Appendix E: Geiger-Muller Counter

A Geiger-Muller counter consists of a tube (aka Geiger–Muller tube) and a ratemeter (aka scaler). The GM Tube is the sensing element. The ratemeter contains the electronics to do the counting and display the reading in count per minute (cpm) or other units.

A Geiger–Müller tube is filled with a low-pressure (~0.1 atm) inert gas. At one end, it has a very thin mica-window through which radiation can enter. The walls of the tube form the cathode. The anode is a wire passing up the center of the tube. A potential difference of several hundred volts is maintained between the cathode and anode.

When an ionizing radiation makes its way through the mica-window into the tube, it may ionize one or a few gas molecules. The resulting charged particles are immediately accelerated by the strong electric field in the tube, and may pick up enough KE so they can in turn ionize more atoms through collisions. If all goes well, a chain reaction will culminate in an avalanche breakdown, which translate into a short, intense pulse of current which passes between the electrodes. These pulses are measured or counted by the ratemeter. The ratemeter usually also includes a loudspeaker which gives a “click” for each pulse. This gives rise to the cute clicking sound you hear.

A few things worth noting:

  • The count rate is not equal to the activity of the radioactive source. Firstly, not all the radiations enter the tube. Secondly, not every radiation produces one pulse. Gamma radiation in particular is only weakly ionising so only a tiny fraction of gamma photons “succeeds” in producing a pulse. It is normal for a low activity beta source to produce more clicks per minute than a high activity gamma source.
  • A pulse is a pulse, whether caused by high or low energy radiation. So the count rate is not a measure of the energy of radiation. In fact, the more energetic the gamma radiation, the more likely it will pass right through the tube. So a high energy gamma source can produce a lower count rate than a low energy one.
  • A pulse is a pulse, whether caused by an alpha particle, beta particle or a gamma photon. A simple trick employed to identify the type of radiation is to insert a suitable absorber between the radiation source and the GM tube. For example, if the count rate is reduced drastically by a piece of paper, we have an alpha source.
  • Even in the absence of any radiation source, GM counters produce a background pulse rate, due to cosmic rays and other background radiation. This background count rate must be subtracted from the measured count rate to obtain count rate due to the radioactive source alone.

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