What is a Photomultiplier?
- an overview or tutorial about the basics of what is a photomultiplier 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.
Photomultiplier construction
Photomultipliers are contained within a glass tube that
maintains a vacuum within the device. There are three main electrodes within a
photomultiplier:
- Photocathode
- Dynodes
- 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 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
Photomultipliers 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 sued 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 the device from excessive
light levels while in operation. High light levels can destroy the devices due
to them being overexcited.
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