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Kappers L.A.,University of Connecticut | Bartram R.H.,University of Connecticut | Hamilton D.S.,University of Connecticut | Lempicki A.,ALEM Associates | And 4 more authors.
Journal of Physics: Conference Series | Year: 2010

Combined radioluminescence, afterglow and thermoluminescence experiments on single-crystal samples of co-doped CsI:Tl,Sm suggest that deeper samarium electron traps scavenge electrons from shallower thallium traps and that electrons subsequently released by samarium recombine non-radiatively with holes trapped as VKA(Tl+) centers, thus providing a mechanism for suppression of trapped-charge accumulation in repetitive applications. In the present investigation, experiments performed on two single-crystal samples of CsI:Tl,Sm with nominal concentrations of 0.11% Tl+ and of 0.2% and 0.05% Sm2+, respectively, support the inference that electrons tunnel freely between samarium ions and are trapped preferentially near V KA(Tl+) centers where non-radiative recombination is the rate-limiting step. © 2010 IOP Publishing Ltd.


Kappers L.A.,University of Connecticut | Bartram R.H.,University of Connecticut | Hamilton D.S.,University of Connecticut | Lempicki A.,ALEM Associates | And 5 more authors.
Radiation Measurements | Year: 2010

Combined radioluminescence, afterglow and thermoluminescence experiments on single-crystal samples of co-doped CsI:Tl,Sm suggest that samarium electron traps scavenge electrons from thallium traps and that electrons subsequently released by samarium recombine non-radiatively with trapped holes, thus suppressing afterglow. Experiments on single crystals support the inference that electrons tunnel freely between samarium ions and are trapped preferentially as substitutional Sm+ near VKA(Tl+) centers where non-radiative recombination is the rate-limiting step. Afterglow in microcolumnar films of CsI:Tl,Sm is enhanced by inhomogeneities which impede tunneling between samarium ions, but is partly suppressed by annealing. © 2009 Elsevier Ltd. All rights reserved.


Bartram R.H.,University of Connecticut | Kappers L.A.,University of Connecticut | Hamilton D.S.,University of Connecticut | Brecher C.,Alem Associates | And 3 more authors.
IOP Conference Series: Materials Science and Engineering | Year: 2015

CsI:Tl is a widely utilized scintillator material with many desirable properties but its applicability is limited by persistent afterglow. However, effective afterglow suppression has been achieved by co-doping with divalent lanthanides. The present report is concerned with observation of multiple thermoluminescence glow peaks in CsI:Tl,Eu and CsI:Tl,Yb, attributed to varying distributions of charge-compensating cation vacancies relative to divalent lanthanide co-dopants, and the subsequent modification of these distributions by repeated observations. It is observed that Yb2+ provides a slightly shallower electron trap than Eu2+, and that it can occupy a face-centered position by virtue of its relatively small ionic radius; the latter observation is confirmed by electrostatic calculations. It is also found that repeated observation of thermoluminescence in these materials has a modest adverse effect on afterglow suppression. © Published under licence by IOP Publishing Ltd.


Cool S.,Radiation Monitoring Devices, Inc. | Miller S.,Radiation Monitoring Devices, Inc. | Brecher C.,ALEM Associates | Lingertat H.,ALEM Associates | And 5 more authors.
IEEE Transactions on Nuclear Science | Year: 2010

We have recently fabricated tellurium-doped zinc selenide (ZnSe:Te) in a robust optical ceramic form, in the first synthesis of this remarkable material from a precursor powder, using powder consolidation techniques.We utilized two techniques in particular-hot uniaxial and hot isostatic pressing-in order to achieve the desired physical and chemical properties, and we are continuing our efforts to refine processing methodologies and parameters to improve this material's physical characteristics and performance. ZnSe:Te promises major advances in radiation imaging and spectroscopy applications with its potentially higher brightness than current scintillators such as CsI:Tl and NaI:Tl, fast emission ( 3 ̃ μs and ̃ 40 μs ), negligible afterglow ( 0.05% at 6 ms), above-average density ( <5.2 g/cm3), and light emission in the visible range (610-650 nm). This paper will describe our fabrication methods and report our preliminary performance characterization results. © 2010 IEEE.


Wang Y.,Radiation Monitoring Devices, Inc. | Baldoni G.,Radiation Monitoring Devices, Inc. | Rhodes W.H.,ALEM Associates | Brecher C.,ALEM Associates | And 5 more authors.
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2012

Lanthanide gallium/aluminum-based garnets have a great potential as host structures for scintillation materials for medical imaging. Particularly attractive features are their high density, chemical radiation stability and more importantly, their cubic structure and isotropic optical properties, which allow them to be fabricated into fully transparent, highperformance polycrystalline optical ceramics. Lutetium/gadolinium aluminum/gallium garnets (described by formulas ((Gd,Lu)3(Al,Ga)5O12:Ce, Gd3(Al,Ga)5O12:Ce and Lu3Al 5O12:Pr)) feature high effective atomic number and good scintillation properties, which make them particularly attractive for Positron Emission Tomography (PET) and other γ- ray detection applications. The ceramic processing route offers an attractive alternative to single crystal growth for obtaining scintillator materials at relatively low temperatures and at a reasonable cost, with flexibility in dimension control as well as activator concentration adjustment. In this study, optically transparent polycrystalline ceramics mentioned above were prepared by the sintering-HIP approach, employing nano-sized starting powders. The properties and microstructures of the ceramics were controlled by varying the processing parameters during consolidation. Single-phase, high-density, transparent specimens were obtained after sintering followed by a pressure-assisted densification process, i.e. hot-isostatic- pressing. The transparent ceramics displayed high contact and distance transparency as well as high light yield as high as 60,000-65,000 ph/MeV under gamma-ray excitation, which is about 2 times that of a LSO:Ce single crystal. The excellent scintillation and optical properties make these materials promising candidates for medical imaging and γ-ray detection applications. © 2012 SPIE.


