33

0.47

0.63

-

-

-

-

0.04

0.09

0.24

0.43

0.65

0.88

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Table VII: Primary X-Ray Shielding Requirements for 0.1 Rem per Week

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Table VIII: Scatter and Leakage X-Ray Shielding Requirements for 0.1 Rem per Week

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Fig. 1: Plan views of door entries to exposure rooms, showing incorrect (a) and correct (b), (c) methods of fitting. (a) Leakage of primary radiation due to incorrectly fitted sliding door; (b) hinged door; (c) sliding door.

Fig. 2: Methods of shielding when pipes ducts, conduits or cables must pass through walls of exposure room.

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(a)

(b) Hinged door (c) Sliding door

Fig. 3: Scatter of radiation through a roof.

Fig. 4: Labyrinth Design of Exposure Room. This is effectively reduced the lead door thickness. Radiation is reduced to approximately 0.1% on each scatter.

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3 m

1 m

1 m

Control

PartialOccupancy

PartialOccupancy

PartialOccupancy

PartialOccupancy

Fig. 5: Broad-beam transmission of X-rays through concrete (= 2.35 g/cm3)

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Fig. 6: Broad-beam transmission of X-rays through lead ( = 11.35 g/cm3)

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Fig. 7: Broad-beam transmission of -rays through concrete (= 2.35 g/cm3)

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Fig. 8: Broad-beam transmission of -rays through concrete (= 2.35 g/cm3)

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Fig. 9: Variation with potential of the absorbed dose rate measured in air due to X-rays scattered at 90 from various materials. Percent scatter is related to primary beam measurements in air at the point of incidence [2].

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Fig. 10: Scattering patterns of diverging X-ray and gamma ray beams normally incident on concrete shield. Percent is related to primary beam measurements in free air at the point of incidence [2].

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