Physics and Engineering of Radiation Detection

6.5. Photodetectors 403

Similarly for the background counts we have

σbg =

√

Nbg

=

√

1. 5 × 102

=12. 2.

The total shot noise due to these two sources is given by

σ =

√

σ^2 d+σbg^2

=

√

223. 62 +12. 22

≈ 224.

A.19EffectofMagneticField....................

One of the most problematic things about PMTs is their vulnerability to the external
magnetic fields. The stem of this problem is the fact that some of the electrons
traveling down the multiplication chain may change their paths due to the magnetic
field. Such stray electrons do not contribute to the output signal and hence reduce
not only PMT sensitivity but may also affect its linearity. This is a serious problem in
situations where high magnetic field is present, such as particle physics experiments.
In such a case there are three options to reduce this effect.

Less Susceptible PMTs:Some PMT structures, such as multi channel type,

are not much affected by external magnetic fields, and can therefore be pre-

ferred over the conventional PMTs.


External Shielding:PMTs can be externally shielded, for example by con-

structing a Faraday cage around them.

Field Compensation: The magnetic field can be compensated by another

magnetic field near the PMTs such that the two cancel out each other. This

method is more suitable for tubes working in low level magnetic fields, such as

that of the Earth. The reason is that application of high compensating field

may not be practically possible or even desirable due to its possible effect on

other parts of the system.

6.5.B PhotodiodeDetectors

PMTs are not the only means of detecting light photons. New advancements in
semiconductor technology have led to the development of the so calledphotodiode
detectors having moderate to extremely high quantum efficiencies. In the previous
Chapter we looked at some specific types of semiconductor detectors, such as PIN
diode, heterojunction diode, and Schottky diode. Each of these detectors can be
used as a photodetector since a photodiode detector is simply a semiconductor diode
detector optimized to work in and around the visible spectrum of radiation.
Before we discuss the specific kinds of semiconductor photodetectors, let us have a
look at the internal gain expected from a general photon sensor. Let us assume that