Recent research in the field of phosphor and scintillator materials and related detectors is reviewed. After a historical introduction the fundamental issues are explained regarding the interaction of x-ray radiation with a solid state. Crucial parameters and characteristics important for the performance of these materials in applications, including the employed measurement methods, are described. Extended description of the materials currently in use or under intense study is given. Scintillation detector configurations are further briefly overviewed and selected applications are mentioned in more detail to provide an illustration.

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Scintillation detectors for x-rays

In this review, the major achievements and research and development (R&D) trends from the last decade in the field of single crystal scintillator materials are described. Two material families are included, namely, those of halide and oxide compounds. In most cases, the host crystals are doped with Ce3+, Pr3+ or Eu2+ rare earth ions. Their spin- and parity-allowed 5d–4f transitions enable a rapid scintillation response, on the order of tens to hundreds of nanoseconds. Technological recipes, extended characterization by means of optical and magnetic spectroscopies, and theoretical studies are described. The latter provide further support to experimental results and provide a better understanding of the host electronic band structure, energy levels of specific defects, and the emission centers themselves. Applications in medical imaging and dosimetry, security measures, high-energy physics and the high-tech industry, in which X(γ)-rays or particle beams are used and monitored, are recognized as the main driving factor for R&D activities in this field.

Recent R&D Trends in Inorganic Single-Crystal Scintillator Materials for Radiation Detection

The absolute light outputs of BGO, CsI(Tl) and some new Ce-doped crystals have been measured to an accuracy of about /spl plusmn/5% using calibrated XP2020Q photomultipliers and standard S3590.03 and S2740.03 photodiodes. The use of small crystals, 9 mm in diameter and 1 mm thick, reduces the corrections for imperfections in the light collection process and in the photoelectron collection by the photomultipliers. The measured light-output of 8500/spl plusmn/350 ph/MeV (photons/MeV) for the BGO crystals agree well with the earlier measurement done by Holl et al. (1988). The carried out study highlighted the importances of the spread in the published emission spectra of the crystals which seems to limit measurement accuracy. Finally, a simple comparative method of measuring light output to an accuracy of /spl plusmn/10%, is proposed using an uncalibrated XP2020 photomultiplier and a 1 mm thick BGO crystal as a standard.

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Absolute light output of scintillators

Properties and Application of Geopolymers Vol. 841 (/MSF.841 /book) Development and Investigation of Materials Using Modern Techniques Vol. 840 (/MSF.840/book) Superplasticity in Advanced Materials ICSAM 2015 Vols. 838-839 (/MSF.838-839/book) 12th International Conference on High Speed Machining Vols. 836-837 (/MSF.836-837/book) Sintering Fundamentals II Vol. 835 (/MSF.835/book) Advanced Machining Technologies: Traditions and Innovations Vol. 834 (/MSF.834/book) Applied Materials and Technologies Vol. 833 (/MSF.833/book) Emerging Functional Materials: Book (/MSF.841/book) Papers (/MSF.841)

Materials Science Forum

For a long time the discovery of new scintillators has been more serendipitous than driven by a deep understanding of the mechanisms at the origin of the scintillation process This situation has dramatically changed since the 1990's with an increased demand for scintillators of better performance for large particle physics experiments as well as for medical imaging It is now possible to design a scintillator for a specific purpose The bandgap can be adjusted, the traps energy levels and their concentration can be finely tuned and their influence can be damped or on the contrary enhanced by specific doping for an optimization of the performance of the scintillator Several examples are given in this paper of such crystal engineering attempts to improve the performance of crystal scintillators used in medical imaging devices An attention is also given to spectacular progress in crystal production technologies, which open new perspectives for large scale and cost effective crystal production with consistent quality

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Development of new scintillators for medical applications

Single crystals of nanoporous 12CaO·7Al2O3 (C12A7) as large as 27 mm in diameter, 120 mm in length, and 207 g in weight have been successfully grown by the Czochralski method. We found that a 2.0% oxygen-containing nitrogen atmosphere and an iridium crucible are the critical factors for success. The crystal ingot exhibited an orange color resulting from the incorporation of Ir4+ ions of ca. 5 × 1017 cm-3, which presumably occupy Ca2+ sites. X-ray diffraction analysis indicated that the ingot obtatined was a single crystal with few domains.

