Question

a) how can instrument resolving time impact the precision of the counting data you obtain in a Geiger-Müller Detector?

b)Why is an end-window GM detector not the detector of choice for alpha and gamma radation?

Answer 1

a) Instrument resolving time (also known as dead time) is the time that Geiger-Muller Detector (GMD) takes to reset itself right after a Geiger discharge. During this time the GMD does not detect any ionizing radiation since the electric field is reduced below the critical level to trigger any chain avalanche. The time that the GMD takes to build the electric field beyond the critical level after a full Geiger discharge is known as the recovery time. If any radiation is detected after the resolving time but before the recovery time, the GMD will produce a pulse. However, this pulse will be smaller in amplitude than a pulse created from a full Geiger discharge. Greater the resolving time less is the precision of the GMD.

Say T is the resolving time of the detector and C is the total number of counts.

Let, t_r be the real time that the GMD is operating. This is the actual time that the detector is on. This is our counting time. It does not depend on the resolving time of the detector, but on how long we actually record the counts.

t_l is the live time that the detector is operating. This is the time that the detector is able to record counts. t_l depends on the resolving time of the detector.

n is the measured counting rate, n = C/t_r

N is the true counting rate, N = C/t_l

Therefore, n/N= (C/t_r)/( C/t_l)=t_l/t_r

This means that the fraction of the counts that we record is the ratio of the live time to the real time. This ratio is the fraction of the time that the detector is able to record counts. The key relationship we need is between the real time, live time, and resolving time. This relationship is given as:

t_l = t_r - CT

This is true since CT is the total time that the detector is unable to record counts during the counting time t_r.

Therefore, T=(t_r-t_l)/C

Thus, for better precision T needs to be small and hence, the difference between the live time and real time must be large. In addition to that, the total number of counts must be large.

b) Alpha radiations are heavy charged particles. Since alpha particles have the shortest range the detector must be placed in close contact with the radiation source. Moreover, it is known that GMD produces a pulse output which is the same magnitude for all detected radiation hence a GMD with an end window tube cannot distinguish between alpha and beta particles. Now, since to detect alpha particles, the detector must be in close contact (approximately within 10mm) with the radiation source, both alpha and beta radiations are detected and thus the detector cannot discriminate between the alpha and beta particles. Hence, end-window GM detector is not used to detect alpha particles.

Gamma radiations are energetic electromagnetic radiation. End-window GM detectors are thin walled detectors. Since GM detectors rely on the interaction of photon radiation with the tube wall materials and the highly energetic gamma radiation can easily go pass through the walls of the end-window GM detector. Hence high energy gamma rays are not detected with end-window detectors. To detect this kind of radiations, windowless tube is used.