The Geiger Muller Tube

A Geiger counter is used to measure alpha, beta and gamma radiation, its main component and sensing element is the Geiger Muller Tube. The Geiger Muller tube is a metal or graphite tube which contains an inert gas such as helium, neon or argon at very low pressure. The inside of the tube wall acts as a cathode and a thin wire passing through the tube an anode, a high voltage is passed across these two electrodes. At one end of the tube is a small window, made of mica, through which passes the ionizing radiation. When the radiation enters the tube it can collide with a gas particle ionizing it (knocking an electron from its shell) creating positive charged atoms or molecules and negatively charged electrons, the electrons are accelerated towards the anode and the cations towards the cathode. As the electrons move towards the anode they gain kinetic energy and can gain enough energy to effectively ionize more gas particles they meet on the way causing an cascade of charged particles. Which results in a short pulse of current to pass from the cathode to the anode, each pulse is recorded by the counter indicating the presence of a particle of radiation.

For the Geiger counter to work accurately an appropriate voltage must be passed across the electrodes. If the voltage is too low then the electric field in the tube is too weak to cause a current pulse, if the voltage is too high then the tube undergoes continuous discharge and can be damaged. An appropriate voltage can be determined by plotting the voltage across the tube against the counting rate. (as done in the first part of this experiment.)

The decay of radioactive material occurs randomly at a rate determined by the stability of the of the material. The number of decays per given time interval will vary due to the randomness of the process. In a statistical sense decay is a rare event because the number of decays is very small compared to the total number of atoms present in the sample. Therefore the Poisson distribution can be used to model the number of decays (counts) in a given interval. The Poisson distribution is a discrete probability distribution which is used to express the probability of a number of events occurring in a given time interval. It requires that each event is individual, is not related to the time since the last event and that the mean number of occurrences remains constant for the duration of the data collection. According the Poisson distribution if