Cosmic muon tomography of pure cesium iodide calorimeter crystals

Scintillation properties of pure CsI crystals used in the shower calorimete r being built for precise determination of the pi(+) --> pi(0)e(+)v(e) deca y rate are reported. Seventy-four individual crystals, polished and wrapped in Teflon foil, were examined in a multiwire drift chamber system speciall y designed for transmission cosmic muon tomography. Critical elements of th e apparatus and reconstruction algorithms enabling measurement of spatial d etector optical nonuniformities are described. Results are compared with a Monte Carlo simulation of the light response of an ideal detector. The dedu ced optical nonuniformity contributions to the FWHM energy resolution of th e PIBETA CsI calorimeter for the pi(+) --> e(+)v 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-ene rgy tail between 5 and 55 MeV was + 0.2%, as opposed to the + 0.3% tail con tribution for the photon of the equivalent total energy. Imposing the 5 MeV calorimeter veto cut to suppress the electromagnetic losses, GEANT-evaluat ed positron and photon lineshape tail fractions summed over all above-thres hold ADCs were found to be 2.36 +/- 0.05 (stat) +/- 0.20 (sys)% and 4.68 +/ - 0.07 (stat) +/- 0.20 (sys)%, respectively. (C) 2000 Elsevier Science B.V. All rights reserved.