Showing papers by "Charles L. Melcher published in 2005"

Perspectives on the future development of new scintillators

Abstract: The search for new scintillators has become increasingly sophisticated and increasingly successful in recent years, driven to a large degree by the rapidly growing needs of medical imaging and high energy physics. Better understanding of the various scintillation mechanisms has led to innovative new materials for both gamma-ray and neutron detection, and the concept of scintillator design and engineering has emerged, whereby materials are optimized according to the scintillation properties needed by specific applications. Numerous promising candidates have been identified during the last few years, and several are currently being actively developed for commercial production. Economical crystal growth often represents a significant challenge in the practical application of new scintillation materials.

Non-proportionality and thermoluminescence of LSO:Ce

Abstract: This work presents the results from experiments performed on Lu/sub 2/SiO/sub 5/:Ce (LSO:Ce). It looks for potential correlations of light yield nonproportionality and other scintillation properties such as intrinsic energy resolution with thermoluminescence properties. The analyzed samples were chosen from various crystal batches having significantly different properties. Single crystal LSO:Ce samples were coupled to an XP2020Q photomultiplier and to large area avalanche photodiodes and measured as a function of /spl gamma/-ray energy between 14.7 and 1770 keV at room temperature and also near liquid nitrogen temperature. To explain the experimental results obtained from spectrometric methods, the properties of the samples were further studied using thermoluminescence techniques within the temperature range from 250K to 600K. The relationships between data obtained from these two types of experiments are reported.

New prospects for time-of-flight PET with LSO scintillators

Abstract: The growing interest in time-of-flight PET triggered the study of the time resolution obtainable with a 4times4times20 mm3 LSO crystal coupled directly to the center of a 52 mm in diameter Photonis XP20D0 photomultiplier as well as the time resolution obtainable with the use of an 11 mm thick lucite light diffuser that simulates the conditions in typical PET block detectors. The LSO crystal directly coupled to the PMT yielded a time resolution of 166plusmn5 ps, while in the case of light readout with the use of the light diffuser it degraded to 196plusmn5 ps and 277plusmn6 ps in the center and at the edge of the PMT, respectively. The light diffuser was coated on the sides with black tape to absorb light and to approximate in this way the realistic performance of a future block detector. Similar time resolution was obtained by coupling the LSO crystal either to the Photonis XP20D0 PMT or to a very fast 25 mm diameter Hamamatsu R5320 PMT. These results illustrate the advantages of the very low time jitter of the Hamamatsu PMT on one side, and high quantum efficiency and a screening grid at the anode of the Photonis PMT, on the other. This study strongly suggests that time-of-flight PET based on LSO crystals is a realistic proposition for the further development

Cerium oxidation state in LSO:Ce scintillators

Abstract: Trivalent cerium ions form the luminescence centers in several important families of scintillation materials including the rare earth oxyorthosilicates, pyrosilicates, and aluminates. When comparing the experimentally determined scintillation properties of cerium-doped scintillators to theoretical models of scintillation mechanisms, there is often speculation regarding the fraction of the total cerium that exists in the radiative trivalent charge state (Ce/sup 3+/) rather than the nonradiative tetravalent state (Ce/sup 4+/). Until now, however, no technique has been developed to quantitatively measure both Ce/sup 3+/ and Ce/sup 4+/ in single crystal scintillators . We report here for the first time direct measurements of Ce/sup 3+/ and Ce/sup 4+/ in Lu/sub 2/SiO/sub 5/:Ce single crystals. Synchrotron radiation was used to measure the X-ray absorption on the M/sub 4/ and M/sub 5/ edges of Ce, and the results were compared to model samples of Ce/sup 3+/ (Ce/sub 2/O/sub 3/) and Ce/sup 4+/ (CeO/sub 2/) which provided clear signatures of the two charge states. The spectra were obtained with a high-resolution superconducting tunnel junction spectrometer on beamline 4.0.2 at the Advanced Light Source synchrotron at Lawrence Berkeley National Laboratory. The results clearly show 100% Ce/sup 3+/, independent of light yield and sample coloration. Therefore, energy migration to the luminescence centers appears to be the determining factor in the scintillation efficiency of these samples, rather than variations in the Ce/sup 3+//Ce/sup 4+/ ratio.

Potential for a fifth generation PET scanner for oncology

Abstract: Four generations of PET scanners have emerged since the first clinical PET system. In this paper we predict the achievable performance of a fifth generation PET scanner, defined as a fully 3D system with no septa, an axial FOV approaching 30 cm with a spatial resolution mainly dictated by the positron physics. The predictions are based on the HIREZ (CPS/Siemens) performance plus current insights in detector technology, electronics and image reconstruction. The HIREZ system uses LSO scintillators, providing fast timing, good energy resolution, high spatial resolution and high system live time. LSO is an excellent candidate for a TOF detector with a proven 400 ps time resolution for detector blocks in coincidence. We have considered and integrated improvements that can be made to the key components necessary to the design of a fifth generation PET scanner. The spatial resolution can be improved by using a finer detector grid. The time window can be reduced. Packing fraction can be increased. The axial extension can be increased, giving more solid angle coverage and therefore better NEC. Based on the predicted gains in reduced randoms, dead time and TOF advantages, we have made initial simulations of the NEC for different axial field of views. An axial extension of /spl sim/32 cm with ECAT EXACT ring geometry gives at least four times higher NEC count rate than for a 16 cm axial FOV system. This will be combined with the improved image quality due to higher spatial sampling and TOF.

Scintillation and optical properties of LuAP and LuYAP crystals

Abstract: In this paper properties of LuAP (LuAlO/sub 3/:Ce), and LuYAP (Lu/sub 0.7/Y/sub 0.3/AlO/sub 3/:Ce) crystals are studied. The previously reported self-absorption has been confirmed, and a possible mechanism is discussed. The thermal stability of the material was evaluated, and both LuAP and LuYAP have been found to readily decompose when heated to a sufficiently high temperature. XRD studies were done to determine that a solid-solid phase transformation occurs under those conditions. In addition, thermoluminescence studies reveal the presence of additional traps in the LuYAP crystal, apparently the result of the yttrium addition to the crystal lattice.

Ce-Doped Lutetium Pyrosilicate Scintillators LPS and LYPS

Abstract: In this paper we present scintillation, optical, and thermoluminescence properties of recently discovered scintillation material, LPS (Lu2Si2O7:Ce) and composition with yttrium LYPS ((Lu,Y)2Si2O7 :Ce). The latter was first grown at Siemens Medical Solutions Molecular Imaging, and had an yttrium concentration of 50%. Both have the thorveitite structure, with monoclinic symmetry, space group C2/m. XRD measurements confirmed the expected crystal structure in which there is a single crystallographic site for lutetium or yttrium ions, with six oxygen neighbors. The trivalent cerium activator ions are assumed to occupy the cation lattice site. The excited 5d state of Ce3+ is split into 2 observable levels with luminescence emission occurring only from the lowest 5d level to the 4f ground state with a Stokes shift of ~2250 cm-1. In this paper, we report on the scintillation properties of LPS and LYPS crystals. The difference in scintillation properties observed between samples is discussed. It was based on thermoluminescence data obtained in the temperature range from 30 to 600 K. These data show that the analyzed samples have surprisingly different sets of traps. Crystal growth and cutting issues of LPS and LYPS are also briefly discussed