Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

where
EXTERNAL RADIATION SAFETY 533
P = allowable air kerma rate, either 0.1 mGy/wk for a controlled area or
0.02 mGy/wk for an uncontrolled area,
T = occupancy factor (Table 10-1),
dP = distance from X-ray tube to point of interest (usually 1 ft., or 0.3 m, from
the barrier),
K 1 P
= unshielded primary air kerma per patient at a distance of 1 m,
U = use factor = fraction of the time that the primary beam is directed towards
a given primary barrier, and
N = number of patients per week.
Using the kVp weighted values for the workload, and using three empirically determined
parameters, mathematical models were fitted to the data obtained for each
of the exposure modalities that had been investigated, and broad-beam transmission
equations were determined for primary and secondary protective barriers. The
thickness of the primary barrier is given by
x = 1
αγ ln
⎡
1 NTUKP ⎢ Pd
⎢
⎣
2 P
1 + β
α
γ
+ β
α
⎤
⎥ , (10.19)
⎦
where α, β, and γ are parameters that depend on the barrier material and on the
operating potential of the X-ray tube, which is implicit in the imaging modality.
NCRP Publication 147 lists the values for these parameters for lead, concrete, gypsum
wallboard, steel, plate glass, and wood. Values for lead, concrete, and gypsum
wallboard for the several modalities that had been studied are listed in Table 10-3.
For calculating the thickness of a secondary barrier, the following empirical equation
is used:
x = 1
αγ ln
⎡
1 NTKsec ⎢ Pd
⎢
⎣
2 γ
sec
1 + β
α
+ β
α
⎤
⎥ . (10.20)
⎦
The parameter values for secondary radiation are different from those for primary
radiation because of the different X-ray energies of the secondary radiation. Table
10-4 lists some of these values.
Some of the physical factors that determine the shielding requirements for protection
from X-ray beams are shown in Figures 10-11, 10-12, and 10-13. A collimated
X-ray beam of area F is directed at patient M (or object to be radiographed, in the
case of nonmedical radiography) from the shielded X-ray tube. The beam passes
through the patient and is attenuated to an acceptable level by the primary protective
barrier before irradiating a person in area 1. The leakage radiation from
the X-ray tube and the scattered radiations are attenuated to an acceptable level by