Biomedical Engineering Reference
In-Depth Information
factors are given in Table 14.1:
w
R
= 1 for the gamma rays and, in this example,
w
R
=
10 for the neutrons. Thus, with the doses expressed in mGy, we find that
E
=
H
T
=
(1
×
8.4 + 10
×
1.2)
=
20 mSv.
(14.9)
Example
During the year, a worker receives 14 mGy externally from uniform, whole-body
gamma radiation. In addition, he receives estimated 50-y “committed” doses of
8.0 mGy from internally deposited alpha particles in the lung and 180 mGy from
beta particles in the thyroid. (a) What is the effective dose for this worker? (b) How
much additional external, uniform, whole-body gamma dose could he receive during
the year without technically exceeding the NCRP/ICRP annual limit? (c) Instead of
the gamma dose in (b), what additional committed alpha-particle dose to the red bone
marrow would exceed the annual effective-dose limit?
Solution
(a) Using the radiation weighting factors from Table 14.1, we obtain the following
equivalent doses for the individual tissues, with the tissue weighting factors from
Table 14.2 shown on the right:
H
Lung
= 8.0 × 20 = 160 mSv
(
w
T
= 0.12)
(14.10)
H
Thyroid
=
180
×
1
=
180 mSv
(
w
T
=
0.05)
(14.11)
H
Whole
-
body
=
14
×
1
=
14 mSv
(
w
T
=
1.00).
(14.12)
The effective dose is, by Eq. (14.4),
E
=
160
×
0.12 + 180
×
0.05 + 14
×
1
=
42 mSv.
(14.13)
(b) In order not to exceed the annual limit, any additional effective dose must be
limited to 50 - 42 = 8 mSv. Therefore, an additional uniform, whole-body gamma
dose of 8 mGy would bring the worker's effective dose to the annual limit of 50 mSv.
(c) We need to compute the dose to the red bone marrow that results in an effective
dose of 8 mSv. The weighting factor for this tissue is, from Table 14.2,
w
T
=
0.12.
Therefore, the committed equivalent dose to the red bone marrow is limited to
H
T
=
(8 mSv)/0.12
67 mSv. Since the radiation weighting factor for alpha particles is
20 (Table 14.1), the limiting average absorbed dose to the red bone marrow is, by
Eq. (14.1),
D
RBM,
α
=
=
3.4 mGy.
Depending on a one's exposure history, it is possible for the cumulative limit in
Table 14.4 to be the limiting factor, rather than the annual limit. The NCRP cumula-
tive limit is 10 mSv times the age of a worker in years. The ICRP cumulative limit is
100 mSv in any 5-y period. (See Problem 15.)
67/20
=
