The Design of Diagnostic Medical Facilities where ... - ResearchGate

) NCRP method
The air kerma from unshielded secondary radiation, K sec
(0), at a distance d sec
for N patients is given by:
K N
K
sec(0)
=
d
1
sec
2
sec
where K 1 sec
= the total unshielded secondary air kerma (mGy) per patient
for leakage plus scatter radiations at 1 m. The values for
various workload distributions, published by the NCRP
(2004), are given in Table 5.4
N = number of patients per week
d sec
= distance from source (m)
The ceiling in this case would be described as radiography room (floor or other barriers). For this
workload distribution, the total unshielded secondary air kerma per patient, K 1 sec
at 1 m is given as 2.3
x 10 -2 mGy. (NCRP, 2004)
At a distance of 2.5 m, this gives:
K sec
(0) = (2.3 x 10 -2 mGy) x 300 patients per week
(2.5) 2
K sec
(0) = 1.104 mGy per week
The level of shielding required to meet the weekly design goal (modified by occupancy) can then be
determined as follows:
The weekly design goal, P =
0.3 mSv
= 0.006 mGy per week
52
Occupancy factor, T = 1
Design goal modified by occupancy, (P/T) = 0.006 mGy per week
The secondary barrier transmission required to reduce the air kerma to a value less than (P/T) is
therefore:
0.006 mGy per week
Bsec (x
barrier)
=
1.104 mGy per week
B
−3
sec barrier)
= 5.4 × 10
(x
Using the graphs from the NCRP to get barrier thickness, x barrier
, gives a lead equivalence of approx. 1.4
mm for the ceiling or 110 mm concrete, which is similar to but not identical with the conclusion from
the BIR method.
The Design of Diagnostic Medical Facilities where Ionising Radiation is used 67