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Antibody-dependent enhancement (ADE) has been shown previously for SARS-CoV-1, MERS-CoV, and SARS-CoV-2 infection in vitro. In this study, the first monoclonal antibody (mAb) that causes ADE in a SARS-CoV-2 in vivo model was identified. mAb RS2 against the SARS-CoV-2 S-protein was developed using hybridoma technology. mAb RS2 demonstrated sub-nanomolar affinity and ability to neutralize SARS-CoV-2 infection in vitro with IC50 360 ng/mL. In an animal model of SARS-CoV-2 infection, the dose-dependent protective efficacy of mAb RS2 was revealed. However, in post-exposure prophylaxis, the administration of mAb RS2 led to an increase in the viral load in the respiratory tract of animals. Three groups of blood plasma were examined for antibodies competing with mAb RS2: (1) plasmas from vaccinated donors without COVID-19; (2) plasmas from volunteers with mild symptoms of COVID-19; (3) plasmas from patients with severe COVID-19. It was demonstrated that antibodies competing with mAb RS2 were significantly more often recorded in sera from volunteers with severe COVID-19. The results demonstrated for the first time that in animals, SARS-CoV-2 can induce antibody/antibodies that can elicit ADE. Moreover, in the sera of patients with severe COVID-19, there are antibodies competing for the binding of an epitope that is recognized by the ADE-eliciting mAb.

We have experimentally characterized the light-output response of a deuterated trans-stilbene (stilbene-d12 ) crystal to quasi-monoenergetic neutrons in the 0.8 to 4.4 MeV energy range. These data allowed us to perform neutron spectroscopy measurements of a DT 14.1 MeV source and a 239PuBe source by unfolding the impinging neutron spectrum from the measured light-output response. The stilbene-d12 outperforms a 1 H − stilbene of similar size when comparing the shape of the unfolded spectra and the reference ones. These results confirm the viability of non-hygroscopic stilbene-d12 crystal for direct neutron spectroscopy without need for time-of-flight measurements. This capability makes stilbene-d12 a well suited detector for fast-neutron spectroscopy in many applications including nuclear reaction studies, radiation protection, nuclear non-proliferation, and space travel.

L’article traite des problèmes qui se posent à l’École supérieure en Russie lors de la formation des spécialistes et propose une nouvelle conception de l’enseignement des langues (ici on parle du français) dont l’aspect principal est la construction du thésaurus professionnel éducatif en langue cible. Cette méthode aide les étudiants, nos futurs professeurs, à assimiler plus facilement le système de concepts professionnels, à mieux maîtriser les techniques et les méthodes éducatives, à lancer et à partager des initiatives de recherche, à répértorier des ressources fiables, et à développer ainsi leur créativité (ici on parle de l’experience des professeurs et des étudiants de l’Université pédagogique Herzen de Russie, St. Pétersbourg). Pour illustrer cette méthode et vous faire partager nos pratiques quotidiennes de terrain, on propose également des fragments d’un microthésaurus et d’un dictionnaire russe-français, faits par nos étudiants pendant les cours de français.

We have developed and evaluated two prototype fast neutron deuterated-stilbene scintillators. The largest scintillator is hexagonal in shape with volume 32 cm3, comprised of deuterated crystalline trans-stilbene (stilbene-d12). These samples were grown using a solution-based crystal growing method developed at Lawrence Livermore Laboratory to grow conventional 1H-based stilbene scintillator crystals. The stilbene-d12 prototype detectors show excellent neutron/gamma discrimination, noticeably better than 1H-stilbene, and appear to have less asymmetry in the response to neutrons incident from different directions. Like other deuterated scintillators, the stilbene-d12 detectors in many applications can provide incident neutron energy spectra, without time of flight, by suitably unfolding the neutron light-output spectra.

