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Radiation Detection and Evaluation - doe-std-1128-98_ch10171
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DOE Standard Guide of Good Practices for Occupational Radiological Protection In Plutonium Facilities
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Personnel Dosimetry cont'd - doe-std-1128-98_ch10173


DOE-STD-1128-98
It is not generally well known that neutron survey instruments have a severe
energy dependence. In fact, some manufacturers claim a 15% response per unit
dose equivalent extending over an energy range of thermal to 15 MeV. The
energy dependence of several commercially available neutron survey meters has
been experimentally measured at the PTB in Germany (Liesecki and Cosack,
1984). Their measurements made with monoenergetic neutrons in low-scatter
conditions demonstrate that a typical moderator-based neutron survey meter
underestimates the dose equivalent by a factor of 2 at an energy of 14 MeV and
overestimates dose equivalent by a factor of 2 to 3 at an energy of 20 keV.
Survey instruments also exhibit changes in response with the direction of
incidence of the neutrons due to absorption and scattering of the neutrons by the
electronics package attached to the moderator/detector. This can also result in
40% variation in response, depending on the direction of incidence. Fortunately,
plutonium compounds emit neutrons in the MeV range, where the problems with
energy and angular responses are minimal. Accuracies of 15% can be achieved
with careful calibration with 252Cf or other fission sources.
6.3.2
Personnel Dosimetry
The detailed requirements of an external dosimetry program are given in the
Implementation Guide, External Dosimetry Program (DOE, 1999e). Explicit
guidance and requirements are given and need not be repeated here. This section
will focus on dosimetry problem areas specific to plutonium facilities and
possible solutions.
Personnel working in plutonium facilities are exposed to both photon and neutron
radiations, and plutonium processing is one of the largest contributors to neutron
exposure in the United States. The response of beta-gamma personnel dosimeters
is well documented and will not be discussed here.
The response of neutron dosimeters to the neutron fields encountered in the
workplace must be evaluated. All existing neutron dosimeters have a severe
energy response problem and must be carefully calibrated for the specific
radiation field in which the neutron dosimeter is worn. Typically, neutron
dosimeters are calibrated to either bare 252Cf or D2O-moderated 252Cf sources in a
low-scatter calibration facility. Then, the neutron dosimeters are worn in
plutonium facilities under high-neutron-scatter conditions, which produce a
completely different energy spectrum than that in which the dosimeter was
calibrated. Accordingly, a typical approach is to perform neutron energy field
characterization surveys at selected areas in a facility. Based on these surveys a
field correction factor is determined which corrects for the difference between
the dosimeter response to the calibration source and the response to the neutron
fields in the workplace. Because of the large response of TLD-albedo dosimeters
to low-energy neutrons (with energies below about 20 keV), the response of the
dosimeter usually 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).
6-20


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