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DOE-STD-1128-98
Guide of Good Practices for Occupational Radiological Protection in Plutonium Facilities
depends on the scattering conditions rather than the initial neutron energy spectrum.
These problems are discussed in detail in several documents, including PNL-3213,
Personnel Neutron Dosimetry at Department of Energy Facilities (Brackenbush et al.,
1980) and PNL-7881, Response of TLD-Albedo and Nuclear Track Dosimeters Exposed
to Plutonium Sources (Brackenbush et al., 1991).
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 acredited in accordance with the requirements of the DOELAP for
6-21


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