Radiation detection requires instruments which can detect nuclear radiation, having at the same time the ability to discriminate among different types of nuclear particles. For many decades, neutron detection has been based on 3He proportional counters sensitive primarily to thermal neutrons. The most common methods for the detection of fast neutrons have been based on liquid scintillators with pulse shape discrimination (PSD). A decade of studies conducted at Lawrence Livermore National Laboratory (LLNL) led to the development of new materials, among which are the first PSD plastics for fast neutron detection manufactured based on high concentrations of 2,5-diphenyloxazole (PPO) dissolved in aromatic polymers. More advantages were introduced by plastics doped with 10B and 6Li that enable combined detection of both thermal and fast neutrons, offering, in addition, a unique “triple” PSD for discrimination between fast neutrons, thermal neutrons, and gamma rays. Studies conducted with these and more recently introduced materials prepared with multiple dyes alternative to PPO produced a deeper understanding of energy transfer and excited state interaction phenomena. Research led to the commercial production of PSD plastics which, due to the deployment advantages and ease of fabrication, create a basis for the replacement of liquid scintillators and widespread use of solid-state scintillators as large-volume and low-cost neutron detectors.

The paper reports results of studies conducted to improve scintillation performance of plastic scintillators capable of neutron/gamma pulse-shape discrimination (PSD). Compositional modifications made with the polymer matrix improved physical stability, allowing for increased loads of the primary dye that, in combination with selected secondary dyes, provided enhanced PSD especially important for the lower energy ranges. Additional measurements were made with a newly-introduced PSD plastic EJ-276, that replaces the first commercially produced EJ-299. Comparative studies conducted with the new materials and EJ-309 liquids at large scale (up to 10 cm) show that current plastics may provide scintillation and PSD performance sufficient for the replacement of liquid scintillators. Comparison to stilbene single crystals compliments the information about the status of the solid-state materials recently developed for fast neutron detection applications.

There has been increased interest in organic semiconductors over the last decade because of their unique properties. Of these, 5, 6, 11, 12-tetraphenylnaphthacene (rubrene) has generated the most interest because of its high charge carrier mobility. In this work, large single crystals with a volume of ~1 cm3 were grown from solution by a temperature reduction technique. The faceted crystals had flat surfaces and cm-scale, visually defect-free areas suitable for physical characterization. X-ray diffraction analysis indicates that solvent does not incorporate into the crystals and photoluminescence spectra are consistent with pristine, high-crystallinity rubrene. Furthermore, the response curve to pulsed optical illumination indicates that the solution grown crystals are of similar quality to those grown by physical vapor transport, albeit larger. The good quality of these crystals in combination with the improvement of electrical contacts by application of conductive polymer on the graphite electrodes have led to the clear observation of alpha particles with these rubrene detectors. Preliminary results with a 252Cf source generate a small signal with the rubrene detector and may demonstrate that rubrene can also be used for detecting high-energy neutrons.

Radiation Portal Monitors (RPMs) based on polyvinyl toluene (PVT) scintillators have been in use since the 1970s. Recently it has been discovered that under certain conditions PVT scintillators can fog at low temperatures. In collaboration with researchers at national laboratories, Eljen Technology has developed methods for the production of Intrinsic Anti-Fogging Scintillators. The production methods have been scaled up to allow for castings of rods and large sheets of scintillators that are deployable in RPMs and other extreme locations. The work is ongoing. The presentation will focus on the properties of the new scintillators and on the ongoing testing results from exposure to extreme environmental conditions.

The development of efficient, robust plastic scintillators with pulse shape discrimination (PSD) has been of much interest in recent years. As is the case with many new technologies, initial formulations for PSD plastics provided new capabilities that have required in-depth research to refine. Herein we describe continued results from extensive optimization studies that have led to the development of PSD plastics with improved scintillation performance and physical properties. Improvements in the formulation for PSD plastics provide a groundwork from which element-loaded PSD plastics can be further advanced. Two separate approaches for incorporating lithium compounds into plastic scintillators are explored based on the identity of the lithium salt used. Aliphatic and aromatic lithium compounds require different methods of incorporation, resulting in different mechanisms of light quenching and reduction in PSD. These quenching mechanisms are explored and discussed for each type of lithium salt. Since at large size physical and scintillation characteristics are much more sensitive to preparation conditions, performance characterization of scaled plastics is described and compared among different formulations. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

