Alexander Gektin

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.

Scintillation and Inorganic Scintillators

This chapter introduces the basic definitions and gives the minimum necessary information about the phenomenon of scintillation and the mechanisms which have to be taken into account for the development of scintillation materials. It starts with an historical brief and describes the sequence of the processes leading to scintillation in a dielectric medium. Definitions are then given of the parameters related to the physical process of light production in the medium, and not dependant on the shape, surface state and optical quality of the scintillator block. After a survey of scintillation mechanisms it is shown that several self-activated scintillators show better scintillation properties when they are doped with appropriate ions. A description is given of the most important activators with a discussion about the conditions for the activator to be efficient in a host matrix. As an example the electron energy level structure of Ce3+ and Pr3+ is described. It is shown that these two ions are good activators with a bright and fast scintillation in many compounds. Several approaches to classify scintillation materials are discussed. This chapter is concluded with a list of the scintillation materials developed so far and of their most important properties.

Scintillation Mechanisms in Inorganic Scintillators

Details of energy transfer phenomena and scintillation mechanisms in luminescent media excited by ionizing radiation are discussed in this chapter. The sequence of relaxation of electronic excitations is described: creation of electron-hole pairs, energy transfer to emitting centers of interest and quantum efficiency of these luminescence centers. The theoretical limit of the light yield is usually much higher than the measured value. The limiting factors at each step of relaxation are considered in self activated, doped and cross-luminescent scintillation materials. The final stage of luminescence centre excitation mechanism in scintillators has a strong influence on the scintillation parameters. It is discussed in detail here. Finally different examples are given of charge transfer and non-radiative relaxation processes of scintillation centers through their coupling with the crystal lattice.

Energy Relaxation in LSO and LGSO Crystals Studied in the VUV Range

The role of energy transfer from Gd3+ to Ce3+ activator in the formation of the luminescence yield and kinetics of Lu2xGd2-2xSiO5:Ce (LGSO) mixed crystals has been studied by VUV spectroscopy including temperature dependent time-resolved measurements of luminescence excitation and emission spectra and luminescence kinetics in the excitation energy range 4 to 25 eV at the SUPERLUMI station of HASYLAB, DESY. Improved scintillation yield as well as suppressed afterglow that can be achieved by appropriate choice of Lu/Gd ratio as well as of Ce-concentration were suggested to result from an additional channel of energy relaxation via Gd-states, which appears in LGSO. The role of Ca-codoping is discussed.

The Role of Core Levels in Scintillation Processes

The Auger decay of a core hole results in appearance of several strongly correlated excitations. This excited region strongly polarizes the lattice and thus the defect creation is possible. In all cases the core hole causes the strong local perturbation of electronic and lattice subsystems. The creation of such excited region with mutual relaxation of correlated electrons and holes can result in the increase of the efficiency of energy transfer to activators, acceleration of the luminescence kinetics, and the appearance of radiation-induced luminescence centers. These effects have been studied using VUV and soft X-ray synchrotron radiation, when the selective excitation of different core levels is possible.

Influence of Crystal Structure Defects on Scintillation Properties

This chapter discusses the influence of different crystal structure defects on the scintillation crystal conversion efficiency, energy transfer, luminescence yield and light collection, as well on their radiation hardness. During the synthesis of crystalline media defects are inevitably produced and are classified according to their size and shape. But defects are also produced in the scintillators under ionizing radiation. Charged particles, even as light as electrons, are producing local defects and charge unbalance in the crystal. Heavier particles, such as protons, α-particles, hadrons and nuclear fragments, loose much more energy when colliding with ions of the crystal lattice and produce relatively large damage area extending over several crystallographic cells. The impact of these radiation induced defects on the scintillation efficiency and crystal transparency is presented. The dynamic of these effects is discussed in detail for the damage building as well as for its recovery. The chapter is concluded with practical considerations on how to improve the scintillator radiation hardness.

Addressing the Increased Demand for Fast Timing

This chapter discusses new developments opening prospects for a fast timing resolution (in the picosecond range) with inorganic scintillation materials. The motivations for the increasing demand for fast timing are given for high luminosity collider experiments as well as for Time-of-Flight PET scanners. The fundamental limits of standard scintillation mechanisms are then explained and possible routes are discussed to exploit transient phenomena and to generate prompt photons in addition to the scintillation photons.

Energy Resolution and Non-proportionality of Scintillators

This chapter concentrates on the energy resolution of inorganic scintillators. It is known since a long time that scintillators suffer non-proportionality of the light yield and deposited energy. Although this phenomenon is not completely understood, the possible mechanisms at the origin of the non-proportionality are discussed in detail in this chapter. A particular attention is given to the structure of the energy deposition from ionizing particles and to phenomena of clustering of excitations. The role of the crystal lattice through phonon coupling in the thermalization process is also discussed.

Examples of Recent Crystal Development

Several examples of recent scintillator development are given in this chapter. They have been chosen in different areas of application to illustrate the common strategies, but also the differences in the approach. Lead tungstate illustrates particularly well how large and very challenging fundamental research projects are instrumental to pushing the limits of detector performances to meet an ambitious scientific goal. On the other hand, new halides and mixed crystals are materials to be used mainly in commercial systems like security and medical imaging devices. It is therefore constrained not only by technical considerations but also by a severe competition environment, as any new commercial product.

How User’s Requirements Influence the Development of Scintillators

In this chapter we discuss practical scintillation parameters which are relevant from a user’s point of view for the pragmatic choice of an existing or the development of a new scintillator. For the majority of applications the most relevant ones are density, operation speed, light yield, identification of particles, production capability, stability under ionizing radiation, durability of operational parameters. We describe five main domains of applications, each of them with its own list of requirements: high energy physics, medical imaging, security applications, physics of the universe, and well and mud logging.