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Question 149: 10 CFR 20.1003 defines the shallow-dose
equivalent as the dose equivalent at a tissue depth of
0.007 cm. (a) Does this mean that the dose to the skin of
the whole body is the sum of the non-penetrating dose
equivalent beta and low energy photons) and the deep dose
equivalent? (b) Is it proper to calculate the extremity
dose by summing the dose equivalent measured on an
extremity dosimeter (which may only be worn for part of the
monitoring period) with the deep dose equivalent?
Answer: General response: 10 CFR 20.1502 requires
monitoring of external dose for individuals who are likely
to receive, in a year, a dose in excess of 10% of the
applicable limits. Requirements to measure / assess the dose
equivalent at depths of 0.007, 0.3, and 1 cm exist under
old Part 20 as well as revised Part 20. In old Part 20,
these requirements are included in the instructions for
Item 5 of NRC Form 5. In the revised Part 20, these
requirements are in Part 20 itself, together with new dose
limits and special names in the definitions for the dose
equivalents at these three depths. The only explicit
requirements concerning the precision and accuracy of
personnel dosimetry are the NVLAP accreditation
requirements, which are the same in old Part 20 [10 CFR
20.202 (c)] and revised Part 20 [10 CFR 20.1501 (c)].
Methods that have been acceptable for measuring / assessing
dose equivalent at these three depths in the past should
continue to be acceptable in the future.
Answers to the specific questions are as follows:
(a) No. The "dose to the skin of the whole body" is the
shallow dose equivalent. The shallow-dose equivalent is
the dose equivalent at a depth of 0.007 cm (7 mg/square cm)
from all types of radiation, whether "penetrating" such as
gamma rays and neutrons) or "non-penetrating" (such as weak
beta radiation and lower energy x-rays).
(b) No, not in general. The question does not make it
clear whether or not the dose summing is for dose during
the same time period. It is never proper to calculate an
extremity dose (shallow-dose equivalent) for a particular
time period by adding a deep dose equivalent to the
shallow-dose equivalent. If the question refers to a
monitoring period during which an extremity dosimeter
(measuring shallow-dose equivalent) was used only part of
the time, but during which a whole-body dosimeter was used
all of the time, the answer depends on the circumstances of
the individual's exposure. It would be acceptable to
assume, for the times during which the extremity dosimeter
is not used, that the extremity dose (shallow-dose
equivalent) is equal to the shallow-dose equivalent
measured by the whole-body dosimeter. If only the
deep-dose equivalent is measured by the whole-body
dosimeter, it would be acceptable to assume, for times
during which the extremity dosimeter is not used, that the
extremity dose is equal to the deep-dose equivalent
(measured by the whole-body dosimeter) if it can be shown
that types and levels of radiation to which the extremity
was exposed would not have resulted in a significantly
higher shallow-dose equivalent to the extremity than the
deep dose equivalent to the whole body. (Reference: 10 CFR
20.1003)