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