396 Chapter 6. Scintillation Detectors and Photodetectors
A.16NoiseConsiderations......................
Since PMTs are very sensitive instruments, their vulnerability to various types of
noise should be carefully considered specially in situations where the count rate is
very low. The most important PMT noise sources are described below.
Thermally Agitated Electron Emission: We saw earlier that the photo-
cathodes and dynodes generally used in PMTs have very low work functions.
This implies that they are susceptible to thermal agitations and might sponta-
neously emit electrons even at room temperature. There are two precautions
that can be taken to minimize this effect: cooling and minimizing the size of
the photocathode. Cooling the photocathode makes the thermal energy un-
available to the electrons. Since the number of thermally emitted electrons also
depends on the area of the photocathode therefore decreasing its size minimizes
this effect.Glass Scintillation:The electron multiplication structure of a PMT is fitted
inside a vacuumed glass enclosure, which itself can act as a scintillator. In
normal operation, the deviation of trajectories of some of the electrons taking
part in the multiplication process is inevitable. Some of these electrons can get
attracted towards the outer glass enclosure, thus producing scintillation light.Field Emission:If a PMT is operated at a very high voltage (near its max-
imum rating), it may suffer from small electrical discharges. The result is
erroneous large random pulses. To avoid such a situation, the PMTs should be
operated at a voltage well below the maximum rating.Leakage Current:No PMT structure can be made absolutely perfect. Small
imperfections in insulations and contaminations are inevitable, causing a small
leakage current to flow in the output circuit. This effect becomes more promi-
nent at low operating voltages. To minimize this, all parts of the tube must be
thoroughly cleaned.Ringing: Any impedance mismatch in the input of the preamplifier and the
PMT can cause error in count rate. This so calledringingeffect is a seri-
ous problem for high rate situations. The best cure, of course, is to ensure
impedance matching.Mechanical Instability:PMT is a sensitive mechanical structure and there-
fore highly susceptible to mechanical vibrations. Such vibrations may cause the
electrons to deviate from their usual trajectories. This may, for example lead
to glass scintillation and consequently the signal-to-noise ratio may decrease
considerably.Inductive Noise:Almost all of the PMTs are sensitive to external magnetic
fields, which inhibit inductive noise in the system. Therefore the PMT struc-
tures should either be electromagnetically shielded or the external field should
be compensated. Some newer designs of PMTs, such as mesh or MCP type,
are less susceptible to magnetic fields and are therefore preferred over other
types if the time response and other requirements allow.