What is a Photomultiplier?

- an overview or tutorial about the basics of what is a photomultiplier or photo multiplier tube used in sensing light and creating images.

Photomultipliers are still in widespread use today. They are extremely sensitive detectors of light including visible light, ultraviolet light and near infrared. As such they are very valuable in detecting all forms of visible and nearly visible light when levels are low or very low.

The great advantage of photomultipliers is their extreme sensitivity. They are able to multiply the signal produced by the incident light by figures up to 100 million. In addition to their very high levels of gain, photomultipliers also exhibit a low noise level, high frequency response and a large collection area. These advantages have meant that despite all the advances in photodiode technology, photomultipliers are still used in virtually all cases when low levels of light need to be detected.

In view of their performance photomultipliers are still used in many areas including particle physics, astronomy, medical imaging and motion picture film scanning.

Photo multiplier tube construction

Photomultipliers are contained within a glass tube that maintains a vacuum within the device. There are three main electrodes within a photomultiplier:

  1. Photocathode

  2. Dynodes

  3. Anode

Within the envelope of the photomultiplier, there is one photocathode, one anode, but there are several dynodes. The anode and dynode are traditional metallic electrodes with coated surfaces, but the photocathode is actually a thin deposit on the entry window.

Photomultiplier operation

Photons enter the photomultiplier tube and strike the photocathode. When this occurs, electrons are produced as a result of the photoelectric effect.

Once the electrons have been generated they are directed towards an area of the photomultiplier called the electron multiplier. As the name suggests, this area serves to increase or multiply the number of electrons by a process known as secondary emission.

The electron multiplier is made up from a number of electrodes, called dynodes. These dynodes have different voltages on them, each one is more positive voltage than the previous one to provide the required environment to produce the electron multiplication effect. This operates by pulling electrons progressively towards the more positive areas in the following way. The electrons leave the photocathode with the energy received from the incoming photon. They move towards the first dynode and they are accelerated by the electric field and they arrive with much greater energy than they left the cathode. When they strike the first dynode more low energy electrons are released, and these are in turn attracted by the greater positive field of the next dynode, and these electrons are similarly accelerated by the greater positive potential of the second dynode, and this process is repeated along all the dynodes until the electrons reach the anode where they are collected.

The geometry of the dynode chain is carefully designed so that a cascade effect occurs along its length with an ever increasing number of electrons being produced at each stage. When the anode is reached, the accumulation of charge results in a sharp current pulse for the arrival of each photon at the photocathode.

Photomultiplier use

Photomultiplier tubes require the use of high voltages for their operation. Typically they require maximum voltages in the region of 1 - 2 kV. In the same way that a thermionic valve or vacuum tube has the cathode as the most negative electrode, the same is true for a photomultiplier. Similarly the anode is the most positive electrode. The dynodes are held at intermediate voltages that are normally generated using a resistive potential divider.

It is also necessary to ensure the photomultiplier is mounted and used with care. Stray magnetic fields can affect their operation as the electron stream can be bent and the operation of the device impaired. To overcome this photomultipliers are normally mounted in a mu-metal screen to prevent stray magnetic fields affecting the device.

It is also necessary to screen a photomultiplier tube from excessive light levels while in operation. High light levels can destroy a photomultiplier because it can become over-excited.

By Ian Poole

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