Page 549 - Instrumentation Reference Book 3E

Detectors 531

Insulator

I:.. Source
wetnane
Figure 22.7 gas flow-type proportional counter system.

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~ 0 1 c~~~~bers Because alpha particles are of 5-50 times. In this case pure methane (CH4) is
very readily absorbed in a small thickness of gas used with the same arrangement as shown in
or solid an early method used in counting them Figure 22.7.
involved the radioactive source being placed The detection efficiency of this system is con-
inside a gas counter with no intermediate win- siderably greater than that provided by the thin-
dow. Figure 22.7 shows the schematic circuit of window counter with the source external to the
such a counter in which the gas, generally pure counter. This is due (1) to the elimination of the
methane (CH4), is allowed to flow through the particles lost in penetrating the window and (2) to
counting volume. The counter can be operated in the improved geometry whereby all particles
the ionization counter region, but more usually which leave the source and enter the gas are
the high-voltage supply is raised so that it oper- counted.
ates in the proportional counter region, allowing
discrimination to be made between alpha parti- Geiger counters The observations made about
cles and beta particles, which may be emitted alpha-particle detection by gas proportional
from the same source, and improving the ratio counters apply also to Geiger counters. This is,
of signal to noise. thin entrance windows or internally mounted
~onizat~on chambers are used in two different
ways. Depending on whether the time constants sources are usabIe and the system of Figure 22.7
is also applicable to Geiger-counter operation.
of the associated electronic circuits are small or However, some differences have to be taken into
large, they are either tin^,'' Le. responding to account. Since operation in the Geiger region
each separate ionizing event, or "integrating," i.e., means that no differences in particle energy are
collecting the ionization over a relatively long measured, all particles are able to trigger the
period of time. The counting mode is little used counter and no energy resolution is possible.
nowadays except for alpha particles.
On the other hand, the counter operating in the
Geiger region is so sensitive that high electronic
Gas proportional counters Thin-window propor- amplification is not required, and in many cases
tional counters allow alphas to be counted in the the counter will produce an output sufficient to
presence of high beta and gamma backgrounds, trigger the scaler unit directly. In order to oper-
since these detectors can discriminate between the ate the system in its optimum condition; some
high ionization density caused by alpha particles amplification is desirable and should be vari-
passing through the gas of the counter and the able. This is to set the operating high-voltage
relatively weak ionization produced by beta par- supply and the amplifier gain on the Geiger
ticles or gamma-ray photons. plateau (see Section 3.2.1.1)> so that subsequent
With counters using flowing gas the source small variations in the high-voltage supply do
may be placed inside the chamber and the voltage not affect the counting characteristics of the
across the detector raised to give an amplification system.