Showing papers by "Charles L. Melcher published in 2011"

Crystal growth and characterization of CsSr1–xEux I3 high light yield scintillators

Abstract: In this work, we report on single crystal growth and characterizations of a new scintillation material: CsSr 1-x Eu x I 3 (0 ≤ x ≤ 1). Single crystals of CsSr 0.99 Eu 0.01 I 3 , CsSr 0.92 Eu 0.08 I 3 and CsEuI 3 were grown via the Vertical Gradient Freeze method. The crystals exhibit good crystal quality, high light yield, and excellent energy resolution. Initial results show that the scintillation light yield of our CsSr 0.92 Eu 0.08 I 3 crystal is ~65,000 ph/MeV with 5.9% energy resolution at 662 keV.

Effect of Ca Co-Doping on the Luminescence Centers in LSO:Ce Single Crystals

Abstract: Experimental studies of LSO:Ce crystals co-doped with various concentrations of Ca are presented. Photoluminescence decay time, excitation and emission spectra, thermal response and X-ray excited luminescence are investigated as a function of Ca concentration. Experimental data show Ca co-doping does not alter the energy level structure of either Ce1 or Ce2, but reduces the relative population of Ce2 to Ce1. Thus, the luminescence from Ce2 is suppressed, which contributes to the fast scintillation decay of Ca co-doped LSO:Ce.

Towards Sub-Minute PET Examination Times

Abstract: The main performance parameters in positron camera system design are sensitivity and spatial resolution. This paper concerns sensitivity, which is a function of the scintillation material, the solid angle subtended by the detector array, and the scintillator packing fraction. The solid angle can be increased by extending the axial extent of cylindrical detector systems. Most commercial positron camera systems are based on rings of detector blocks with lutetium oxyorthosilicate, LSO:Ce or LYSO:Ce, as the scintillator of choice. By adding more rings, the solid angle and thus the absolute sensitivity increases while the singles detection efficiency remains fairly constant assuming the same crystal thickness. It has been shown that Ca co-doping of LSO:Ce reduces the scintillation decay time to ~30 ns with a light output over 30000 ph/MeV. This improvement may give a time-of-flight (TOF) advantage with time resolution of 500 ps or less. If the count rate sensitivity of a large axial field-of-view (AFOV) system is combined with the TOF sensitivity increase, we have the means to create examination times in the sub-minute range with no compromise in image quality. In the present study we have compared the existing Siemens molecular CT (mCT) systems to future 6, 8, 12, 20 and higher block ring systems with and without TOF. The mCT 4 block ring system has been used as a reference. The time for acceptable image quality with this system is then extrapolated to other systems based on planar sensitivity. However, the planar sensitivity is related to the solid angle, and reaches saturation for large AFOVs. This implies that there is an upper count rate sensitivity limit. A 20 block ring system may cover a 70 cm examination range at a certain planar count rate and could provide acceptable quality images in approximately 10 seconds by combining the high planar sensitivity count rate provided by the multi-ring feature, the high stopping power of LSO and the TOF gain due to the improved timing resolution. The increased sensitivity may be used to reduce patient dose.

Chloride Scintillator for Radiation Detection

Abstract: A halide scintillator material is disclosed. The material is single-crystalline and has a composition of the formula A3MBr6(1-x)Cl6x (such as Cs3CeBr6(1-x)Cl6x ) or AM2Br7(1-x)Cl7x (such as CsCe2Br7(1-x)Cl7x ), 0≤x≤ 1, wherein A consists essentially of Li, Na K, Rb, Cs or any combination thereof, and M consists essentially of Ce, Sc, Y, La, Lu, Gd, Pr, Tb, Yb, Nd or any combination thereof. Furthermore, a method of making halide scintillator materials of the above-mentioned compositions is disclosed. In one example, high-purity starting halides (such as CsBr, CeBr3, CsCl and CeCl3) are mixed and melted to synthesize a compound of the desired composition of the scintillator material. A single crystal of the scintillator material is then grown from the synthesized compound by the Bridgman method. The disclosed scintillator materials are suitable for making scintillation detectors used in applications such as medical imaging and homeland security.

