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4.2 Detector assembly 49
dent photon of the same energy. The scintillation yield L/E decreases
with increasing particle energy E. This leads to more produced scintillation
photons for Compton scattered gamma rays than for photons absorbed
directly by the photo effect. The full energy line width depends
on the contribution of Compton scattering and the photo effect. It gets
wider the more both processes contribute. With an increasing scintillator
size the contribution by Compton effect rises. Therefore the intrinsic
resolution decreases while the amount of fully absorbed gamma rays
increases. For high count rates scintillators of small crystal sizes have
better energy resolution.
The efficiency for the collection of scintillation light depends in general
on the scintillator dimensions, the size and position of the readout device
and the opacity of the crystal. Thus, it also depends on the wave
length of the emission spectrum, the refractive index of the scintillator,
on the optical coupling, on the refractor and on the entrance window
of the readout device at these wavelengths. Ultimately, absorption and
loss reduce the detectable scintillation light and worsens the resolution.
Readout devices at opposite sides of the scintillator increase the
collection efficiency in general and increase the uniformity of the light
collection, leading to decreasing variance.
Birks [27] quoted that larger crystals have worse resolution due to difficult
uniform light collection. Non-proportional scintillator response
contributes in the same order of magnitude to a decrease in resolution
as nonuniform light collection.
The intrinsic resolution of the readout device is determined by the photoelectron
emission efficiency and the electron multiplication process.
The quantum efficiency of the photocathode limits the amount of produced
photoelectrons. It depends on the wavelength of the incident
photon and on the point of impact. A nonuniformity of the photocathode
leads only for very low count rates to a wider line width and does
not contribute otherwise. Thus large diameter PMTs should be avoided
in this case. Finally variations in the multiplication process of the photoelectron
in the case of PMT, APD and Si-PMT contribute to the resolution.
All readout divices loose resolution due to dark current and
PIN diodes especially due to electronic noise [27, 30].
For the simulation, described in the next chapter, the energy resolution,
reported by Kim et al. [43], was used. The KIMS Collaboration
used CsI:Tl crystals with surfaces from 55x55 to 70x70 mm 2 and 300
mm length. The crystals were wrapped in two 0.2 mm thick Teflon
layers and couplet to two three inch PMT’s type D726UK, with RbCs