Showing papers by "Charles L. Melcher published in 2008"

Effects of $\hbox {Ca}^{2+}$ Co-Doping on the Scintillation Properties of LSO:Ce

Abstract: In addition to desirable physical properties including a density of 7.4 g/cm3 , an effective atomic number of 66, and no hygroscopicity, Lu2SiO5:Ce has well-known scintillation properties of ~30 900 photons/MeV, an emission peak near 420 nm, and a decay time of ~43 ns. These scintillation properties are achieved with Ce doping concentrations roughly in the range of 0.05 to 0.5 atomic percent relative to Lu. These properties make Lu2SiO5:Ce a widely used scintillator in positron emission tomography, in particular. We have found that both the light output and decay time may be improved by a combination of optimized crystal growth atmosphere and co-doping with divalent cations such as Ca. Scintillation light output of ~38 800 photons/MeV has been achieved as well as scintillation decay time as short as 31 ns with no long components. The relationship between growth conditions, dopant concentration, decay time, and light output is well defined, thus allowing one to reliably "tune" the crystal to the desired combination of light output and decay time. Possible explanations of the underlying mechanism are being explored and include compensation of oxygen vacancies, alteration of the relative occupancies of the cerium lattice sites, and suppression of trapping centers. In addition to higher count-rate capability and better coincidence timing, the improved decay time is expected to be particularly advantageous for time-of-flight positron emission tomography. Also, phoswich detectors comprising "standard" LSO (~43 ns decay time) and "fast" LSO (~31 ns decay time) become an attractive alternative to typical phoswich designs that often suffer from problems of mismatched light outputs and indices of refraction or the absorption of one scintillator's light by the other.

Potentials for large axial field of view positron camera systems

Abstract: By extending the axial field-of-view (FOV) in positron emission tomography (PET), the volume sensitivity and the planar sensitivity increase However, given the current level of commercial block detector technology for the positron camera design, there seems to be little interest in going beyond a 6 or 8 block ring system, corresponding to an axial FOV of 35 to 45 cm, mainly due to cost issues such as the scintillator material and the photomultipliers A large axial FOV system of the order of one meter or more facilitates a whole body exam in a single bed position, so that multiple organs can be examined simultaneously Therefore, the dynamic uptake and the bio-distribution of a tracer in different organs could be followed and whole-body kinetics could be studied The extended axial FOV for PET systems based on Resistive Plate Chambers (RPCs) has been discussed recently RPC systems provide very good intrinsic spatial and timing resolution We developed a simple RPC system model by extending our previously described multiblock-ring LSO model to a ∼2 meter axial FOV system and reducing the LSO scintillator depth to provide a singles sensitivity relevant for a RPC detector The model estimates absolute and planar sensitivity Based on the planar sensitivity, we estimated the acquisition time needed for the RPC system to achieve comparable image quality with current Siemens Biograph TruePoint and TruePoint TrueV systems (3 and 4 block-rings, respectively) In addition we compared the noise-equivalent count rates (NEC) according to the NEMA NU2-2001 protocol

Combinatorial Thin Film Synthesis of Cerium Doped Scintillation Materials in the Lutetium Oxide—Silicon Oxide System

Abstract: The search for new scintillator crystals can be limited by the time consuming nature of the crystal growth process. In this paper, we demonstrate the use of a combinatorial thin film synthesis technique that is being used to explore new scintillator materials. The combinatorial synthesis process utilizes up to four individual R.F. magnetron sputtering sources which can be simultaneously powered to generate a wide composition space of binary, ternary, or quaternary material systems. In this work, we have investigated the lutetium oxide (Lu2O3)-silicon oxide (SiO2) material system doped with cerium. Lu2SiO5-doped with cerium has been thoroughly characterized due to its use in PET imaging and provides a good benchmark for our proposed approach. Thin film, cerium doped lutetium oxide-silicon oxide gradients were synthesized to investigate the phases of the material system that exhibit scintillation properties and the results were compared to the results of bulk crystal samples. We have found that the emission spectra of the thin film materials have similar characteristics compared to the bulk crystals. Additionally, X-ray diffraction measurements have been correlated to the anticipated phases of the equilibrium phase diagram, and the intensity of the luminescence emission spectra have been correlated with the corresponding phases of the system.

Implementation of wavelength shifters in phoswich detectors

Abstract: A phoswich device for determining depth of interaction (DOI) includes a wavelength shifting layer between first and second scintillators of different scintillation materials and having different decay time characteristics. The wavelength shifting layer allows a true phoswich device to be constructed where the emission wavelength of one scintillator is in the peak excitation band of the other scintillator, by shifting the scintillation light outside of this excitation band to prevent scintillation light of one scintillator from exciting a response in the other scintillator, thus enabling unique identification of the location of a gamma photon scintillation event. The phoswich device is particularly applicable to positron emission tomography (PET) applications.

Energy resolution of calcium co-doped LSO:Ce scintillators

Abstract: Co-doping Lu 2 SiO 5 :Ce (LSO:Ce) with Ca divalent cations changes the scintillation properties of the crystal. In the present work an influence of Ca2+ co-doping on energy resolution, light output and non-proportionality was investigated for samples with 0, 0.1, 0.2, 0.3, 0.4 atomic percent Ca with respect to Lu. A substantial improvement of energy resolution in the co-doped crystals was found and higher light output by about 10% was observed. The best energy resolution of 7.35±0.15% was measured for LSO with 0.2% Ca. Contrary to our expectations, the change in the measured energy resolution of Ca2+ co-doped LSO samples is not reflected in the non-proportional characteristic of the studied crystals as the non-proportionality curves are independent of Ca concentration. Possible explanations of the underlying mechanism of improving the energy resolution include afterglow suppression via Ca co-doping. Earlier studies showed that calcium co-doping significantly reduces the trap population, hence the decay time of LSO is shortened and the afterglow is substantially quenched. In the current work, the integrated afterglow intensities as well as the afterglow effective decay times correlate with the concentration of Ca2+. Since the afterglow was measured about 30 ms after the crystal was irradiated by a strong X-ray source, the integral intensity does not include the faster components of afterglow. Hence, the correlation between afterglow intensity and energy resolution treated in this work is very preliminary.