Rhodes W.H.,ALEM Associates | Wang Y.,Radiation Monitoring Devices, Inc. | Brecher C.,ALEM Associates | Gary Baldoni J.,Radiation Monitoring Devices, Inc.
Journal of the American Ceramic Society | Year: 2011

Pr-doped Lu 3Al 5O 12 (LuAG) transparent ceramic, a potential scintillator material, was fabricated by sinter/hot isostatic pressing (HIP). The specimens were subjected to various post-densification heat treatments and the evolution of porosity and its relationship to transparency was monitored and studied. Annealing was necessary to remove discoloration and to restore stoichiometry, but when performed at too high a temperature it caused a severe decrease in transparency. Transparency was restored by reHIPing, indicating that some nanometer-size pores remained even after the original HIP cycle, which expanded in size during annealing and contracted again during re-HIPing. Annealing at a lower temperature restored stoichiometry without serious transparency degradation, due to a favorable difference in diffusion rates for mass transfer and O 2- diffusion. This phenomenon illustrates a fundamental difference between residual porosity in ceramics consolidated by pressureless and pressure-assisted processes. © 2011 The American Ceramic Society.


Van Loef E.V.,Radiation Monitoring Devices, Inc. | Wang Y.,Radiation Monitoring Devices, Inc. | Miller S.R.,Radiation Monitoring Devices, Inc. | Brecher C.,ALEM Associates | And 6 more authors.
Optical Materials | Year: 2010

In this paper we report on the fabrication and characterization of SrHfO3:Ce ceramics. Powders were prepared by solid-state synthesis using metal oxides and carbonates. X-ray diffraction measurements showed that phase-pure SrHfO3 is formed at 1200 °C. Inductively coupled plasma spectroscopy confirmed the purity and composition of each batch. SrHfO3 exhibits several phase changes in the solid, but this does not appear to be detrimental to the ceramics. Microprobe experiments showed uniform elemental grain composition, whereas aluminum added as charge compensation for trivalent cerium congregated at grain boundaries and triple points. Radioluminescence spectra revealed that the light yield decreases when the concentration of excess Sr increases. The decrease in the light yield may be related to the change of Ce3+ into Ce4+ ions. For stoichiometric SrHfO3:Ce, the light yield is about four times that of bismuth germanate (BGO), the conventional benchmark, indicating great potential for many scintillator applications. © 2010 Elsevier B.V. All rights reserved.


Roy S.,Boston University | Lingertat H.,ALEM Associates | Brecher C.,ALEM Associates | Sarin V.,Boston University
Optical Materials | Year: 2013

Polycrystalline cerium activated lutetium oxyorthosilicate (LSO:Ce) is highly desirable technique to make cost effective and highly reproducible radiation detectors for medical imaging. In this article methods to improve transparency in polycrystalline LSO:Ce were explored. Two commercially available powders of different particulate sizes (average particle size 30 and 1500 nm) were evaluated for producing dense LSO:Ce by pressure assisted densification routes, such as hot pressing and hot isostatic pressing. Consolidation of the powders at optimum conditions produced three polycrystalline ceramics with average grain sizes of 500 nm, 700 nm and 2000 nm. Microstructural evolution studies showed that for grain sizes larger than 1 μm, anisotropy in thermal expansion coefficient and elastic constants of LSO, resulted in residual stress at grain boundaries and triple points that led to intragranular microcracking. However, reducing the grain size below 1 μm effectively avoids microcracking, leading to more favorable optical properties. The optical scattering profiles generated by a Stover scatterometer, measured by a He-Ne laser of wavelength 633 nm, showed that by reducing the grain size from 2 μm to 500 nm, the in-line transmission increased by a factor of 103. Although these values were encouraging and showed that small changes in grain size could increase transmission by almost three orders of magnitude, even smaller grain sizes need to be achieved in order to get truly transparent material with high in-line transmission. © 2012 Elsevier B.V. All rights reserved.


Roy S.,Boston University | Lingertat H.,ALEM Associates | Brecher C.,ALEM Associates | Sarin V.K.,Boston University
IEEE Transactions on Nuclear Science | Year: 2012

While polycrystalline ceramic of Ce +3 doped lutetium oxyorthosilicate (LSO) has demonstrated scintillation characteristics equivalent to those of single crystal material, it lacks in optical quality. It is projected that if their grain size could be reduced to the nanometer range they would be smaller than the wavelength of light thereby minimizing scattering and substantially improving optical quality. In this investigation ceramic LSO:Ce that is much more transparent than would be expected from a highly anisotropic material, has been successfully produced by hot pressing at 75 MPa in a graphite die and furnace. The conditions necessary for powder processing and densification were optimized so as to produce dense LSO:Ce ceramic discs with an average grain size of 700 nm. Appreciable improvement in optical properties was observed, with decay and emission levels comparable with LSO single crystals, the light output was some 20% below that of single crystal. The degradation of light output in the nanoceramic is attributed to the formation of quenching centers associated with the loss of oxygen during densification, to which such nanomaterials are highly susceptible. © 2012 IEEE.

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