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Light response of pure CsI Calorimeter crystals painted with wavelength shifting lacquer

We have measured scintillation properties of pure CsI crystals used in the shower calorimeter built for a precise determination of the Pi+ -> Pi0 e+ mue decay rate at the Paul Scherrer Institute (PSI). All 240 individual crystals painted with a special wavelength-shifting solution were examined in a custom-build detection apparatus (RASTA—radioactive source tomography apparatus) that use 137Cs radioactive gamma source, cosmic muons and a light emitting diode as complementary probes of the scintillator light response. We have extracted the total light output, axial light collection nonuniformities and timing responses of the individual CsI crystals. These results predict improved performance of the 3Pi sr PIBETA calorimeter due to the painted lateral surfaces of 240 CsI crystals. The wave length-shifting paint treatment did not affect appreciably the total light output a nd timing resolution of our crystal sample. The predicted energy resolution for positrons and photons in the energy range of 10­100MeV was nevertheless improved due to the more favorable axial light collection probability variation. We have comp ared simulated calorimeter ADC spectra due to 70MeV positrons and photons with a Monte Carlo calculation of an ideal detector light response.

computed tomography ; scintillation detectors ; Monte Carlo simulations

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