A. Lempicki

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.

CE-DOPED SCINTILLATORS : LSO AND LUAP

Abstract In this paper we compare the scintillator performance of the two of the leading contenders for Positron Emission Tomography, both cerium doped: Lu 2 SiO 5 (LSO) and LuAlO 3 (LuAP). LSO crystals come in two categories, characterized by high (H) or low (L) light output. Such a distinction is not found in LuAP, but its output is consistently lower than that of LSO-H. Based on decays, thermoluminescent data and dependence of light output on temperature, we conclude that the principal reason for the superior properties of LSO-H is its virtual absence of shallow traps. LuAP also suffers from an additional handicap in the form of a parasitic absorption of unknown origin, which causes a light output dependence on the thickness of the sample.

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.

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.

Effect of shallow traps on scintillation

The paper establishes the mathematical connection between the process of scintillation in Ce-doped materials and their thermoluminescence. A set of kinetic equations describes the competing processes of radiative recombination and trapping by a single trap. The more general case of second-order kinetics is simplified to first order, allowing an analytic solution. Second-order kinetics is also solved numerically without such simplification and both are successfully applied to explain the temperature dependence of the scintillation light output on temperature, in the range where glow peaks occur. The order of the kinetics predicts rather different shapes of decay, but are difficult to distinguish experimentally.

Cerium compounds as scintillators

Stoichiometric Ce-materials with negligible Ce-Ce interactions should have superior scintillator properties. The authors present two materials: CeF/sub 3/ and Ce/sub x/La/sub 1-x/P/sub 5/O/sub 14/. While cerium trifluoride is a known scintillator, pentaphosphate is of a limited usefulness, except as a remarkable model material. It is shown that quenching in fluoride is responsible for loss of 50% of the light output and is the cause of the so-called ultrafast component (2 ns). The light output of fluoride (about 50% of BGO) could be significantly improved. It is concluded that deeper understanding of Ce systems is needed to fully exploit their potential.<<ETX>>

Electron multiplication in scintillators and phosphors

Abstract An energy-dependent loss parameter, K(E), is calculated from a model dielectric response function. The rapid variation of K(E) suggests definition of an effective threshold displacement, ΔE, appreciable for semiconductors but negligible for insulators. The model is applied both to scintillator efficiency and to multi-photon phosphors.

Scintillation materials for medical applications

Scintillators are beginning to attract renewed attention because modern High Energy Physics accelerators are placing unprecedented demands of quantity and quality of detector materials and Positron Emission Tomography (PET), used by the medical field. Both applications required materials for scintillator detectors with properties beyond those delivered by traditional scintillators. Thallium doped halides are very efficient, but slow and chemically unstable. Two modern developments, namely the very fast BaF[sub 2], which owed its success to the newly discovered crossover transitions, and CeF[sub 3], which carried a promise of fast components, more practical wavelengths and attractive efficiency. Since traditional scintillators (Tl doped halides) are very efficient, and could be even more efficient at larger concentrations of Tl, if it were not for concentration quenching. However Tl transitions are spin forbidden and slow. Both ills could be remedied by replacing Tl with Ce, whose transitions are allowed and which is known to form fully concentrated compounds of high photoluminescent efficiency and no quenching. These materials, plus new Ce-doped materials, exhibiting highly promising properties for medical applications, became the target of our studies.