The NADs are used to determine the neutron and photon dose at various
locations in the plutonium facility, as well as providing spectral and calibration
data for PNADs. A typical NAD used at the Hanford Site is shown in Figure 6.4.
This unit is fixed to the wall or posted at locations around plutonium storage
areas where it is easily recovered in the event of a criticality. The "candle" insert
contains neutron- and gamma-sensitive TLDs as well as activation foils
positioned at the center of the detector. Tests at the Health Physics Research
Reactor at Oak Ridge have shown that this arrangement gives accurate estimates
of "deep" dose for both neutrons and gamma rays. A set of foils identical to those
used in the PNAD dosimeter is positioned above the moderator. These foils
provide an estimate of the average cross-section or response per unit dose, so that
the neutron dose from the foils in the PNAD can be more accurately evaluated
for the incident neutron spectrum.
The PNAD dosimeter typically consists of several activation foils. In the case of
the Los Alamos/Hanford design (Vasilik and Martin, 1981), the activation foils
consist of ½-in.- diameter foils of bare and cadmium-covered gold, bare and
cadmium-covered indium, cadmium-covered copper, and a sulfur pellet.
Algorithms have been developed to unfold an approximate neutron energy
spectrum from the measured neutron activation products, so that neutron doses
can be calculated. Criticality dosimeters containing various activation foils are
available from vendors, but some of the commercial products do not contain
sufficient material to measure neutron doses as low as 10 rad, which is the
recommended lower detection limit for personal criticality accident dosimeters.
In the past, the Health Physics Research Reactor (HPRR) at Oak Ridge National
Laboratory was available to calibrate criticality accident dosimeters, and several
intercomparisons were held to demonstrate the accuracy and lower detection
limit of criticality alarms and nuclear accident detector systems (Sims and Raga,
However, the HPRR has been dismantled. Some dosimeter testing capability is
being established at pulsed reactor facilities at Sandia (Albuquerque, New
Mexico) and at a Department of Defense pulsed reactor at the Aberdeen Proving
Ground in Maryland.