A solution-based technique for growing large-volume stilbene scintillators was developed in 2013; crystals up to diameters of 10 cm, or larger, have been grown while preserving excellent pulse shape discrimination (PSD) properties. The goal of this study is to evaluate the PSD capabilities of 5.08 by 5.08-cm stilbene crystals grown by Lawrence Livermore National Laboratory and Inrad Optics when exposed to a 1000 to 1 gamma ray-neutron ratio and operating at a 100-kHz count rate. Results were compared to an equivalent EJ-309 liquid scintillation detector. 252 Cf neutron pulses were recorded in two experiments where 60 Co and 137 Cs sources created the high-gamma field. The high count rate created numerous double pulses that were cleaned using fractional and template approaches designed to remove double pulses while preserving neutron counts. PSD was performed at a threshold of 42 keVee (440-keV proton) for stilbene and 60 keVee (610-keV proton) for EJ-309 liquid. The lower threshold in stilbene resulted in a neutron intrinsic efficiency of approximately 14.5%, 10% higher than EJ-309 liquid, for bare 252 Cf and 13% for 252 Cf in the high-gamma field. Despite the lower threshold, the gamma misclassification rate in stilbene was approximately 3×10 −6 , nearly a factor-of-five lower than what we found with the EJ-309 liquid.

Transparent plastic scintillators with pulse shape discrimination containing 6Li salicylate have been synthesized by bulk polymerization with a maximum 6Li loading of 0.40 wt%. Photoluminescence and scintillation responses to gamma-rays and neutrons are reported herein. Plastics containing 6Li salicylate exhibit higher light yields and permit a higher loading of 6Li as compared to previously reported plastics based on lithium 3-phenylsalicylate. However, pulse shape discrimination performance is reduced in lithium salicylate plastics due to the requirement of adding more nonaromatic monomers to the polymer matrix as compared to those based on lithium 3-phenylsalicylate. Reduction in light yield and pulse shape discrimination performance in lithium-loaded plastics as compared to pulse shape discrimination plastics without lithium is interpreted in terms of energy transfer interference by the aromatic lithium salts.

Organic single crystals (OSCs) have ideal qualities (well defined structure and morphology, lack of grain boundaries, high purity, 3D long range order, good electronic transport properties) for several technological applications, in particular as key components for electronic devices. It is only recently that OSCs have been considered as ionizing radiation detectors, and the latest developments in this field are here reported. In the first section, various methods for OSC growth are described, with emphasis on cost-effective, solution-based approaches capable of delivering large volume, well performing crystals. The second section is focused on the use of solution-grown OSCs as scintillators (i.e., as high energy photon to UV-vis photon conversion), highlighting the ability of cm-scale OSCs to effectively detect neutrons and to carry out neutrons-gamma pulse-shape discrimination tasks. Finally, the third section describes the use of semiconducting, solution-grown OSCs as effective solid state direct detectors (i.e., directly converting high energy photons into charge carriers), evidencing extremely promising performances in terms of operability in environmental conditions (i.e., no need for encapsulation), radiation hardness, linear response and low operating voltage. The latest developments in the application of organic single crystals (OSCs) as ionizing radiation detectors, both as scintillators and as solid state direct detectors, are reviewed. Properties of OSCs relevant to this task are discussed together with the most effective and convenient methods for their growth.

Single crystals of trans -stilbene, C 14 H 12, with properties suitable for high-energy neutron detection were grown from solution in anisole and toluene by the temperature reduction method with growth rates up to 6 mm/day. From these crystals, slices of appropriate orientation and thickness of 2–4 mm were cut and studied by X-ray diffraction topography applying the Lang method using CuKα radiation. The topographs exhibit growth defects such as liquid inclusions, dislocations, striations, and faulty growth-sector boundaries. These defects occur in the same typical arrangements and geometries as is observed in all kinds of crystals grown on habit faces from solution. Besides growth dislocations originating from inclusions and propagating with the growth front, many plastic glide dislocations in the shape of loops or half-loops emitted from inclusions by stress relaxation are observed. The glide system underlying this plasticity is discussed.

