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

EXTERNAL RADIATION SAFETY 519
Figure 10-3. Geometry for calculating
the variation of dose with distance
from a plane source of radiation.
If activity is given in Ci/m 2 and Ɣ in R-m 2 /Ci-h, and since an exposure of 1
R corresponds to a dose equivalent of 1 rem, the dose-equivalent rate in rems/h
can be calculated with Eq. (10.10) by using the traditional units instead of the
SI units.
H ˙
rem · m2
= Ɣ × Ca
Ci · h
Ci
m2 × π × lnr 2 + h2 h2 rem
. (10.11)
h
The ratio of the dose-equivalent rate at a distance h to the dose-equivalent rate at
any other distance is given by
H1 ˙
=
H2 ˙
2 2
(r + h1 ln
)
h 2
1
2 2
(r + h2 )
ln
h 2 2
W Example 10.3
. (10.12)
Fifty MBq of 24 NaCl solution spilled over a circular area 50 cm in diameter. What is
the gamma-ray dose-equivalent rate at a height of
(a) 30 cm?
(b) 1m?
Solution
(a) From Table 6-3, we find the specific gamma-ray emission constant for 24 Na
to be 4.36 × 10 −7 Sv-m 2 /MBq-h. The areal concentration Ca = 50 MBq/π
(0.25 m) 2 = 254.65 MBq/m 2 . Substituting the respective values into