Czochralski Growth of 12CaO·7Al2O3 Crystals

Growth rate and surface morphology of 4H–SiC crystals grown from Si–Cr–C and Si–Cr–Al–C solutions under various temperature gradient conditions

Several LGSO samples with different Ce doping were studied in comparison to a GSO crystal. The light output, non-proportionality of the light response, energy resolution and intrinsic resolution were measured and compared to the intensity of the afterglow recorded in the second range of time. A high light output of 17000 plusmn 1700 ph/MeV and a good energy resolution of 6.5 plusmn 0.2% for 662 keV gamma rays from a 137Cs source were obtained for the best sample, doped with 1 mol% of Ce. The energy resolution and, in particular, the intrinsic resolution varied with the concentration of Ce in the tested crystals and were not correlated with their common non-proportionality characteristics. In contrast, the observed correlation of the intrinsic energy resolution of the LGSO crystals and the intensity of their afterglow suggests that the energy resolution of scintillation detectors may be affected by a strong afterglow of the crystals. A further improvement of the crystals exhibiting a strong afterglow can be expected due to the improvement of its quality.

Energy Resolution of LGSO Scintillators

Growth of cerium-doped gadolinium pyrosilicate (Gd2Si2O7:Ce) single crystals, which show 2.5 times greater light output for gamma-rays and five times greater light output for alpha particles than GSO single crystals, is accomplished using floating zone growth method (FZ method). Although growth of Gd2Si2O7 (GPS) single crystal is considered to be difficult because it melts incongruently according to the phase diagram in a Gd2O3-SiO2 system, we attempted crystal growth of Ce:GPS because the possibility exists that heavy Ce doping changes the phase diagram. Transparent single crystals were obtained, but cracks were observed in the crystals. The crystal structure was triclinic with space group P/1 and density of 5.5 g/cm3. Two peaks were observed by photoluminescence spectrum measurement at 374 nm and 394 nm caused by 5d-4f transition in Ce3+ ion. Decay times of Ce:GPS were 46 ns for gamma-ray and 39 ns for alpha particles; its density was 5.5 g/cm3. We consider that the energy resolution of 23% will be improved by fabrication of large crystals and improvement of crystal perfectability.

Floating zone growth and scintillation characteristics of cerium-doped gadolinium pyrosilicate single crystals

Single crystal Ce-doped Gd/sub 2/SiO/sub 5/ (GSO) has a good balance of scintillation properties such as high density, intense light output, and short decay time, and also has an excellent temperature response and excellent radiation hardness. We studied causes for cracks in single-crystal GSO which have been a major problem in GSO growth, and tried to optimize the growth conditions by using a two-zone furnace which reduced thermal gradient. We were thus able to grow a crack-free single-crystal of GSO that was 80 mm in diameter and 280 mm long. Specimens with 20/spl times/20/spl times/200 mm/sup 3/ sampled from the large GSO boules with 0.5 and 1.5 mol% Ce have been evaluated for high energy physics application.

Kazuhisa Kurashige

Papers

Czochralski Growth of 12CaO·7Al2O3 Crystals

Growth rate and surface morphology of 4H–SiC crystals grown from Si–Cr–C and Si–Cr–Al–C solutions under various temperature gradient conditions

Energy Resolution of LGSO Scintillators

Floating zone growth and scintillation characteristics of cerium-doped gadolinium pyrosilicate single crystals

Scintillation performance of large Ce-doped Gd/sub 2/SiO/sub 5/ (GSO) single crystal