A.J. Wojtowicz

LuAlO/sub 3/:Ce and other aluminate scintillators

A new scintillator, LuAlO/sub 3/:Ce, has been found to have properties that put it in the forefront when stopping power and timing is of importance. This material, an extension of other well known cerium-doped scintillators, the yttrium-based orthoaluminate and garnet, can be readily pulled from the melt, and displays particularly promising performance. We summarize the results achieved when Y is replaced by Lu in this class of oxide crystals. >

Luminescence properties of Ce-activated YAG optical ceramic scintillator materials

Abstract Scintillation and luminescence characteristics of a highly dense transparent YAG: Ce-ceramic are reported and compared to those of a single crystal. When excited with gamma-rays both types of materials display the same dominant ≈85 ns decay, but the ceramic also shows a new rapid component of ≈20 ns that is completely absent in the single crystal. The scintillation output from the ceramic reaches about 50% of that from the single crystal, having been diminished by unusual loss processes caused by the deformed lattice at the grain boundary interfaces. A phenomenological model involving distortion of the band structure is proposed to explain the results of kinetic measurements.

Fundamental limits of scintillator performance

Abstract In this paper we consider the basic physical processes involved in the scintillation process and discuss the limitations imposed on two important performance parameters, namely efficiency (light output) and speed. Light output is determined by the product of efficiencies of energy conversion, energy transfer and luminescence processes. We propose a procedure by which these partial efficiencies can be obtained for any scintillator and use it to evaluate some known materials. Limits of speed are set by the value of the Einstein coefficient A for the luminescent emission and by transfer rates. The singular significance of the transfer step is illustrated by some Ce-based scintillators. A figure of merit and an “efficiency-speed” diagram are introduced in order to compare different scintillators.

PROPERTIES OF THE NEW LUAP:CE SCINTILLATOR

Abstract The scintillation properties of LuAP (lutetium aluminum perovskite, LuAlO 3 ) have been investigated at three different levels of Ce doping: ≥ 0, 0.035 and 0.105 mol%. The light yield, in photoelectrons per MeV, was measured as 122±20, 1300±100 and 2850±200, respectively. The light pulse shapes were largely exponential, with a decay constant of 16.5±1 ns for all the samples studied. In all cases, however, an additional slow component, amounting to about 10±3% of the total light, was also found, characterized by a time constant of 74±7 ns. The sample doped with 0.105 mol% Ce showed an energy resolution of 9.3% for the 662 keV full energy peak from a 137 Cs source. The high detection efficiency of the material for γ-rays (because of its high density of 8.4 g/cm 3 ) is confirmed by a photofraction of about 13% for a specimen with a volume of only 0.05 cm 3 . The time resolution for 60 Co γ-rays at a 1 MeV threshold was measured as 160 ps, somewhat poorer than expected. Nevertheless, the high light yield, fast light pulse, high detection efficiency for γ-rays and excellent time resolution make this material a very attractive scintillator, particularly in positron emission tomography.

Cerium-doped orthophosphates: new promising scintillators

The authors report initial results for a new class of scintillating materials, cerium-doped rare earth orthophosphates. The most promising is Ce:LuPO/sub 4/, which under gamma -excitation shows fast, single-exponential decay (25 ns at room temperature), and a decent light output (17200 photons per 1 MeV). These features, combined with a reasonable density (6.53 g/cm/sup 3/), make it a strong contender for many applications. >

Electron traps and scintillation mechanism in YAlO3:Ce and LuAlO3:Ce scintillators

Abstract In this paper we present the results of thermoluminescence, isothermal decay and scintillation light yield measurements on two isostructural scintillator materials, YAlO 3 xa0:xa0Ce and LuAlO 3 xa0:xa0Ce. In addition to the variety of deep traps identified by thermoluminescence and isothermal decays, scintillation light yield experiments demonstrate the presence in both materials of a number of relatively shallow traps. While the deep traps may reduce the scintillation light yield, they do not influence the kinetics of the process. The shallow traps, on the other hand, by interfering with the process of radiative recombination of charge carriers via Ce 3+ ions, can strongly affect not only the yield of the scintillation process but its kinetics as well. The presence of shallow traps provides a consistent explanation for a number of poorly understood relationships between the two scintillator materials, including a higher room temperature scintillation light yield and longer scintillation decay time in YAlO 3 xa0:xa0Ce, and a longer scintillation rise time in LuAlO 3 xa0:xa0Ce. Theoretical analysis indicates that elimination of these traps would make the two materials nearly identical in scintillator performance. Although the specific identity of all traps remains elusive, the performance of both scintillator materials is now, in practical terms, fully understood.

Recombination and scintillation processes in

This paper reports spectroscopic and scintillation studies of the well established scintillator material . Standard measurements of luminescence emission and excitation spectra have been accompanied by investigations of thermoluminescence and scintillation light yield over a wide temperature range, and by decay measurements under pulsed gamma and VUV excitations at various temperatures. These measurements are interpreted in the framework of a model that includes a recombination centre and a number of electron traps. We demonstrate that some unusual and largely unexplained features of the scintillator, such as a substantial disparity between scintillation and radiative decay times, the presence of slow components in scintillation decays, and a strong temperature variation of scintillation light yield between 150 and 300 K, have their origin in the processes of charge carrier capture and emission by electron traps. Although the nature of these traps remains elusive, most of the trap parameters, such as frequency factors, energy depths, and relative populations, have been estimated. This makes it possible to predict the characteristics of trap-free material and thereby the potential improvements that could be achieved.

Fundamental limitations of scintillators

Abstract Speed and light output are the two most important parameters characterizing scintillation materials. While the limit of speed is set by the Einstein coefficient A of the relevant luminescent transition, light output is determined by efficiencies of three processes acting in sequence: energy conversion, transfer and luminescence. We will present a scheme to characterize any given material and apply it to some known and established materials as well as to new materials utilizing d-f transitions on Ce 3+ ions. We will demonstrate that, while the conversion efficiency may, in some cases, be low because of high losses to optical phonons, it is by far the transfer step, which is responsible for vast differences in light outputs of different Ce materials.

Stoichiometric cerium compounds as scintillators, II. CeP/sub 5/O/sub 14/

For pt.I see NSSMI Conf. Record, Nov., 1991, NM, USA. Results showing the relative importance of several processes which are responsible for the efficiency and speed of a scintillator are presented. Arguments for concentrated materials are presented and illustrated in the case of Ce-pentaphosphate, (CeP/sub 5/O/sub 14/). The low density of this material does not make it a viable scintillator, but its remarkable simplicity and relatively easy technology make it perfect to study basic physical mechanisms of scintillation in concentrated materials. Based on studies of a series of Ce/sub x/La/sub 1-x/P/sub 5/O/sub 14/ crystals, the authors formulate general conclusions relating to the relative importance of the process starting with direct ionization of the emitting centers (Ce/sup 3+/) versus long range gathering of electron-hole pairs. These conclusions provide guidance for optimizing the speed and efficiency of future concentrated scintillators. Some peculiarities of the Ce-pentaphosphate luminescence are reported and interpreted. >

Lutetium aluminate: spectroscopic and scintillation properties

The present status of the LuAlO/sub 3/:Ce (LuAP) scintillator is reviewed and its performance compared to that of YAlO/sub 3/:Ce (YAP). The light output of LuAP, at this stage of development, appears to be limited by two factors: insufficient Ce concentration and the presence of a parasitic absorption. Both LuAP and YAP show a dependence of light output on thickness, indicating that YAP also suffers from parasitic absorption. Measurements on undoped material offers a clue to the identity of the parasite.