Everything about Photomultiplier totally explained
Photomultiplier tubes (
photomultipliers or
PMTs for short) are extremely sensitive detectors of light in the
ultraviolet,
visible and near
infrared. These detectors multiply the signal produced by incident light by as much as 10
8, from which single
photons can be resolved. The combination of high gain, low noise, high frequency response and large area of collection have meant that these devices still find applications in
nuclear and
particle physics,
astronomy,
medical imaging and motion picture film scanning (
telecine).
Semiconductor devices like
avalanche photodiodes have replaced photomultipliers in some applications, but photomultipliers are still used in most cases.
Structure and operating principles
Photomultipliers are constructed from a glass
vacuum tube which houses a
photocathode, several
dynodes, and an
anode. Incident
photons strike the
photocathode material which is present as a thin deposit on the entry window of the device, with
electrons being produced as a consequence of the
photoelectric effect. These electrons are directed by the focusing
electrode towards the
electron multiplier, where electrons are multiplied by the process of
secondary emission.
The electron multiplier consists of a number of electrodes, called dynodes. Each dynode is held at a more positive voltage than the previous one. The electrons leave the photocathode, having the energy of the incoming photon (minus the
work function of the photocathode). As they move towards the first dynode they're accelerated by the electric field and arrive with much greater energy. On striking the first dynode, more low energy electrons are emitted and these, in turn, are accelerated toward the second dynode. The geometry of the dynode chain is such that a cascade occurs with an ever-increasing number of electrons being produced at each stage. Finally the anode is reached where the accumulation of charge results in a sharp current pulse indicating the arrival of a photon at the photocathode.
Usage considerations
Photomultiplier tubes typically require 1000 to 2000 volts for proper operation. The most negative voltage is connected to the cathode, and the most positive voltage is connected to the anode. Negative high voltage supplies are usually preferred. Voltages are distributed to the dynodes by a resistive
voltage divider, though variations such as active designs (with transistors or diodes) are possible. The divider design influences aspects such as the frequency response and rise time, and therefore may be critical to an application.
While powered, photomultipliers must be shielded from ambient light to prevent their destruction through overexcitation. If used in a location with high magnetic fields (which will curve electron paths), they're usually shielded by a layer of
mu-metal.
Further Information
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