Biomedical Engineering Reference
In-Depth Information
Radiation Measuring Devices
The reverse-biased n-p junction constitutes an attractive radiation detector. The
depletion region, which is the active volume, has high resistivity, and ions produced
there by radiation can be collected swiftly and efficiently. It can serve as a rate meter
or to analyze pulses. The number of electron-hole pairs produced in a pulse is
proportional to the energy absorbed in the active volume, and so the junction can
be used as a spectrometer. The “
W
values” for Si and Ge are, respectively, 3.6 eV
and 3.0 eV per electron-hole pair, as compared with the corresponding figure of
∼
30
eV per ion pair in gases. Statistically, the relatively large number of charge
carriers produced per unit energy absorbed in semiconductors endows them with
much better energy resolution than other detectors. Unique among detectors is
the fact that the physical size of the depletion region can be varied by changing the
bias voltage. For measuring alpha and beta radiation, junctions are fabricated with a
very thin surface barrier between the outside of the device and the depletion region.
Several examples of semiconductor radiation instruments will be briefly described.
Electronic dosimeters have advanced rapidly in recent years. They are in wide-
spread use, particularly in nuclear power-plant, home-security, and military applica-
tions. Figure 10.22 shows a silicon diode personal electronic dosimeter for photons
in the energy range 50 keV to 6 MeV. Other models in the series measure photons
down to 20 keV and beta radiation. The unit operates as both a passive and an active
dosimeter. It has adjustable dose and dose-rate warning and alarm levels. It can be
used either in an autonomous or in a satellite mode, with remote computer inter-
facing with exposure-records management software. The device performs regular
internal operating checks and reports dose increments, date, and time at specified
intervals. A microprocessor counts pulses, converts them to dose, and calculates
dose rate.
High-purity germanium (HPGe) detectors are available for a wide variety of tasks
(Fig. 10.23). The crystals are housed in a vacuum-tight cryostat unit, which typically
contains the preamplifier in a cylindrical package. Depending on the intended ap-
plication, germanium detectors come in a number of different planar and coaxial
configurations.
A light-weight, rugged, portable multichannel analyzer for gamma spectra is
shown in Fig. 10.24. It is controlled from a key pad connected through an interface
module to the HPGe detector. The unit has 16k channels and gives a live display
of data being acquired. It holds 23 spectra in its internal memory. Nuclide ID and
activity calculations are performed by using stored calibration information. The in-
strument can also interface with a computer to utilize other software applications.
CZT
isthenamegivento
cadmium zinc telluride
semiconductors, which oper-
ate at normal temperatures. The relatively high-density crystal is advantageous for
stopping secondary electrons. The resolution is intermediate between Ge and Si.
With its high sensitivity for gamma detection, CTZ applications include homeland
security, waste effluent monitoring, and first-responder technology. Detectors, such
as the one shown in Fig. 10.25, are small, rugged, programmable, and operate with
very low energy consumption.
