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44 4 Scintillation detectors
AVALANCHE PHOTO DIODES (APDs) are diodes with an internal multiplication
process. The small amount of charge, which is produced in
conventional diodes, is increased with the avalanche effect. The charge
carriers are accelerated sufficiently between collisions with the lattice
to produce additional electron hole pairs. The gain is independent of
the amount of incident light for applied voltages higher than the breakdown
voltage. APDs are used with applied reverse voltage near below
the breakdown voltage. There, the gain is proportional to the applied
voltage. With a gain of a few hundred a signal is produced that is bigger
than the electronic noise level. The quantum efficiency is 80 %. The
signal level is very sensitive to temperature and the applied voltage.
Statistical variations of the avalanche effect lead to considerable fluctuations
in the multiplication process. These are larger than the fluctuations
in the multiplication process of PMTs and can significantly contribute
to the energy resolution. It becomes usually inferior to conventional
silicon photodiodes. A better energy resolution for lower particle
energies, as can be achieved with conventional diodes, is nevertheless
possible, because the noise level of the preamplifier is usually much
higher than the noise level of diodes. The dark current depends on the
diode size and is between 0.5 and 50 nA [29, 41].
SI-PMT are pixelated avalanche diodes used in Geiger mode. This is
the operation at break down voltage, where the gain is independent of
the bias voltage. Si-PMT are with a gain of 10 5 to 10 6 competitive to
PMTs. The signal of a Si-PMT is the sum of each APD pixel output.
Single photons produce detectable charge avalanches. This allows the
counting of single photons or the detection of pulses of multiple photons.
Due to the limited pixel number, saturation occurs for high count
rates. Room temperature operation is possible. They have good time
resolution and can work in magnetic fields.
4.2.2 Detector mounting
Having a scintillator and an electronic readout device, the latter must
be mounted onto the scintillator in a way that allows the detection of
as much scintillation light as possible. Self-absorption and losses at the
scintillator surfaces impede the collection of the entire produced scintillation
light. The amount of self-absorption depends on the scintillator
size and opacity. It becomes a non-negligible factor for scintillator
length that is in the order of the attenuation length of the crystal. This
can be, depending on the quality of the crystal, up to meters.