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 (+) --> pi ( o)e(+)nu (e) decay rate at the Paul Scherrer institute (PSI). All 240 indiv idual crystals painted with a special wavelength-shifting solution were exa mined in a custom-built detection apparatus (RASTA - radioactive source tom ography apparatus) that uses a Cs-137 radioactive gamma source, cosmic muon s and a light-emitting diode as complementary probes of the scintillator li ght response. We have extracted the total light output, axial light collect ion nonuniformities and timing responses of the individual CsI crystals. Th ese results predict improved performance of the 3 pi sr PIBETA calorimeter due to the painted lateral surfaces of 240 CsI crystals. The wavelength-shi fting paint treatment did not affect appreciably the total light output and timing resolution of our crystal sample. The predicted energy resolution f or positrons and photons in the energy range of 10-100 MeV was nevertheless improved due to the more favorable axial light collection probability vari ation. We have compared simulated calorimeter ADC spectra due to 70 MeV pos itrons and photons with a Monte Carlo calculation of an ideal detector ligh t response. (C) 2001 Elsevier Science B.V. All rights reserved.