Biomedical Engineering Reference
In-Depth Information
Fig. 10.11
Portable survey and count-rate meter with optional
GM pancake probe. See text. (Courtesy Fluke Biomedical.)
is used to advantage in monitoring for neutrons in mixed gamma-neutron fields.
The charged recoil nuclei from which the neutrons scatter generally produce large
pulses compared to those from the Compton electrons and photoelectrons pro-
duced by the photons.
Geiger-Mueller counters are very convenient and reliable radiation monitors,
providing both visual and audible responses. They usually come equipped with a
removable shield that covers a thin window to enable the detection of beta and al-
pha particles in addition to gamma rays. Readout can be, e.g., in counts per minute
or in mR h
-1
with
137
Cs calibration. With the latter, special energy compensation
of the probe is needed to flatten the energy response for low-energy photons. Fig-
ure 10.11 shows an example of a counter with a pancake GM probe. The instrument
is also compatible with other kinds of probes, and can be used to detect alpha, beta,
gamma, and neutron radiation. Examples of various GM and scintillation (Sec-
tion 10.3) probes employed in a variety of applications are shown in Fig. 10.12.
Ideally, after the primary discharge in a GM tube, the positive ions from the
counter gas drift to the cathode wall, where they are neutralized. Because of the
high potential difference, however, some positive ions can strike the cathode with
sufficient energy to release secondary electrons. Since these electrons can initiate
another discharge, leading to multiple pulses, some means of quenching the dis-
charge must be used. By one method, called external quenching, a large resistance
between the anode and high-voltage supply reduces the potential difference after
each pulse. This method has the disadvantage of making the tube slow (
∼
10
-3
s)
in returning to its original voltage. Internal quenching of a GM tube by addition
of an appropriate gas is more common. The quenching gas is chosen with a lower
ionization potential and a more complex molecular structure than the counter gas.
When a positive ion of the counter gas collides with a molecule of the quench-