The scintillation process in plastic scintillators has been studied with mixed systems containing high concentrations of multiple fluorescence dyes. It has been shown that the triplet–triplet interaction phenomena leading to the production of delayed light and pulse shape discrimination (PSD) can vary depending on the triplet energies of the interacting dyes. At small differences between the first excited triplet state energies, found to be below 0.1 eV, a pair of interacting dyes of different molecular species can be involved in heterogeneous excitation migration and triplet–triplet annihilation that result in the enhancement of delayed light and PSD, due to the engagement of both dyes in the process. At differences above ∼ 0.27 eV, excitation trapping on the lower energy triplet molecules leads to conditions when triplet–triplet migration and annihilation can proceed only homogeneously between molecules of the same species. The results explain the different effects produced by the selection of fluorescent dyes in previously reported PSD systems containing one primary and one secondary dyes. They also demonstrate a non-traditional approach to the design of new plastic scintillators with multiple dyes that, at improved scintillation and PSD performance, may simplify the production process.

To provide efficient and reproducible response to nuclear radiation, scintillating plastics should be prepared as materials of high optical transparency and homogeneity that should remain intact during the lifetime of their operation. A problem with traditionally used polyvinyltoluene (PVT) or polystyrene (PS) plastic scintillators arises from their exposure to repeated large temperature swings that cause surface and volume defects, resulting in decreased sensitivity of the detection properties. The problem can be avoided by composition modifications that enable preparation of plastics with properties that are resistant to the continuous variations in environmental conditions. This paper describes improvements in physical properties that can be achieved by the addition of crosslinkers (such as divinylbenzene, DVB) and oxygen-containing compounds or copolymers (e.g. polymethylamethacrylate, PMMA) to the traditional PVT or PS polymer matrices. Results of characterization show that the new materials can be prepared with the scintillation performance equal to that of commercial plastic scintillators, such as EJ-200.

Polyvinyl toluene (PVT) scintillator plastic may degrade in the field due to inward water diffusion at elevated temperatures, which exceeds the saturation limit at lower temperatures, and after long periods of time (e.g., many years), leads to the formation of disk-like defects that attenuate scintillation light, leading to detector degradation. Using fractography and high-magnification optical and electron microscopy to characterize the water-induced defects, a model of the fogging process is hypothesized as follows: excess water present at low temperatures diffuses to spheroids to minimize contact with the hydrophobic polymer. Through the hydrophobic effect, the entropy of water increases by forming nanoclusters which minimizes the contact between the water and the PVT. As the water nanoclusters grow, they break and fold the polymer into densely-packed crystalline regions creating more space for water within the spheroids. The polymer outside the spheroid resists the shrinkage which builds up tension within the spheroid. Once the tensile stress exceeds the yield strength of the plastic, the spheroid is torn in half resulting in a defect. Excess water then drains into the cavities along the disk thereby further increasing its entropy. Slower cooling (over 1 day) leads to larger spheroids and hence, larger “permanent” defects. Freezing causes some defects to further grow due to the expansion of water to ice. These findings imply that the remaining lifetime of scintillator plastic in the field could be predicted using temperature and humidity data thereby mitigating security risks of degraded radiation portal monitors.

We have developed a science-based predictive model for fogging of the scintillator plastics deployed in radiation portal monitors. The only input required is the weather conditions [i.e., daily high and low, for temperature and relative humidity (R.H.)]. The code utilizes this information, together with a thermodynamic model of the water content in equilibrium with the plastic surface, a diffusion model of the water transport into the interior of scintillator plastic panel, and a comparison of the resulting paired “water-content/night-temperature” points as a function of position in the panel against an empirical “fog-line” that establishes a water/temperature criterion for the onset of fogging. We have analyzed the behavior of several representative portal monitor sites, and the output of the model analysis is reasonably consistent with observation.

