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Personnel Dosimetry - doe-std-1128-98_ch10172
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DOE Standard Guide of Good Practices for Occupational Radiological Protection In Plutonium Facilities
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Extremity Dosimetry - doe-std-1128-98_ch10174


DOE-STD-1128-98
Thermoluminescent dosimeters are the most widely used neutron dosimeters in
plutonium facilities. The energy response of a typical TLD-albedo dosimeter is
shown in Figure 6.2. At neutron energies below about 20 keV, the energy
response is almost constant. Above 20 keV, the response per unit dose equivalent
drops dramatically by almost three orders of magnitude at 10 MeV. Almost all
neutrons emitted by plutonium have energies in the MeV range. However, about
50% of the neutrons striking a thick concrete wall or floor are reflected back into
the room at lower energies, and neutrons typically are reflected two or three
times before being absorbed. Thus, the low-energy scattered neutrons are often
more important in determining TLD-albedo dosimeter response than the high-
energy neutrons emitted by the plutonium source. The TLD-albedo dosimeters
are often calibrated in specific facilities by exposing them on phantoms at
locations where the dose equivalent has been carefully determined from dose and
spectrometric measurements (Brackenbush et al., 1991).
Nuclear track dosimeters are also being used for personnel dosimetry in
plutonium facilities. These dosimeters have the advantage of a much more
constant response per unit dose equivalent, as shown in Figure 6.3. Nuclear track
dosimeters operate on the principle that a fast neutron interacts with plastic to
produce a proton recoil that damages the polymer. Under special etch conditions,
the damaged areas are removed to produce a distinct track, which is easily
observed under a microscope. The neutron dose equivalent is then determined
from the track density. Nuclear track dosimeters have a distinct threshold, usually
about 100 keV.
In conclusion, the combination of TLD-albedo and nuclear track dosimeters can
provide a more uniform response with energy and more accurate personnel
dosimetry. This combination of dosimeters may be an appropriate solution to
neutron dose monitoring in DOE facilities with significant neutron exposure.
It is important to verify and document that personnel dosimetry systems provide
accurate measurements and records of the occupational radiation doses received
by workers in plutonium facilities (McDonald et al., 1992). To provide a level of
confidence in dosimetry services in DOE facilities, the DOELAP accreditation
program was established. 10 CFR 835 402(b) (DOE 1998a) requires that
personnel dosimetry programs implemented to demonstrate compliance with the
dose limits established in Subpart C shall be accredited in accordance with the
requirements of the DOELAP for Personnel Dosimetry. The National Institute of
Standards and Technology (NIST) has also established the National Voluntary
Laboratory Accreditation Program (NVLAP) for testing and accreditation of
dosimeter processors serving commercial industry and medical facilities.
Because the dosimetry needs at many DOE facilities, particularly those
processing plutonium, are different from commercial industries, the DOE
initially established a broader and more stringent accreditation program. Both
DOELAP and NVLAP accreditation programs use performance tests that
evaluate the accuracy and precision of personnel dosimetry measurements. The
accuracy is determined by comparing the measured dose equivalent to the
"conventionally true dose equivalent" derived from calibration standards directly
traceable to NIST in carefully controlled conditions.
6-21


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