Improvement of photon collection uniformity from an NE213 scintillator using a light guide

The uniformity in collecting scintillation photons to the photomultiplier is one of the key parameters to carry out precise measurement using scintillators. A simple method is proposed to improve the collection uniformity of the scintillation photons just by inserting a light guide between a scintillator and a photomultiplier where the sidewall of the light guide is surrounded by a black paper as a photon absorber. This light guide insertion is expected to have a function to reduce the collection efficiency of the scintillation photons generated closer to the photomultiplier due to the larger transmission loss, since a decrease of the angle of incidence raises the fraction of the photons escaping from the sidewall of the light guide. The experiments using an NE213 liquid organic scintillation detector and gamma-ray sources revealed that the light guide functioned as a compensator for the original tendency of the photon collection and improved the photon collection uniformity. The details of the physical mechanism were examined by simulating the behavior of the gamma rays using the EGS4 electron-photon transport Monte-Carlo code and the light propagation process using the ray-tracing program GuideM. The effects of the light guide were also examined on the whole energy resolution and the discrimination capability to separate gamma rays and neutrons.