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

EXTERNAL RADIATION SAFETY 551
TABLE 10-7. Primary Barrier Tenth Value Layers (TVLs) for Ordinary Concrete
(2.35 g/cm 3 ), Steel (7.87 g/cm 3 ) and Lead (11.35 g/cm 3 )
ENDPOINT ENERGY MATERIAL TVL1(cm) TVLe(cm)
4 Concrete 35 30
Steel 9.9 9.9
Lead 5.7 5.7
6 Concrete 37 33
Steel 10 10
Lead 5.7 5.7
10 Concrete 41 37
Steel 11 11
Lead 5.7 5.7
15 Concrete 44 41
Steel 11 11
Lead 5.7 5.7
18 Concrete 45 43
Steel 11 11
Lead 5.7 5.7
20 Concrete 46 44
Steel 11 11
Lead 5.7 5.7
25 Concrete 49 46
Steel 11 11
Lead 5.7 5.7
30 Concrete 51 49
Steel 11 11
Lead 5.7 5.7
Co-60 Concrete 21 21
Steel 7.0 7.0
Lead 4.0 4.0
The first (TVL1) and equilibrium (TVLe) tenth-value layers are used to account for the spectral changes in the radiation as
it penetrates the barrier.
Reproduced with permission of the National Council on Radiation Protection and Measurements from Structural
Shielding Design and Evaluation for Megavoltage X-and Gamma-Ray Radiotherapy Facilities. Bethesda, MD: National
Council on Radiation Protection & Measurement; 2005, NCRP Report 151.
Since the concrete block wall attenuates the secondary radiation to 1.3 ×
10 −1 , we will need additional shielding. The additional transmission factor is
calculated with Eq. (10.23):
1.3 × 10 −1 × Badd = 5.4 × 10 −3
Badd = 4.2 × 10 −2 .
In Figure 10-17. we see that 0.72-mm lead will reduce the radiation level in
the adjacent room to the design level of 0.02 mGy/wk. Table 10-7 shows that
we specify 0.79-mm (1/32-in.) thick lead sheeting to affix to the wall.
(c) Ceiling—The shielding design distance for the floor above is 1.7 m above
the floor, which corresponds to 3.2 m above the isocenter. Since the room
is fully occupied and not controlled, P = 0.02 mGy/wk and the use factor,