Solid angle compensation in gas proportional scintillation counters using an annular anode with azimuthal geometry

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Abstract
A new, simple and robust design for a gas proportional scintillation counter (GPSC) is studied. One sole electrode, the anode, is used to define the electric field in the drift and scintillation regions of the detector volume. The anode has an annular shape aligned with the photosensor axis. Such design allows to keep constant the solid angle subtended by the photosensor relative to the different positions of the scintillation region. Having an oblong anode with a 10 cm inner diameter and a 5 cm photomultiplier tube placed 5 cm below the anode, an energy resolution of 12.0 % FWHM has been achieved for a 10 kV anode bias, the maximum voltage that could be applied to the anode in the present prototype. According to simulations, energy resolutions of ~10 % can be achieved for anode voltages of ~13 kV, a value comparable to the 9–10 % achieved in GPSC using solid angle variation compensation. Independently of having a constant solid angle, the absolute value of the solid angle must be considered, a lower number of EL photons detected at the photosensor due to a reduced solid angle may contribute to the GPSC energy resolution degradation. In addition, non-uniformities present in the photosensor or in the GPSC must be taken into account, since they will contribute to a dependence of the pulse amplitude on the radiation interaction position despite the constant solid angle subtended by the photosensor relative to the scintillation region.

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