Cosmic muon tomography of pure cesium iodide calorimeter crystals

TL;DR

This study investigates the scintillation properties and optical nonuniformities of pure CsI crystals used in a calorimeter for precise decay measurements, employing cosmic muon tomography and simulations to evaluate energy resolution and tail contributions.

Contribution

It introduces a novel cosmic muon tomography method for assessing optical nonuniformities in CsI crystals used in calorimeters, with detailed analysis and comparison to Monte Carlo simulations.

Findings

Optical nonuniformity contributions to energy resolution are quantified.

The study establishes upper limits for optical nonuniformity corrections.

Tail fractions for positron and photon lineshapes are precisely evaluated.

Abstract

Scintillation properties of pure CsI crystals used in the shower calorimeter being built for precise determination of the pi+ -> pi0 e+ nu decay rate are reported. Seventy-four individual crystals, polished and wrapped in Teflon foil, were examined in a multiwire drift chamber system specially designed for transmission cosmic muon tomography. Critical elements of the apparatus and reconstruction algorithms enabling measurement of spatial detector optical nonuniformities are described. Results are compared with a Monte Carlo simulation of the light response of an ideal detector. The deduced optical nonuniformity contributions to the FWHM energy resolution of the PIBETA CsI calorimeter for the pi+ -> e+ nu 69.8 MeV positrons and the monoenergetic 70.8 MeV photons were 2.7% and 3.7%, respectively. The upper limit of optical nonuniformity correction to the 69.8 MeV positron low-energy tail between 5 MeV and 55 MeV was +0.2%, as opposed to the +0.3% tail contribution for the photon of the equivalent total energy. Imposing the 5 MeV calorimeter veto cut to suppress the electromagnetic losses, GEANT-evaluated positron and photon lineshape tail fractions summed over all above-threshold ADCs were found to be 2.36+-0.05(stat)+-0.20(sys)% and 4.68+-0.07(stat)+-0.20(sys)%, respectively.