Page 241 - Radiochemistry and nuclear chemistry

Detection atut Measurement Techniques 225

8.7.3. Single channel analyzers
The pulse obtained from many detectors is directly proportional to the energy absorbed
in the detector. If all of the energy of the nuclear particle is absorbed in the detector, which
is possible for all kinds of ionizing radiation, and only one particle or photon interacts at
a time with the detector, the energy of the particle or photon can be determined from the
size of the generated pulse. In single-channel analyzers (SCA) only one channel exists
which serves as a "window" to accept only pulses of a certain size corresponding to a
limited range of energy as indicated in Figure 8.5(b). This window can be moved in steps
through the entire energy range, thereby obtaining a measure of the count rate of particles
having different energies. In the figure the window position and width are set so as to cover
only the middle peak. By narrowing the window, and moving it from zero to maximum
pulse amplitude, the whole particle energy spectrum is obtained. Spectrometry represents
one of the principal techniques whereby the energy of ionizing radiation is measured. The
single channel spectrometer is a system for energy determinations, although multichannel
systems are simpler to use, more accurate and faster.
The main use of SCAs is to select events within a given energy interval. Thereby the
background count rate is very much reduced which increases sensitivity. One of the most
important routine uses of SCAs is in simultaneous 3H and 14C counting using liquid
scintillators. Two SCAs are normally used, one set to cover pulses corresponding to the
very soft ~- from 3H and the other set to cover the more energetic pulses caused by the
~- from 14C.

8.7.4. Counters and rate meters

Ultimately the amplified pulses must often be accumulated over a fixed time interval and
the resulting number displayed. The device used is based on a simple electronic register
which is incremented by one each time a pulse arrives at its input. Such devices are often
called counters or scalers. Counters can usually be operated in two modes, preset time or
preset count. Counting is started and stopped by manual push buttons or by logic signals
from other units, e.g. the logic stop signal from a timer in the preset time mode. In the
preset count mode, the scaler stops at a given count and produces a logic output signal
which can be used by other units, e.g. to stop a timer.
In some applications a continuous display or record of the count rate is desired, e.g. in
a survey meter. This is achieved by use of a count rate meter. Count rate meters can be
analog (based on a diode pump) or digital (based on a recycling scaler-timer-display unit).
In either case, the value obtained follows the input rate with a certain time-lag, determined
by the time constant used. A high time constant gives a smooth reading as it averages input
pulse rate fluctuations over a longer time interval, but a very sluggish response to true input
rate changes. Correspondingly a short time constant gives a rapid response but also a very
fluctuating reading.