6.3.3 Photomultiplier tubes and detection circuitry

The visible photons are collected by an array of photomultiplier tubes behind the crystal. These convert each visible photon to an electron and then multiply the number of electrons sufficiently to give a voltage pulse. Because the number of visible photons is proportional to the energy of the incoming gamma ray, the height of the pulse depends on this energy. This gives a method of counting the numbers of gamma photons at different energies that reach the crystal.

A resistive network connected to the photomultiplier tubes gives information on the spatial position (x,y) of each incoming gamma ray. The so-called ‘z coordinate’ corresponds to the pulse height and therefore the energy of the gamma ray.