Personnel Dosimetry - doe-std-1128-98

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).

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