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

538 CHAPTER 10
TABLE 10-5. Unshielded Primary and Secondary Air Kerma (mGy per Patient) for the
Indicated Workload, Wnorm (mA-min/wk), normalized to a primary beam distance of 1 m
WORKLOAD DISTRIBUTION W a
norm K 1 P b K 1 sec c K 1 sec d
Rad room (chest bucky) 0.6 2.3 5.3 × 10 −3 7.3 × 10 −3
Rad room (floor or other barriers) 1.9 5.2 2.3 × 10 −2 3.3 × 10 −2
Rad tube (R &Froom) 1.5 5.9 2.9 × 10 −2 4.0 × 10 −2
Chest room 0.22 1.2 2.7 × 10 −3 3.6 × 10 −3
Cardiac angiography 160 N/A 2.7 3.8
aFor the indicated clinical installations, Wnorm is the average workload, mA per patient.
bThese values for the primary air kerma, mGy per patient, ignore the attenuation available in the radiographic table and
image receptor.
cLeakage and side scatter.
dLeakage and forward/backscatter.
Reproduced with permission of the National Council on Radiation Protection and Measurements from Structural
Shielding Design for Medical X-Ray Imaging Facilities. Bethesda, MD: National Council on Radiation Protection &
Measurement; 2004. NCRP Report 147.
per patient, 250 patients per week, 1 for the use factor, and 3 m for the distance
to the nearest point to be protected into Eq. (10.21), we have
KP(0) =
mGy · m2 patients
1.2 × 250
patient wk
(3 m) 2
× 1
= 33.3 mGy/wk.
The required transmission factor for the primary barrier is calculated from
BP =
P mGy/wk
. (10.22)
KP(0)
Since the area to be protected is not controlled, the limiting radiation dose
there is 0.02 mGy/wk (2 mrems/wk). The required primary transmission
factor therefore is
0.02 mGy/wk
B P =
33.3 mGy/wk = 6 × 10−4 .
The wall consists of two 5/8-in.-thick gypsum wallboards, or a single wallboard
with a total thickness of 28 mm. We will first determine the transmission of
the gypsum wallboard to see whether it is sufficient. If not, we will determine
the thickness of additional lead sheeting that must be hung on the wall to
transmit no more than 6 × 10 −4 , or 0.06%, of the incident radiation.
Based on the workloads listed in Table 10-2, broad-beam transmission curves
for the primary X-ray spectra and for the secondary (leakage and scattered)
radiation generated by each of the imaging applications have been published
in NCRP 147 for lead, concrete, gypsum wallboard, steel, plate glass, and
wood. The curves for gypsum and concrete are reproduced in Figures 10-14
and 10-15.
Figure 10-14 shows that 28 mm (the two gypsum wallboards, 14 mm each)
gypsum wallboard transmits 0.35 (35%) of the “chest room” incident radiation.
However, we need a transmission no greater than 6 × 10 −4 (0.06%).