Piotr Szupryczynski

Optimization of a LSO-Based Detector Module for Time-of-Flight PET

We have explored methods for optimizing the timing resolution of an LSO-based detector module for a single-ring, “demonstration” time-of-flight PET camera. By maximizing the area that couples the scintillator to the PMT and minimizing the average path length that the scintillation photons travel, a single detector timing resolution of 218 ps fwhm is measured, which is considerably better than the ~385 ps fwhm obtained by commercial LSO or LYSO TOF detector modules. We explored different surface treatments (saw-cut, mechanically polished, and chemically etched) and reflector materials (Teflon tape, ESR, Lumirror, Melinex, white epoxy, and white paint), and found that for our geometry, a chemically etched surface had 5% better timing resolution than the saw-cut or mechanically polished surfaces, and while there was little dependence on the timing resolution between the various reflectors, white paint and white epoxy were a few percent better. Adding co-dopants to LSO shortened the decay time from 40 ns to ~30 ns but maintained the same or higher total light output. This increased the initial photoelectron rate and so improved the timing resolution by 15%. Using photomultiplier tubes with higher quantum efficiency (blue sensitivity index of 13.5 rather than 12) improved the timing resolution by an additional 5%. By choosing the optimum surface treatment (chemically etched), reflector (white paint), LSO composition (co-doped), and PMT (13.5 blue sensitivity index), the coincidence timing resolution of our detector module was reduced from 309 ps to 220 ps fwhm.

Scintillation Non-Proportionality of Lutetium- and Yttrium-Based Silicates and Aluminates

This study compares the light yield as a function of gamma-ray energy for several Lu- and Y-based silicate and aluminate scintillators. These comparisons illustrate the effects of crystal matrix, growth atmosphere, activator concentration, and co-doping on proportionality. The radioisotopic sources used in this study produce gamma-rays and X-rays from 22 keV to 1.3 MeV. The samples investigated are 10 times 10 times 10 mm in size and have the following single-phase compositions: LSO:Ce; LSO:Ce, Ca; LPS:Ce; LuAG:Pr; YSO:Ce; and YSO:Ce, Ca. The light yield vs. gamma-ray energy response is similar for scintillators with similar crystal structures, despite significant variations in growth atmosphere and activator concentration. Additionally, sample-to-sample variation is significantly reduced in the Ce-doped oxyorthosilicates when calcium is added as a co-dopant. Overall, the silicates measured in this study exhibit similar non-proportional responses, and in general are less proportional in the lower energy range compared to LuAG and other aluminates.

Scintillation and optical properties of LuAP and LuYAP crystals

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.

A 36-Pixel Tunnel Junction Soft X-Ray Spectrometer for Scintillator Material Science

We have built a 36-pixel superconducting tunnel junction (STJ) spectrometer for high-resolution soft X-ray spectroscopy at the synchrotron. It has an energy resolution between ~10 and ~20 eV FWHM for X-ray energies below ~1 keV, and can be operated at maximum total count rates of ~106 counts/s. For increased sensitivity, we have modified the detector cold finger the so that the array now subtends a solid angle of Omega/4pi ap0.1%. The spectrometer is currently used at the Advanced Light Source synchrotron for speciation measurements of dilute samples by fluorescence-detected X-ray absorption spectroscopy. We present measurements of the oxidation state of dilute cerium activators in novel scintillator materials, determine the Ce3+/Ce4+ ratio, and discuss the sensitivity of the instrument for quantitative speciation measurements.

Cerium-doped mixed-alkali rare-earth double-phosphate scintillators for x- and gamma-ray detection

Previous measurements of the scintillation properties of members of the single-alkali, rare-earth double-phosphate family have demonstrated high light output and fast decay times when exposed to ionizing radiation. Cerium-doped K3Lu(PO4)2 and Rb3Lu(PO4)2 scintillators have exhibited light outputs of 32,500 and 28,200 photons/MeV respectively and decay times of 37 and 34 nanoseconds respectively. Because of the ease with which the alkali constituents (Li, Na, K, Rb, Cs) of the crystal matrix may be interchanged (e.g. K2CsLu(PO4)2 and CsLi2Lu(PO4)2), the rare-earth double-phosphate family of scintillators provides an ideal system for the study of matrix effects on scintillation efficiency and kinetics. In order to better understand and to ultimately optimize the scintillation properties of these scintillators, new members of the rare-earth double-phosphate family have been synthesized by high temperature flux growth. These new samples, represented by the general formula (A,B)3Lu(PO4)2:Ce where A and B are alkali elements, incorporate mixed alkali rather than single alkali components and varying levels of Ce doping. Light output, scintillation decay times, and photoluminescence measurements for the most promising of the samples to date are reported. In this paper, we identify promising samples and results that clearly demonstrate outstanding light output, up to 270% of BGO, fast decay times, 29-39 nanoseconds, and peak emission wavelengths of ~ 400 nm for many of the samples.

Ce-Doped Lutetium Pyrosilicate Scintillators LPS and LYPS

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