Personnel Dosimetry cont'd - doe-std-1128-98

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.

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