Preston College Cosmic Ray Group

Demonstrating electromagnetic cascades in lead

This is a small experiment requiring nothing more than three GM tubes, an AND gate and some sheets of lead. The axes of the three GM tubes are arranged so that a single particle cannot trigger them all together. The counters are connected to the AND gate so that a count is recorded when all three are triggered together (a triple coincidence).

In the 1930's, Bruno Rossi found that placing sheets of lead above the counters caused an increase in the count rate rather than decreasing it as expected. It was soon recognised that high energy particles could cause particle avalanches in the lead and these avalanches accounted for the increased count rate.

It is found that as the as the number of lead sheets placed above the counters increased, the coincidence rate increased to a maximum at about 2cm thickness and then decreased again (graph).

The thickness corresponding to the peak in the graph can be used to make an estimate of the original energy of the of the avalanche initiating particles. The average energy is found to be of the order 70 MeV - impressive when you consider that ordinary radioactive sources emit particles in the energy range 1 - 5 MeV.

Link to more technical detail

The experiment is described in a little more detail in: Physics Education Vol34(1) January 1999 pp19-27