PHOTOMULTIPLIER (PM) TUBE

Typical Number in Hospital:

Cost Bands:

References: 3

Photomultiplier tubes produce electric currents which are proportional to light intensity, and can detect extremely low levels of light. An important use in hospitals is in scintillation counters. In this application they detect the flashes of light occurring in a scintillation crystal due to ionizing radiation. These flashes cause the release of one or more electrons at the tube cathode and these are drawn towards the next electrode (dynode) by a high positive voltage. When these electrons strike the dynode more electrons are displaced by 'secondary emission' and they are attracted to the next dynode, and so on. By the time this sequence has been repeated several times the number of electrons is large enough to be measured. The construction of a photomultiplier tube is a series of curved dynodes (sometimes called venetian blind construction) in an evacuated glass envelope. They require a very stable high voltage supply since the output pulse height is dependent on the energy of the original radiation and on the voltage applied to the tube.

PM tubes are also used in some photometers in the pathology department.