Theory : A GM counter is a pulse type radiation detector which gives rise to negative
pulses due to radiations incident on it. The negative pulses are counted using the counter.
It consists of a cylindrical metallic tube filled with an inert gas like Argon or Neon mixed
with some organic vapor at pressure of about 10 cm of mercury. When any radiation is
incident on the GM counter, it sets out a primary ionizing event in the sensitive volume
of the counter. The electrons so released are accelerated by the electric field and cause
further ionizations. The collective effect of such phenomena develops into a progressive
avalanche spreading throughout the length of anode causing momentary discharge. This
spontaneous discharge lowers down the potential of the anode, giving rise to a negative
pulse. During the interval of this negative pulse, which is of the order of l00-200 μ
seconds, the counter is inoperative for any radiation incident on it. Hence it is called dead
time. This is approximately equal to resolving time. For any accurate assessment of
activity, the observed count rate must be corrected for the resolving time.