Dariusz Wisniewski

LuAlO/sub 3/:Ce and other aluminate scintillators

A new scintillator, LuAlO/sub 3/:Ce, has been found to have properties that put it in the forefront when stopping power and timing is of importance. This material, an extension of other well known cerium-doped scintillators, the yttrium-based orthoaluminate and garnet, can be readily pulled from the melt, and displays particularly promising performance. We summarize the results achieved when Y is replaced by Lu in this class of oxide crystals. >

Development of Novel Polycrystalline Ceramic Scintillators

For several decades most of the efforts to develop new scintillator materials have concentrated on high-light-yield inorganic single-crystals while polycrystalline ceramic scintillators, since their inception in the early 1980s, have received relatively little attention. Nevertheless, transparent ceramics offer a promising approach to the fabrication of relatively inexpensive scintillators via a simple mechanical compaction and annealing process that eliminates single-crystal growth. Until recently, commonly accepted concepts restricted the polycrystalline ceramic approach to materials exhibiting a cubic crystal structure. Here, we report our results on the development of two novel ceramic scintillators based on the non-cubic crystalline materials: Lu2SiO5:Ce (LSO:Ce) and LaBr3:Ce. While no evidence for texturing has been found in their ceramic microstructures, our LSO:Ce ceramics exhibit a surprisingly high level of transparency/translucency and very good scintillation characteristics. The LSO:Ce ceramic scintillation reaches a light yield level of about 86% of that of a good LSO:Ce single crystal, and its decay time is even faster than in single crystals. Research on LaBr:Ce shows that translucent ceramics of the high-light-yield rare-earth halides can also be synthesized. Our LaBr3:Ce ceramics have light yields above 42000 photons/MeV (i.e., >70%of the single-crystal light yield).

Electron traps and scintillation mechanism in LuAlO3:Ce

In this paper we report measurements of thermoluminescence in the temperature range of 20–370 K, isothermal decays, pulsed vacuum ultraviolet and γ-excited luminescence time profiles at various temperatures on cerium-activated orthoaluminate (LuAlO3:Ce, LuAP), a new and promising scintillator material. We demonstrate that results of all these experiments can be consistently explained by assuming a recombination mechanism of scintillation light production in the LuAP scintillator. Using a simple first-order kinetic model that includes Ce3+ ions as recombination centres and a number of electron traps, we extract from experimental data the basic trap parameters (energy depths and frequency factors). Consequently we identify nine traps that are responsible for undesired features of the LuAP scintillator, such as a reduced scintillation light output, a relatively long scintillation rise time and slow scintillation components (afterglow) at room temperature. We demonstrate that some of these traps are responsible for large variations of the scintillation light yield with temperature as reported earlier. Although the deepest traps do not alter scintillation time profiles, they are responsible for a significant scintillation light loss and are, therefore, detrimental to scintillation performance of the material. We observe that there is an apparent correlation between trap depths and frequency factors for at least five of the traps that may fit some more general pattern involving various groupings of all the traps. This, in turn, would indicate that traps in LuAP are not unrelated and are due, most likely, to a series of native defects in the LuAP crystal structure. Although the specific identity of traps remains unknown, the performance of the LuAP scintillator is now, in practical terms, fully understood and can be described numerically at any temperature using a model and a set of parameters given in this paper. It is clear that any major improvement of the material would require that traps are eliminated or that their influence on the scintillation process is minimized.