Investigating the luminescence properties as a function of activator concentration in single crystal cerium doped Lu2SiO5: Determination of the configuration coordinate model

Abstract: The effect of cerium concentration on single crystal lutetium orthosilicate (Lu2SiO5, LSO) was investigated with temperature-dependent steady state and time resolved luminescence spectroscopy. The results were used to determine the thermal quenching activation energies and the phonon energies responsible for the thermal broadening of the luminescence spectra. The measured phonon values were used to calculate configuration coordinate (CC) diagrams of the luminescence centers. The nature of single crystal LSO:Ce concentration quenching was determined to be due to radiative energy transfer by self-absorption. The observed broadening of the excitation spectra and narrowing of the emission spectra with increasing cerium was explained via a CC model. A combination of the CC model and concentration quenching explained the measured thermal quenching activation energies.

Phoswich solutions for the PET DOI problem

Abstract: A high spatial resolution in PET can be achieved by using small detector elements. To maintain good sensitivity these elements have to be quite long, thus introducing parallax error and making the spatial resolution non-uniform over the image volume. Uniformity of spatial resolution can be improved by utilizing depth-of-interaction (DOI) information to reduce the parallax error. In the present study we have focused on phoswich approaches based on interacting scintillators, that is, a phoswich combination in which one scintillator emits light in the excitation band of the other. We have looked at LaBr 3 :Ce and LaCl 3 :Ce and the interactions of those two scintillators with LSO:Ce, GSO:Ce and YSO:Ce. The reasons to use the two Lanthanum scintillators are twofold: light output is high and the two different emission wavelengths, 350 nm (LaCl 3 :Ce) and 380 nm (LaBr 3 :Ce) may produce different interactions with the three oxyorthosilicate scintillators. In addition a possible DOI detector comprising LuAG:Pr pixels with a thin LSO:Ce layer at one end has been evaluated. A Bollinger–Thomas set-up was used to measure luminescence rise and luminescence decay time characteristics in all cases. When using LaCl 3 :Ce, the phoswich combinations with YSO:Ce and GSO:Ce showed phoswich decay time characteristics as expected for a simple convolution of the decay times of the two phoswich components. A correction was needed, however, for the LaCl 3 :Ce–LSO:Ce phoswich due to the LSO:Ce intrinsic activity. For the LaBr 3 :Ce–LSO:Ce phoswich, corrections were needed for non-interacting LaBr 3 :Ce light in addition to the expected phoswich interaction.

Characterization of Ca co-doped LSO:Ce scintillators coupled to SiPM for PET applications

Abstract: Scintillators suitable for PET applications must be characterized by a high efficiency for gamma-ray detection, determined by a high density and atomic number of the crystal; a fast light signal that allows to achieve a good time resolution and to cope with high counting rates; a high light yield for a good energy and time resolution; a good linearity of the light output as a function of the energy to preserve the intrinsic energy resolution of the scintillator. Recently developed LSO:Ce scintillators, co-doped with Ca, have been produced by the University of Tennessee group. They are characterized by the improved performance of most the above-mentioned characteristics. The crystals, initially tested with PMTs, showed a higher light output, faster light pulse, improved energy resolution and reduced afterglow, as compared to the standard LSO:Ce crystals. Even though the PMTs still represent the gold standard photodetectors, the recently available SiPMs are now valid candidate to replace PMTs in the next generation of PET scanners thanks to their compactness, high spatial resolution performances, low bias operating voltage and, most important for combined PET/MRI systems, insensitivity to static and RF fields. In this work we present the performance of Ca co-doped LSO:Ce samples coupled to SiPMs and PMTs. In particular we have assessed their performances by evaluating the energy and time resolution.

Synthesis and scintillation properties of CsGd2Cl7:Ce3+ for gamma ray and neutron detection

Abstract: In this work, we report on optical and scintillation properties of a new scintillation material CsGd 2 Cl 7 :Ce Crystals were grown in vacuum sealed ampoules, and the response to gamma rays and thermal neutrons was characterized The scintillation light yield was ∼38,000 ph/MeV, and the primary decay time was 60 ns under γ-ray excitation A thermal neutron response resulting from capture by 157 Gd and 155 Gd and subsequent emission of conversion electrons and X-rays was observed The crystals exhibited a layered structure with cleavage planes and relatively low hygroscopicity