Biomedical Engineering Reference
In-Depth Information
Fig. 10.39
Left: closed TLD dosimeter. Right: open dosimeter,
showing four filter areas, chips removed. (Courtesy Thermo
Electron Corp.)
tons from the elastic scattering of fast neutrons could be counted and analyzed.
Figure 5.1 shows an example of alpha- and beta-particle tracks in photographic
film. In the cloud chamber, moisture from a supersaturated vapor condenses on
the ions left in the wake of a passing charged particle, rendering the track visible.
In the bubble chamber, tiny bubbles are formed as a superheated liquid starts to
boil along a charged particle's track. Another device, the spark chamber, utilizes a
potential difference between a stack of plates to cause a discharge along the ionized
path of a charged particle that passes through the stack.
Track etching is possible in some organic polymers and in several types of
glasses. A charged particle causes radiation damage along its path in the mater-
ial. When treated chemically or electrochemically, the damaged sites are attacked
preferentially and made visible, either with a microscope or the unaided eye. Track
etching is feasible only for particles of high LET. The technique is widely used in
neutron dosimetry (e.g., CR-39 detectors). Although neutral particles do not pro-
duce a trail of ions, the tracks of the charged recoil particles they produce can be
registered by techniques discussed here.
Optically Stimulated Luminescence
Optically stimulated luminescence
(OSL) shares some similarities and some contrasts
with thermoluminescence. A number of materials exhibit both phenomena. Under
irradiation, electrons become trapped in long-lived excited states of doped crystals.
With TLDs, dose is inferred from the amount of light emitted under thermal stim-
