Detection of Scintillation Photons with Photodiodes

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Abstract
A statistical theory on the efficiency of collection of scintillation photons from a scintillator by a photon detector was developed. If A0 is the total surface area of the scintillator, A the surface area of the portion of the scintillator to which the detector is coupled, η the diffuse reflectivity of the material into which the scintillator is packed, K the probability that a photon impinging on A is absorbed by the photon detector, the photon collection efficiency is ε=KAA0{11−η[1−(KA/A0)]}. An experimental determination of the value of ε was made with a photodiode CsI(Tl) unit, which was constructed primarily as a cosmic ray detector used in space vehicles. The CsI(Tl) crystal was packed in a case of high reflectance surface (η=94%) and coupled to two Au-Si surface barrier photodiodes with LTV 602 clear silicone potting compound. The experimental value is about 30% which is in reasonable agreement with the theory. The dispersion of the signal out (full width at half-maximum) of this unit for the detection of 40 MeV α-particles is 75 keV at 495 keV. It is comparable with what is obtained with a CsI(Tl)-photomultiplier tube combination.

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