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4.2 Detector assembly 41
4.2 Detector assembly
The other important component of a scintillation detector is the electronic
readout. It converts the part of the scintillation light that reaches
the device, typically just a few hundred photons, into a usable electric
signal. This can be done with photomultiplier tubes or diodes. In
the following section, possible devices are introduced, afterwards the
aspects of detector mounting and the light collection are discussed in
section 4.2.2—“Detector mounting” and finally the energy resolution of
scintillator is discussed in section 4.2.3—“Energy resolution”.
4.2.1 Electronic readout devices
PHOTO MULTIPLIER TUBES (PMTs) are evacuated glass or metal tubes,
consisting of a front glass with photocathode, followed by the dynode
string that multiplies the photoelectron (electron cascade) and an anode
that collects the multiplied electrons. The final PMT signal is proportional
to the number of incident photons, until a saturation at high
photon fluxes.
Roughly every fourth initial photon that hits the photocathode produces
a photoelectron. In this case the initial photon is absorbed by
the cathode material and its energy is transferred to the atomic electrons.
The electrons can then migrate towards the surface and escape
from the photocathode if their energy is higher than the work function
of the material. Higher quantum efficiencies are achieved with cathode
materials that have a negative electron affinity (see Knoll [29] p. 271).
The quantum efficiency of modern photocathodes, which is the ratio of
emitted photoelectron per incident photon, is 20 to 30 %. It is mainly
limited by the escape width of the photoelectrons, because photocathodes
of this thickness are still semitransparent for photons.
The resolution of PMTs can be limited for low count rates by dark current.
At room temperature 10 6 −10 8 electrons per m 2 s are emitted from
the photocathode. This number increases with temperature. Especially
large diameter PMTs can have less resolution due to variations of the
thickness of the photocathode. The quantum efficiency also depends
on the wavelength of the incident photon. The detection of photons
with long wavelength is limited by the height of the potential barrier
and at short wavelength by the opacity of the entrance window material.
The cut-off lies for glass at 350 nm and for fused silica or quartz
at 160 nm. This is shown in figure 4.10. Multialkali (Na2KSb) cathodes