Fixed and Personnel Nuclear Accident Dosimeters

DOE-STD-1128-98

Guide of Good Practices for Occupational Radiological Protection in Plutonium Facilities

Neutron-Measuring Component of Dosimeter. Criticality accidents create a wide

range of neutron energies. Since the neutron dose per unit fluence is strongly

dependent on neutron energy, knowledge of the neutron energy spectrum is important

in accident dosimetry. In criticality accidents, neutrons with energies greater than

about 100 keV contribute most of the dose; therefore, measurement of the fast neutron

dose is of most importance. See Delafield (1988) for a review of the different types of

neutron dosimeters available for accidents.

Gamma-Measuring Component of Dosimeter. Delafield (1988) noted that the ratio

of the gamma rays to neutron dose will vary according to the type of critical assembly

and whether or not additional shielding is present. For unshielded assemblies, the

gamma-to-neutron ratio can range from 0.1 for a small heavy metal system up to about

3 for a small hydrogen-moderated solution system. A concrete or hydrogenous

shielding material will increase the gamma-to-neutron ratio. Gamma dose can be

determined by TLD, film, or radiophotoluminescent glass.

Dosimeter Comparison Studies. Sims and Dickson (1979) and Sims (1989) present

a summary of nuclear accident dosimetry intercomparison studies performed at the

Oak Ridge National Laboratory Health Physics Research Reactor. The most recent

summary (Sims, 1989) showed that of the 22 studies conducted over 21 years, 68% of

the neutron dosimeter results were within the 25% accuracy standard and 52% of the

gamma dosimeter results were within the 20% accuracy standard. Most

measurements that failed to meet the accuracy standards overestimated the actual dose.

Some of their other findings include the following:

Doses from hard neutron energy spectra are more accurately measured than those

from soft energy spectra

The threshold detector unit (TDU) is the most accurate type of nuclear Accident

neutron dosimeter; however, its use is declining due to increasingly strict control of

small quantities of fissionable materials

Activation foils (ACT) are the most popular nuclear accident neutron dosimeter

For gamma dosimeters, TLDs are the most popular and the least accurate, and film

is the least popular and the most accurate.

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