PROPORTIONAL COUNTER CIRCUITRY
Figure 11 Single Channel Analyzer Output
Since the single channel analyzer can be set so that an output is only produced by a certain pulse
size, it provides for the counting of one specific radiation in a mixed radiation field.
This output is fed to a scaler which counts the number of pulses it receives. A timer gates the
scaler so that the scaler counts the pulses for a predetermined length of time. Knowing the
number of counts per a given time interval allows calculation of the count rate (number of counts
per unit time).
Proportional counters can also count neutrons by introducing boron into the chamber. The most
common means of introducing boron is by combining it with tri-fluoride gas to form Boron
Tri-Fluoride (BF3). When a neutron interacts with a boron atom, an alpha particle is emitted.
The BF3 counter can be made sensitive to neutrons and not to gamma rays.
Gamma rays can be eliminated because the neutron-induced alpha particles produce more
ionizations than gamma rays produce. This is due mainly to the fact that gamma ray-induced
electrons have a much longer range than the dimensions of the chamber; the alpha particle energy
is, in most cases, greater than gamma rays produced in a reactor. Therefore, neutron pulses are
much larger than gamma ray-produced pulses.