Analysis of the last years theoretical studies and track simulations to conclusion that primary stages (electron scattering and e-h thermalization) play the key role in the following scintillator efficiency. The long thermalization length comparing to Onsager radius is the main reason for geminate pair concentration decrease and later luminescence losses. The easiest way for thermalization length decrease is the scintillation crystal doping or even transfer to the mixed crystals (solid solution). The simple model of modification of electrons scattering and e-h pairs thermalization for the mixed crystals is proposed. It is shown that solid solutions have higher light output independently on the crystal type. Analysis of experimental data confirmed this conclusion. This phenomenon is found for halide, oxide and sulfates scintillators. The similar behavior is typical for mixed anion and/or cation systems. The key role of initial track formation stages is illustrated by the same trend for activated scintillators and pure crystal with intrinsic luminescence. These estimations and experimental data lead to the conclusion that the scintillation efficiency improvement by mixed crystal use can play an important role in the search and development of new scintillators.

Scintillation Efficiency Improvement by Mixed Crystal Use

A detector structure having one or more inorganic scintillating crystals (210) coupled through wavelength shifting optical fibers (WLSFs) (310, 320) to position sensitive photomultipliers (PMTs) (330). By using WLSFs the detector structure can operate with a significant reduction in the number of PMTs and electronic channels, while potentially improving the resolution of the system by allowing three-dimensional reconstruction of energy deposition positions.

Three-dimensional imaging detector employing wavelength-shifting optical fibers

Spectral and kinetic properties of the fast intrinsic ultraviolet luminescence of CsI single crystals either pure or doped by Br, Cl, Rb, or K, studied under different types of excitation at temperatures from 80 to 550 K, are presented. Since conventional models of intrinsic luminescence in their ‘‘pure’’ form fail to explain the observed properties of the UVL, a new one should be worked out taking into account collective interaction of neighboring excitations and defects, created by ionizing particles, which form clusters of excitations. It has been shown that luminescence decay time can be controlled by doping with homologs.

Time‐resolved XEOL spectroscopy of new scintillators based on CsI

Scintillation properties of pure CsI crystals used in the shower calorimeter being built for precise determination of the π + →π 0 e + ν e 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 π + → e + ν 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 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.

Cosmic muon tomography of pure cesium iodide calorimeter crystals

Fast U.V. scintillations in CsI-type crystals

Scintillation characteristics of a mixed CsICsBr crystal have been studied. The crystal luminescence spectrum is 280–340 nm; the decay time is 11 ns; the relative light yield is 0.18 of NaI(Tl). The parameters of the γ-quantum hodoscope based on CsICsBr and hodoscopic PMTs (HPMTs) have been measured. The hodoscope spatial resolution is shown to be 2 σ = 2.4 mm. A design of a positron emission tomograph ring on the basis of CsICsBr and HPMTs is proposed. It is suggested that the CsICsBr scintillator may be used in electromagnetic calorimeters at counting rates greater than 10 7 s −1 .

CsI-based scintillators in γ-detection systems

A hodoscopic PMT (HPMT) is proposed for the design of a positron emission tomograph (PET). The spatial resolution of a HPMT-based PET element is measured. It amounts to 3.6 and 1.75 mm for the BGO and GSO, respectively. HPMT-based single- and multilayer PET ring designs are suggested.

A.I. Gorelov

Papers

CsI-based scintillators in γ-detection systems

Position-sensitive detector for γ-quanta for PET