Polyvinyl toluene (PVT) plastic is often used to make gamma ray sensitive scintillators and is used in many important applications. For example, PVT is used to scan recycled steel going into a plant to be processed. It is also used in personnel portals to scan employees, and is used at the borders of many countries to scan cargo and cars passing through. In recent years, it was discovered that PVT can become degraded when it absorbs water and then is subjected to daily temperature swings, as is evidence by a reduction in scintillation light. Water absorption by PVT was notionally associated with temporary fogging (i.e., a rapid decrease in opacity which was reversible over time) and permanent fogging (i.e., permanent crack-like defects) and was for some time suspected to be the leading cause of the fogging due to circumstantial evidence. However, definitive proof was not established and in particular the specific mechanisms by which water entered the plastic and ultimately created temporary and permanent point-like defects was not fully understood. This paper is an overview of an effort initiated to reveal the fundamental root cause and also the dynamics of the fogging and degradation processes. This understanding is important not only to solve degradation problems for PVT in fielded systems, but also (1) to provide direction for future procurements for new equipment, and (2) to provide direction for future R&D efforts into advanced plastics with enhanced spectroscopic capabilities or dual particle gamma/neutron capabilities. Unexpected outcomes of this investigation included (1) the capability to predict the onset of fogging based on models using weather data alone, and (2) new modified formulations of PVT and PS which are fog-resistant. This paper gives an overview of the water uptake and associated mechanisms for various compositions, characterization of resulting temporary and permanent defects, predictive modeling of fogging, and possible solutions including encapsulation, heaters and new formulations. While this paper is an overview of the root cause analysis and solution, five accompanying papers (Lance et al., 2018, Myllenbeck and Feng, 2018, Payne et al., 2018, Zaitseva et al. 2018, Kouzes et al., 2018) from the same conference (SORMA 2018) delve further into the details and also expand upon the scope of what will be reported herein.

We compare the pulse-shape-discrimination performance of a large Stilbene detector, manufactured at the Lawrence Livermore National Laboratory, to a similarly sized liquid scintillator, made by Eljen Technologies, Inc. (type EJ309), under various high-count-rate scenarios, such as can be expected during Active Interrogation of objects using an electric neutron generator for detecting the presence of Special Nuclear Materials and contraband. Pulse Shape Discrimination (PSD) is performed with an extension of previously described wavelet and event fitting algorithms, which are also used to reject pileup events and to correct the energy calibration for event signals clipped during digitization. We find that strong PSD and pileup rejection cuts are required to function in this high-count-rate environment. Whereas the Stilbene detector has intrinsically better pulse-shape discrimination performance, the pulses of events from the liquid scintillator are somewhat narrower, reducing pileup. We use an event fitting cut that provides simultaneous PSD identification and pileup rejection. When applying the same cut to data taken with both detectors, we find that Stilbene has higher event use efficiency by about a factor of two and retains a better gamma rejection ratio compared to EJ309.

Detection of special nuclear materials (SNM) requires instruments that can detect and characterize uranium and plutonium isotopes, having at the same time the ability to discriminate among different types of radiation. For many decades, neutron detection has been based on 3He proportional counters sensitive primarily to thermal neutrons. The most common methods for detection of fast neutrons have been based on liquid scintillators with pulse shape discrimination (PSD). The shortage of 3He and handling issues with liquid scintillators stimulated a search for efficient solid-state PSD materials. Recent studies conducted at LLNL led to development of new materials, among which are organic crystals with excellent PSD and first PSD plastics for fast neutron detection. More advantages are introduced by plastics doped with neutron capture agents, such as 10B and 6Li, that can be used without moderation for combined detection of both thermal and fast neutrons, offering, in addition, a unique “triple” PSD for signal separation between fast neutrons, thermal neutrons, and gamma-rays. More recent studies have been focused on development of deuterated scintillators that can be used for neutron spectroscopy without time-of-flight (ToF). Among commercially produced materials are large-scale (>10 cm) stilbene crystals grown by the inexpensive solution technique, and different types of PSD plastics that, due to the deployment advantages and ease of fabrication, create a basis for the widespread use of solid-state scintillators as large-volume and low-cost neutron detectors.

