Development of a Low Energy Proton Detector Using a Ruby Scintillator

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A magnetic bottle of ultracold neutrons (UCN) based on a multiple cusp configuration using permanent magnets has been proposed by N. Inoue, H. Nihei and N. Akiyama. Confinement of UCN can be confirmed by detecting protons produced by the decay of UCN in the bottle. The protons are collected efficiently and detected by a proton detector which is installed on the ceiling of the bottle and biased at about -50 〜 -100kV. It is necessary to develop a low-energy proton detector system insensitive to neutron and γ-ray, since the detector is used near an atomic reactor.The detector used is a ruby scintillator containing chromium (Al_2O_3(Cr_2O_3)). In this scintillator the (n, p) reaction rate is very small, and the effect of γ-ray also can be made very small since the scintillator can be laminated. The thickness of the scintillator is 0.02 mm. The results obtained in this research are described. (1) Alpha-particles (5.5 MeV) emitted from ^<241>Am source and protons (0.2 〜 1.0 Mev) accelerated by a Van-deGraph accelerator were irradiated to the scintillator installed on the surface of a low noise type photomultiplier. When a single charged prticle has been irradiated, the output signal from the photomultiplier consists of many isolated signals. The gross decay time of the signal has been about 2.5 ms. (2) The luminescence spectrum having some different wavelengths was observed by a spectrometer. This result is consistent with the data reported elsewhere. (3) The sensitivity of the detector system to neutron and γ-ray was tested in the reactor room of KUR (Kyoto Univ. Reactor). It has been confirmed that the detector system is almost insensitive to neutron and γ-ray. (4) It is given as a conclusion that by combining the characteristics obtained by the above mentioned experimental results, it is possible to make a low-energy proton detector with high S/N ratios.