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4.2 Detector assembly 47
drops sharply at 400 nm and the reflectivity of Al also decreases (figure
4.11). MgO is a suitable reflector also for light of lower wavelength.
These external reflectors yield a good light collection efficiency which
can be further improved by increasing the internal reflection. The material
surrounding the scintillator should therefore have a low refractive
index. A gap of air between the scintillator and the refractor is a convenient
and effective solution. With such an assembly θB reduces to
42 ◦ for nscint = 1.5. The surface of the scintillator can be polished or
ground leading to diffuse or specular internal reflection [29, 28].
OPTICAL COUPLING. The coupling between the scintillator and the
electronic readout should transmit as much light as possible. A layer
of air in between should be avoided. A small air gap between a CsI:Tl
crystal (nCsI:T l=1.79) and a photodiode (nP D=1.5) reduces the electron
yield by 15 % [39]. An optical glue, grease or oil with an refraction index
between the scintillator and the PMT or diode window is the optimal
optical coupling (see [39] for more details). The PMT window mostly
consists of glass or quartz with an refraction index of nwindow=1.5. The
refraction index of silicon matches very good with organic scintillators.
Finding a suitable optical grease for inorganic scintillator is more complicated
[28, 29].
LIGHT GUIDES are coupled in between the scintillator and the readout.
They are used if the direct coupling is impossible or not desired due to
lack of space, an inconvenient scintillator shape, the presence of magnetic
fields or for a reduction of the radioactive background induced by
the readout electronics. Photons, entering the light guide reach through
internal reflexion to the readout. The light guide surface is often polished
and good results are obtained with external reflectors. R. L. Garwin
declared with phase space arguments that the photon flux density
in light guides is not compressible. Accordingly, due to the Liouville
Theorem a flux of light with the intensity Iin, incident at an area Ain,
cannot be compressed into a smaller cross section area at the output
Aout. Due to loss, the light intensity at the output is only in the ideal
case Iin · Aout/Ain and in general smaller. [28, 29, 30]
WAVE LENGTH SHIFTER (WLS) play an important role for big scintillator
volumes. They permit the collection of scintillation light from big
areas and enable the detection of the scintillation light with readout
electronics of smaller area. Wave length shifters are essentially light
guides with an admixture of a fluorescent wave length shifting material.
They absorb incident scintillation light and reemit light with
a longer wavelength. The WLS is coupled optically to the electronic
readout, but is usually separated from the scintillator crystal by an air