Personnel Dosimetry - STD-1128-98master0184

DOE-STD-1128-98

Guide of Good Practices for Occupational Radiological Protection in Plutonium Facilities

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

252

Cf 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

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