Biomedical Engineering Reference
In-Depth Information
istered. One then measures the count rate at different operating voltages of the
tube, leaving the discriminator level set. The resulting count rate from the alpha-
beta source will have the general characteristics shown in Fig. 10.10. At low volt-
ages, only the most energetic alpha particles will produce pulses large enough to
be counted. Increasing the potential difference causes the count rate to reach a
plateau when essentially all of the alpha particles are being counted. With a further
increase in voltage, increased gas multiplication enables pulses from the beta par-
ticles to surpass the discriminator level and be counted. At still higher voltages, a
steeper combined alpha-beta plateau is reached. The use of proportional counters
for neutronmeasurements is described in Section 10.7. Gamma-ray discrimination
Fig. 10.9
Gas-flow proportional counter for monitoring tritium
activity concentration in air. See text. (Courtesy Berthold
Technologies USA, LLC.)
Fig. 10.10
Count rate vs. operating voltage for a proportional
counter used with discriminator for counting mixed alpha-beta
sources.
