A.N. Vasil'ev

Luminescence properties and scintillation mechanisms of cerium- and praseodymium-doped lutetium orthoaluminate

Absorption, reflection as well as luminescence emission, excitation, and decay curves for single crystals of and grown by the Bridgman technique have been measured at various temperatures. The fluorescence spectra photo-excited over a wide energy domain ranging from the UV to the x-ray region, and the kinetics are typical of the cerium and praseodymium ions. These experimental results show that the exciton transfer to the dopant occurs at around 8 eV, and the energy transfer via sequential hole and electron trapping is dominant at higher energy. This process must be considered as the main scintillation mechanism in this crystal. The high efficiency of this mechanism is explained by the small energy difference between the 4f level of the dopant and the top of the valence band, estimated from XPS measurements.

EXTENSIVE STUDIES ON CEF3 CRYSTALS, A GOOD CANDIDATE FOR ELECTROMAGNETIC CALORIMETRY AT FUTURE ACCELERATORS

Abstract In the framework of its search for new heavy, fast and radiation hard scintillators for calorimetry at future colliders, the Crystal Clear Collaboration performed a systematic investigation of the properties and of the scintillation and radiation damage mechanisms of CeF 3 monocrystals. Many samples of various dimensions up to 3 × 3 × 28 cm 3 were produced by industry and characterised in the laboratories by different methods such as: optical transmission, light yield and decay time measurements, excitation and emission spectra, gamma and neutron irradiations. The results of these measurements are discussed. The measured light yield is compared to the theoretical expectations. Tests in high energy electron beams on a crystal matrix were also performed. The suitability of CeF 3 for calorimetry at high rate machines is confirmed. Production and economical considerations are discussed.

Recombination of Correlated Electron–Hole Pairs With Account of Hot Capture With Emission of Optical Phonons

Electron thermalization and electron-hole recombination in scintillating crystals is simulated with and without account for Coulomb field created by a hole using both analytical estimations and Monte-Carlo approach. The Monte-Carlo simulation is performed both for crystals with one and two branches of longitudinal optical phonons to check the role of additional branches of these phonons. The results of numerical simulation show that the account for Coulomb field at all stages of the thermalization and capture significantly increases the probability of the geminate electron-hole binding in case of high values of optical phonon energies.

From Luminescence Non-Linearity to Scintillation Non-Proportionality

The problem of non-linearity of luminescence is reviewed on the basis of kinetic equations for concentrations and two-particle correlation functions. The reasons of time dependence of the rates of reactions between excitations are discussed. The non-linearity of luminescence and non-exponentiality of its decay is attributed to initial correlation between excitations created by photons, and to non-homogeneous spatial distribution of deposited energy. The case of excitation by ionizing particle is discussed. The equation for the scintillation yield and decay kinetics with account for either small energy losses (plasmons, excitons and electron-hole pairs) or large energy losses (delta-electrons and Auger cascade) is obtained.

Scintillation Efficiency Improvement by Mixed Crystal Use

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.

Time-resolved luminescence of CeF3 crystals excited by X-ray synchrotron radiation

Abstract Under X-ray synchrotron radiation (SR) of storage ring VEPP-3 excitation time-resolved luminescence spectra and decay curves for two different CeF 3 crystals have been measured. The temperature dependence of the decay kinetics of emission at 300 nm of new pure CeF 3 crystal has been studied. The maximum of the intensity of this emission was observed at 390 K under SR X-ray excitation. The results are compared with data obtained under UV SR excitation and are discussed in terms of two models of luminescence quenching.

Fast luminescence of undoped PbWO4 crystal

Abstract The results of an investigation of PbWO4 luminescence properties obtained by time-resolved techniques under VUV, X-ray synchrotron radiation and γ-radiation excitation are presented. The fast luminescence in the 430 nm band with fwhm of 0.5 eV is characterized by a decay time of about 4–5 ns under X-ray excitation. A slow luminescence with decay time of more than 1 μs can be attributed to a green emission band. An intermediate decay time was also detected. This time changes from 20 ns (for 360 nm emission) to 50 ns (for 550 nm). The PbWO4 fast band excitation spectrum and reflectivity were measured in the fundamental absorption region using VUV synchrotron radiation (4–150 eV). A comparison between these two spectra allowed us to suppose the excitonic type of the excitation mechanism of the fast emission band. The role of Pb2+ ions in the PbWO4 luminescence is discussed.

The Origins of Scintillator Non-Proportionality

Recent years have seen significant advances in both theoretically understanding and mathematically modeling the underlying causes of scintillator non-proportionality. The core cause is that the interaction of radiation with matter invariably leads to a non-uniform ionization density in the scintillator, coupled with the fact that the light yield depends on the ionization density. The mechanisms that lead to the luminescence dependence on ionization density are incompletely understood, but several important features have been identified, notably Auger-like processes (where two carriers of excitation interact with each other, causing one to de-excite non-radiatively), the inability of excitation carriers to recombine (caused either by trapping or physical separation), and the carrier mobility. This paper reviews the present understanding of the fundamental origins of scintillator non-proportionality, specifically the various theories that have been used to explain non-proportionality.

Multiscale Approach to Estimation of Scintillation Characteristics

This work is directed to analysis and description of the factors which determine the efficiency of inorganic scintillators. The interconnection of different stages of track formation and relaxation is analyzed. The hierarchy of the scales of different processes in scintillators is discussed. The main attention is paid on the evolution of energy and spatial distribution of excitations in the excited region in scintillators. Different types of recombination - geminate and stochastic - and their features are considered, both in cases of low and high concentration of excitations and impurities. The spatial structure of the track after thermalization is discussed, with discriminating high-energy and low-energy part of the track of ionizing particle. Main features of thermalization and formation of thermalization length in different types of crystals with simple and complex structure and with different phonon spectrum are considered. The peculiarities of these processes in binary and complex halides and the estimation of limits of scintillator efficiency in these crystals are reviewed. Possible mechanism based on Auger relaxation of core hole which enhances the yield in CsI is proposed. The estimations of limiting factors for traditional alkali halide crystals and the perspectives of complex halides are discussed.

Progress in Studying Scintillator Proportionality: Phenomenological Model

We present a model to describe the origin of non-proportional dependence of scintillator light yield on the energy of an ionizing particle. The non-proportionality is discussed in terms of energy relaxation channels and their linear and non-linear dependences on the deposited energy. In this approach, the scintillation response is described as a function of the deposited energy deposition and the kinetic rates of each relaxation channel. This mathematical framework allows both a qualitative interpretation and a quantitative fitting representation of scintillation non-proportionality response as function of kinetic rates. This method was successfully applied to thallium doped sodium iodide measured with SLYNCI, a new facility using the Compton coincidence technique. Finally, attention is given to the physical meaning of the dominant relaxation channels, and to the potential causes responsible for the scintillation non-proportionality. We find that thallium doped sodium iodide behaves as if non-proportionality is due to competition between radiative recombinations and non-radiative Auger processes.