Photoelectric Detectors (Ground Based Astronomy)

There is a group of substances that will absorb photons and subsequently emit the electrons excited by them. These materials are said to be PHOTOEMISSIVE and they are used in PHOTOELECTRIC DETECTORS. The emitted electrons are called PHOTOELECTRONS. Materials such as caesium antimonide and gallium arsenide are good photoemitters and can have an efficiency as high as 40 per cent. Photoemissive detectors vary in the way that the photo-electrons are themselves detected and recorded.

In most detectors, the photoemissive substance is deposited as a very thin, semi-transparent layer onto glass. An electric field is applied to attract the photoelectrons away from the sensitive surface, which is known as the PHOTOCATHODE. Without this electric field drawing them away, the photoelectrons would eventually be reabsorbed by the photocathode.

One of the simpler photoelectric devices is the PHOTOMULTIPLIER. It is impossible or difficult to detect single photoelectrons, and therefore in the photomultiplier it is arranged that the emitted electrons are accelerated to an electrode coated with an electronemissive material. When the electron hits this material a burst of several SECONDARY ELECTRONS is produced. These electrons are themselves accelerated towards another electrode coated with the same material, so that an even larger burst of secondary electrons is produced in a cascade process. The photomultiplier consists of a series of these electrodes, and it gets its name from the way that the number of electrons in the stream is multiplied at each collision with an electrode. Eventually all the electrons are collected by an electrode called the ANODE. The output current at the anode is then a measure of the number of photoelectrons generated by the photocathode. The whole process from the emission of a photo-electron to the production of the output current takes place in a fraction of a microsecond. If the photons arrive at a rate less than a few million per second, it is possible to detect individual photo-electrons with very fast electronic circuits. In this way we may count individual photons from astronomical objects.

In the same way that glass lenses can focus light from an object to produce an image, it is possible to arrange electric and magnetic fields to produce electron images. We can arrange for all the photoelectrons emitted from any one spot on the photocathode to be accelerated and focused by electromagnetic lenses into another single spot where the electron image is to be formed. If they are focused onto a special material called a PHOSPHOR, a bright spot of light is emitted by the phosphor where the electrons strike. This happens in the same way as in a television tube where electrons are accelerated and focused to produce a bright spot when they hit the phosphor.

Devices known as IMAGE INTENSIFIED makes use of this phenomenon. With these, a two-dimensional picture is focused onto the photocathode. The emitted photoelectrons are then accelerated down the tube by an electric field produced by a series of ring electrodes. The electrons are also focused by the magnetic field provided by the focusing coil or SOLENOID. An electron image is formed at the output window, which is coated with a thin layer of phosphor that produces a bright visible image. Because the photoelectrons have been considerably accelerated, the output image can be 10-100 times brighter than the input image. The intensified image can be photographed. The overall efficiency for photons from the astronomical object is set mainly by the efficiency of the photocathode (10-40 per cent) and not by that of the photographic plate (0.1-1 per cent).

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