Biomedical Engineering Reference
In-Depth Information
Fig. 10.38
Schematic views of beta-gamma (left) and
neutron (right) TLD dosimeters. (Courtesy Thermo Electron
Corp.)
trons, some will be moderated by the body and detected as slow neutrons only in
chip 4 of the dosimeter, thus furnishing evidence of neutron exposure.
When a potential for exposure to neutrons exists, a special TLD dosimeter, such
as that shown on the right in Fig. 10.38, should be employed. Readings from the
pairs of TLD-600 and TLD-700 elements, one sensitive and the other insensitive
to neutrons, can be compared. Since their responses to gamma rays are identical,
differences can be attributed to neutrons. The cadmium filters for chips 1 and 2
absorb incident thermal neutrons. Differences in their readings, therefore, are as-
sociated with fast neutrons. Without the cadmium filters, differences between ele-
ments 3 and 4 indicate total (fast-plus-thermal) neutron exposure.
The beta-gamma dosimeter of Fig. 10.38 and its system of filters is displayed
in Fig. 10.39. Thermoluminesent dosimeters can be processed by automated read-
out systems, which can transfer results to a central computer system for dosime-
try records. Computer algorithms have been written to unfold the required dose
assessments for individuals from the readings obtained from the different chips.
Laboratory accreditation is provided through the National Voluntary Laboratory Ac-
creditation Program (NVLAP).
10.6
Other Methods
Particle Track Registration
A number of techniques have been devised for directly observing the tracks of in-
dividual charged particles. Neutron dosimetry with the film badge described in
Section 10.4 utilized neutron-sensitive emulsions in which the tracks of recoil pro-