Exploratory Research on the Development of Novel

We report the discovery of a new family of Ce3+-activated phosphate glass scintillators that can be formed either with or without the addition of 6Li, for neutron or X-ray/gamma-ray radiation detection, respectively. Trivalent cerium can be efficiently introduced into these phosphate glasses in surprisingly high concentrations in the form of anhydrous cerium tri-chloride. Additionally, these glasses can be melted and poured at the relatively low temperatures of 1000-1050degC (i.e., substantially lower than silicate glasses), and to retain the cerium in the trivalent state it is not necessary to maintain highly reducing conditions during the synthesis process. The family of alkaline-earth-alkali phosphate glasses investigated here represents a system with two dissimilar cations - thereby offering a large range of potential compositional variations, substitutions, and combinations. In order to alter the scintillator characteristics, we have explored part of that compositional space by studying Ca-Na, Ca-Li, Ca-Cs, Ca-Rb, Ca-K and Ca-Ba-Na phosphate glasses, as well as various co-doping and post-synthesis thermal processing schemes. A series of experiments under X-ray, gamma-ray, and neutron excitations was carried out. The broad, peaking at about 354 nm, UV scintillation of these glasses is well suited for applications that use common photomultipliers with bi-alkali photo-cathodes. Pulse shape measurements show that the primary component of the scintillation in most of these glasses corresponds to 75-90% of the emitted photons, and it decays with a time constant of 30 to 40 ns, which classifies these materials as reasonably fast scintillators. Although the gamma-induced light yield of these new scintillating phosphate glasses is, thus far, only about 30% of that of commercial GS20 silicate glass, due to the generally faster scintillation, the initial amplitude of the scintillation pulse of these glasses is close to that of the above-mentioned GS20 scintillator.

{\rm Ce}^{3+}

ZnO-based scintillators are particularly well suited for use as the associated particle detector in a deuterium-tritium (D-T) neutron generator. Application requirements include the exclusion of organic materials, outstanding timing resolution, and high radiation resistance. ZnO, ZnO:Ga, ZnO:In, ZnO:In,Li, and ZnO:Er,Li have demonstrated fast (sub-nanosecond) decay times with relatively low light yields. ZnO:Ga has been used in a powder form as the associated particle detector for a D-T neutron generator. Unfortunately, detectors using powders are difficult to assemble and the light yield from powders is less than satisfactory. Single-crystal ZnO of sufficient size has only recently become available. New applications for D-T neutron generators require better timing resolution and higher count rates than are currently available with associated particle detectors using YAP:Ce as the scintillator. Recent work suggests that ZnO-based scintillators can provide alpha-particle-excited light yields comparable to YAP:Ce scintillators. ZnO-based polycrystalline ceramic scintillators offer the advantages of high light yield, ease of fabrication, low cost, and robust mechanical properties. Precursor powders used in these studies include ZnO and ZnO:Ga powders synthesized using solution-phase, urea precipitation, and combustion synthesis techniques as well as ZnO powder from a commercial vendor. Precursor powders have been sintered using uniaxial hot pressing and spark plasma sintering techniques. Photoluminescence measurements have confirmed that, for most samples, the emissions from these sintered bodies consist primarily of slow, visible emissions rather than the desired sub-nanosecond near-band-edge emissions. Subsequent hydrogen treatments have shown significant improvements in the luminescence characteristics of some ceramic bodies, while other samples have shown no change in luminescence.

-Activated Phosphate Glass Scintillators

A new scintillator material consisting of a methanol adduct of cerium trichloride with the composition CeCl3(CH3OH)4 has been discovered and crystallized in the form of large single crystals by solution growth in methanol. The peak emission of the x-ray-excited luminescence spectrum occurs at ∼364 nm. A light yield of up to ∼16 600 photons/MeV and an energy resolution of 11.4% were obtained using 662 keV gamma-ray photons. The scintillator decay time for 662 keV gamma-ray excitation was measured using the time-correlated, single-photon-counting method, and a nominal value of 64.4 ns was obtained. The molecular adduct CeCl3(CH3OH)4 represents the first example of a rare-earth, metal-organic scintillator that is applicable to gamma ray, x ray, and neutron detection.

Investigation of ZnO-Based Polycrystalline Ceramic Scintillators for Use as

A new scintillator material for the detection of fast neutrons that consists of a methanol adduct of cerium trichloride with the composition CeCl₃(CH₃OH)₄ has been characterized using 14.1 MeV neutrons from a deuterium-tritium neutron generator and fast neutrons from a bare instrumented ²⁵²Cf source. The timing resolution of the scintillator for fast neutrons was found to be 1~ ns. Neutron interactions in the CeCl₃(CH₃OH)₄ composition were simulated using the MCNP-PoliMi code. These simulations indicate that proton recoils account for most of the deposited energy in CeCl₃(CH₃OH)₄. The crystalline molecular adduct CeCl₃(CH₃OH)₄ represents a rare-earth metal-organic scintillator that can be applied to both fast neutron and gamma-ray detection.

