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4.2 Detector assembly 43
secondary electrons are accelerated to the second dynode and so on.
PMTs have typically 9 to 16 dynodes and an overall multiplication factor
(gain) of 10 6 up to 10 9 . The final electron pulse is collected at the
anode and has a time delay of 20-50 ns and a width of a few nanoseconds.
The detection of single photons is possible. The Hamamatsu
PMT R669 shows a temperature coefficient of -0.3 %/K at 550 nm [39].
The time resolution of a PMT is mainly determined by the escape velocity
of the photoelectron and the distance between the emission point
and the hitting point at the first dynode. It is typically below one ns
[28, 29, 30].
PIN diodes consist of a p-doped primary material that is heavily
n-doped on one surface. A controlled diffusion of the donator atoms
leads to a n-doped layer at the surface and an internal i-region, with
nearly intrinsic properties. There the number of acceptor and donator
atoms is identical and the specific resistance of 10 5 Ωcm corresponds
to the intrinsic resistance of silicon. The depletion zone must be thick
enough so that incident particles can create enough ionisation in the
sensitive region to form a larger signal than the noise level. The
thickness of the depletion zone is limited to 10 to 15 mm and is much
bigger than that of diodes with pn-junction. This is suitable for beta
or low-energy photon detection. The small p-layer of the diode is the
entrance window. The incident ionising particles can produce direct
or indirect electron hole pairs. Those are produced in the i-region and
the induced charge is processed to an attached preamplifier to produce
the output signal at the anode. With a required energy for excitation of
3.6 eV in Silicon and 2.8 eV in Germanium, they are able to detect the
typical scintillation light of 3-4 eV with a high quantum efficiency of
60-80 %. Only in the best case, as much electron hole pairs as incident
scintillation photons are produces. These are typically just a few
hundred. Without any multiplication process the signal is 8 orders of
magnitude lower than that of PMTs. Electronic noise is a problem due
to this very small signal. PIN diodes have a factor 2 worse resolution
than PMTs and can compete only for high energetic particles [29]
The temperature dependence of photodiodes (Hamamatsu S2744-08)
is very small with 0.01 %/K at 550 nm [39].
The greater sensitive region leads to a greater thermal noise than normal
diodes. Thus cooling is necessary to gain a high resolution. The
dark noise can also be reduced by using materials with a larger band
gap than silicon, like mercury iodide crystals. Such PIN diodes are superior
to PMTs and achieve energy resolutions of 4.58 % with CsI:Tl
scintillators [28, 29, 30, 41].