Shielding Calculation Method For X-Ray Room

General Shielding Concepts 

The objective of a shielding calculation is to determine the thickness of the barrier that is sufficient to reduce the air kerma in an occupied area to a value ≤P/T, the weekly shielding design goal modified by the occupancy factor for the area to be shielded. The broad-beam transmission function [B(x)] is defined as the ratio of the air kerma behind a barrier of thickness x to the air kerma at the same location with no intervening radiation barrier. An acceptable barrier thickness (xbarrier) is one in which the value of the broad-beam transmission function is:

where d is the distance between the radiation source and the individual beyond the barrier, K1 is the average unshielded air kerma per patient at 1 m from the source, and N is the expected number of patients examined in the room per week. The transmission characteristics of broad-beam x-ray sources are discussed in Appendix

transmission curves are provided; and parameters (α, β and γ) are provided for a model that permits an algebraic solution for xbarrier as:

Note that the broad-beam transmission fitting parameters (α, β and γ) depend on the material of the barrier, as well as the workload distribution as a function of kVp.

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Shielding for Primary Barriers

The barrier transmission factor (BP) sufficient to decrease KP(0) (the air kerma from unshielded primary radiation at a distance dP) to P/T is given by:

Appropriate values for , the unshielded primary air kerma per patient at 1 m, are provided for each of the clinical workload distributions in Table

 P is the weekly shielding design goal as

"Shielding design goals are used in the design or evaluation of barriers constructed for the protection of employees and members of the public. The weekly shielding design goal for a controlled area is an air-kerma value of 0.1 mGy week–1. The weekly shielding design goal for an uncontrolled area is an air-kerma value of 0.02 mGy week–1"

T is the occupancy factor

 U is the use factor in

dP is the distance from the source to the location of the maximally exposed individual beyond the primary barrier in

The distance (d) to the occupied area of interest should be taken from the source to the nearest likely approach of the sensitive organs of a person to the barrier. For a wall this may be assumed to be not <0.3 m. For a source located above potentially occupied spaces, the sensitive organs of the person below can be assumed to be not >1.7 m above the lower floor, while for ceiling transmission the distance of at least 0.5 m above the floor of the room above is generally reasonable. In some special cases, such as a nursing station or outdoor sidewalk, the distance from the barrier to the nearest routinely occupied area may be considerably greater

The primary beam transmission functions [BP(xbarrier)] for each workload distribution for a variety of shielding materials have been derived and are shown in Appendix , These were calculated by summing the air kerma in each kVp interval transmitted through a given barrier thickness and dividing that by the total air kerma expected with no barrier. These primary beam transmission curves are shown in Figures for lead, concrete, gypsum wallboard, steel, and plate glass .

The structural barrier thickness (xbarrier) required to adequately shield against primary radiation may be calculated by determining the total shielding thickness required (xbarrier + xpre), and then if applicable, subtracting the equivalent “preshielding” thickness xpre given in Table

to obtain xbarrier. Alternatively, an algebraic solution for xbarrier, given in Equation

The fitting parameters (α, β and γ) for primary radiation generated by the clinical workload distributions are given in 

Shielding for Secondary Barriers

The barrier transmission factor [Bsec(xbarrier)] that reduces Ksec(0) (the air kerma from unshielded secondary radiation at a distance dsec) to P/T for secondary radiation is:

 Appropriate values for the unshielded secondary air kerma per patient at 1 m, are provided for each of the clinical workload distributions in

P is the weekly shielding design goal (Same as Primary shield )

T is the occupancy factor (Same as Primary shield )

dsec is the distance from the source of the secondary radiation to the location of the maximally-exposed individual beyond the secondary barrier in

The thickness xbarrier satisfying Equation can be graphically determined from

As before, an algebraic determination of xbarrier may also be made. The secondary transmission [Bsec(xbarrier)] has been fitted to the form of Equations

 with fitting parameters given in Table

 Substituting Bsec(xbarrier) from

 Equation yields

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