Light response of pure CsI calorimeter crystals painted with wavelength-shifting lacquer

TL;DR

This study evaluates the scintillation properties of pure CsI crystals painted with wavelength-shifting lacquer, demonstrating improved axial light collection and energy resolution for calorimeter applications in particle physics experiments.

Contribution

It provides detailed measurements of light response and collection nonuniformities of painted CsI crystals, predicting enhanced calorimeter performance.

Findings

Wavelength-shifting paint did not significantly alter total light output or timing resolution.

Axial light collection probability was improved, leading to better energy resolution.

Predicted energy resolution for 10-100 MeV particles showed notable improvement.

Abstract

We have measured scintillation properties of pure CsI crystals used in the shower calorimeter built for a precise determination of the pi+ -> pi0 e+ nu 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 uses a 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 3 pi sr PIBETA calorimeter due to the painted lateral surfaces of 240 CsI crystals. The wavelength-shifting paint treatment did not affect appreciably the total light output and timing resolution of our crystal sample. The predicted energy resolution for positrons and photons in the energy range of 10-100 MeV was nevertheless improved due to the more favorable axial light collection probability variation. We have compared simulated calorimeter ADC spectra due to 70 MeV positrons and photons with a Monte Carlo calculation of an ideal detector light response.