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We have investigated the applicability of phosphate glasses as host systems for the formation of rare-earth-activated gamma- and x-ray scintillators. Glass scintillators have generally suffered from low light yields, usually attributed to inefficient energy transfer from the glass matrix to the luminescent center. Our research on these phosphate glasses has shown that their structural properties can be readily varied and controlled by compositional alterations. The melting and pouring temperature of ~1050°C for these phosphate glasses is significantly lower than the processing temperatures generally associated with the formation of silicate glass scintillators. The calcium-sodium phosphate glasses will tolerate relatively high cerium concentrations based on the initial melt compositions, and the light yield for gamma-ray excitation at 662 keV was determined as a function of cerium concentration up to the saturation level. The rare-earth-activated Ca-Na phosphate glass primary-component decay time was in the range of 32 to 42 nsec for various Ce concentrations with the contribution of the light output of the primary component ranging from 80 to 90%. Studies of the effects of co-doing with both Ce and Gd were also carried out in the case of the Ca-Na phosphate glass hosts. The effects of post-synthesis thermochemical treatments in a variety of atmospheres and at various processing temperatures were also investigated for the Ce-activated Ca-Na phosphate scintillators.

-Particle Detectors

Cerium activated rare-earth tri- halides represent a well-known family of high performance inorganic rare-earth scintillators - including the high-light-yield, high-energy-resolution scintillator, cerium-doped lanthanum tribromide. These hygroscopic inorganic rare-earth halides are currently grown as single crystals from the melt - either by the Bridgman or Czochralski techniques - slow and expensive processes that are frequently characterized by severe cracking of the material due to anisotropic thermal stresses and cleavage effects. We have recently discovered a new family of cerium-activated rare-earth metal organic scintillators consisting of tri-halide methanol adducts of cerium and lanthanum - namely CeCl3(CH3OH)4 and LaBr3(CH3OH)4:Ce. These methanol-adduct scintillator materials can be grown near room temperature from a methanol solution, and their high solubility is consistent with the application of the rapid solution growth methods that are currently used to grow very large single crystals of potassium dihydrogen phosphate. The structures of these new rare-earth metal-organic scintillating compounds were determined by single crystal x-ray refinements, and their scintillation response to both gamma rays and neutrons, as presented here, was characterized using different excitation sources. Tri-halide methanol-adduct crystals activated with trivalent cerium apparently represent the initial example of a solution-grown rare-earth metal-organic molecular scintillator that is applicable to gamma ray, x-ray, and fast neutron detection.

Single-crystal CeCl3(CH3OH)4: A new metal-organic cerium chloride methanol adduct for scintillator applications

Ceramic materials show significant promise for the production of reasonably priced, large-size scintillators. Ceramics have recently received a great deal of attention in the field of materials for laser applications, and the technology for fabricating high-optical-quality polycrystalline ceramics of cubic materials has been well developed. The formation of transparent ceramics of non-cubic materials is, however, much more difficult as a result of birefringence effects in differently oriented grains. Here, we will describe the performance of a few new ceramics developed for the detection of gamma- and x-ray radiation. Results are presented for ceramic analogs of three crystalline materials - cubic Lu2O3, and non-cubic LaBr3, and Lu2SiO5 or LSO (hexagonal, and monoclinic structures, respectively). The impact of various sintering, hot-pressing and post-formation annealing procedures on the light yield, transparency, and other parameters, will be discussed. The study of LaBr3:Ce shows that fairly translucent ceramics of rare-earth halides can be fabricated and they can reach relatively high light yield values. Despite the fact that no evidence for texturing has been found in our LSO:Ce ceramic microstructures, the material demonstrates a surprisingly high level of translucency or transparency. While the scintillation of LSO:Ce ceramic reaches a light yield level of about 86 % of that of a good LSO:Ce single crystal, its decay time is even faster, and the long term afterglow is lower than in LSO single crystals.