6.5. Photodetectors 367
Both of these detector types have their own pros and cons. for example photo-
multiplier tubes have sensitive mechanical structures that are prone to damage in
mechanically unstable environments while the photodiodes are made of semiconduc-
tor materials and are therefore mechanically stable. On the other hand the response
time of PMTs is much smaller than photodiodes and are therefore preferred in timing
applications. Of course these are only two parameters and the choice of a photode-
tector is in reality a compromise between a number of factors including cost. We will
learn about these factors as we discuss these two types of detectors in the following
sections.
6.5.A PhotomultiplierTubes......................
Photomultiplier tubes are sensitive devices that are capable of converting light pho-
tons into a very large number of electrons. The basic building blocks of a com-
plete PMT are a photocathode, an electron multiplication structure, and a readout
electrode. The photons incident on the photocathode get converted into electrons
through the process of photoelectric effect. The electron is then made to acceler-
ate and strike a metallic structure calleddynode, which results in the emission of
more electrons. The newly produced electrons are again accelerated towards another
dynode, where even more electrons are produced. This process of electron multi-
plication continues until the electrons reach the last dynode where the resulting
current is measured by some electronic device. This process is graphically sketched
in Fig.6.5.1.
Before we go on to the discussion of individual components of a PMT, let us
have a look at the characteristics that make PMT based photodetectors desirable
for radiation detection. The most important of these characteristics are listed below.
High sensitivity.Good signal-to-noise ratio.Fast time response.Large photosensitive area.The reader should bear in mind that here we are not comparing PMTs with pho-
todiodes. With rapidly developing technology, new photodiode detectors are now
being developed that match or even surpass these and other properties.
A.1 Photocathode..........................
We learned about the process of photoelectric effect in Chapter 2 and saw that some
particular materials emit electrons when they absorb photons of energy above a
certain threshold. This threshold energy is called work function and is characteristic
of the material. The material itself is called photocathode. If we assume that no
energy is lost during this process then the energy of the emitted photon would simply
be equal to the difference between the photon energy and the work function of the
material, that is
Ee=hc
λ