Cosmic muon tomography of pure cesium iodide calorimeter crystals

Scintillation properties of pure CsI crystals used in the shower calorimeter being built for precise determination of the π→πeνe decay rate are reported. Seventyfour 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 π+→e+ν 69.8MeV positrons and the monoenergetic 70.8MeV photons were 2.7% and 3.7%, respectively. The upper limit of optical nonuniformity correction to the 69.8MeV positron low-energy tail between 5MeV and 55MeV 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. Preprint submitted to Elsevier Preprint 7 February 2008 PACS Numbers: 87.59.F; 29.40.Mc; 24.10.Lx