Neutron-Measuring Component of Dosimeter

DOE-STD-1128-98

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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