Vapor Phase Proportional Counting
A proportional counter consists simply of a center wire which is positively charged with respect to a coaxial cylindrical case. A sample may be counted by introducing it into the counter as a vapor, usually mixed with a counting gas such as methane. Electrons produced by radiation in the counter are attracted to the center wire at a sufficiently high velocity to produce a secondary electron shower by collision with gas molecules. Thus, each pulse of radiation produces an electrical pulse sufficiently large to be detected by the amplifier-scaler circuit with essentially 100% efficiency . Unlike a Geiger counter, however, a proportional counter does not produce pulses so large as to induce appreciable dead times, with consequent limitations on the amount of activity that can be counted. Proportional counters thus combine the advantage of simplicity with maximum sensitivity and applicability to a very wide range of counting rates. They also retain the advantages of pulse counters in that results are available in a continuously integrated form, and in that for the highest-sensitivity applications, they may be used with anticoincidence background-reduction apparatus.