Pulse shape discrimination in plastic scintillators has been of much interest in recent years. As with many innovative technologies, initial formulations for PSD plastics provided new capabilities that required much in-depth research to fully develop and refine. Herein we describe results from extensive optimization studies which have led to the development of PSD plastics with markedly improved scintillation performance and physical properties. Results of exploring different plastic matrices as well as a variety of secondary dyes are reported and optimum components are described. Due to the large concentration of additives required to manifest optimal PSD properties in plastic scintillators, the physical stability can be limited and the mechanical properties of PSD plastics are inferior to standard engineering plastics. Practical and theoretical solutions have been developed to address the physical stability and mechanical deformation problems in PSD plastics, and this work has resulted in physically stable scintillators with robust mechanical properties. Performance deterioration on increasing the size of PSD plastics is also addressed. At large sizes, physical and performance characteristics are much more sensitive to preparation conditions and compositional alterations as compared with small scintillators, and efforts to improve these properties are described. Finally, efforts to incorporate aromatic lithium compounds into PSD plastics are summarized and the effects of the lithium compounds on scintillation, stability, and attenuation are discussed.

We analyzed the frequency with which intraligand contacts occurred in a set of 1300 protein-ligand complexes [ Plewczynski et al. J. Comput. Chem. 2011 , 32 , 742 - 755 .]. Our analysis showed that flexible ligands often form intraligand hydrophobic contacts, while intraligand hydrogen bonds are rare. The test set was also thoroughly investigated and classified. We suggest a universal method for enhancement of a scoring function based on a potential of mean force (PMF-based score) by adding a term accounting for intraligand interactions. The method was implemented via in-house developed program, utilizing an Algo_score scoring function [ Ramensky et al. Proteins: Struct., Funct., Genet. 2007 , 69 , 349 - 357 .] based on the Tarasov-Muryshev PMF [ Muryshev et al. J. Comput.-Aided Mol. Des. 2003 , 17 , 597 - 605 .]. The enhancement of the scoring function was shown to significantly improve the docking and scoring quality for flexible ligands in the test set of 1300 protein-ligand complexes [ Plewczynski et al. J. Comput. Chem. 2011 , 32 , 742 - 755 .]. We then investigated the correlation of the docking results with two parameters of intraligand interactions estimation. These parameters are the weight of intraligand interactions and the minimum number of bonds between the ligand atoms required to take their interaction into account.

The scintillation properties of trans -stilbene crystals grown for the first time by application of the solution growth technique to the scale of 10 cm are reported. Measurements of the scintillation light output, pulse shape discrimination, and neutron detection efficiency were made with sets of crystals cut as 50 cm diameter cylinders of different lengths from 0.3 to 10 cm. Comparison to liquid scintillators and traditional melt-grown stilbene showed that at increasing sizes new solution-grown crystals exhibit better scintillation performance that makes them promising for use in large scale neutron detectors. Results are discussed in relation to structural imperfections attributed to different methods of growth.

Recently it has been demonstrated that plastic scintillators have the ability to distinguish neutrons from gamma rays by way of pulse shape discrimination (PSD). This discovery has lead to new materials and new capabilities. Here we report our work with the effects of aromatic, non-aromatic, and mixed aromatic/non-aromatic matrices have on the performance of PSD plastic scintillators.

Development of the plastic scintillator with neutron sensitivity from thermal to multi-MeV and pulse shape discrimination (PSD) has been demonstrated. Incorporation of 10B-containing compounds into the plastic scintillator with PSD capability leads to detector improvement in regard to neutron detection efficiency while preserving the discrimination between neutrons and γ-rays. Effects of boron loading on scintillation and pulse shape discrimination properties are discussed. A PSD figure-of-merit value of 1.4±0.03 has been achieved for events in a thermal neutron energy domain, 50–100 keVee, for PSD plastic loaded with 5 wt.% of m-carborane.

The detection of neutrons in the presence of gamma-ray fields has important applications in the fields of nuclear physics, homeland security, and medical imaging. Organic scintillators provide several attractive qualities as neutron detection materials including low cost, fast response times, ease of scaling, and the ability to implement pulse shape discrimination (PSD) to discriminate between neutrons and gamma-rays. This talk will focus on amorphous organic scintillators both in plastic form and small-molecule organic glass form. The first section of this talk will describe recent advances and improvements in the performance of PSD-capable plastic scintillators. The primary advances described in regard to modification of the polymer matrix, evaluation of new scintillating dyes, improved fabrication conditions, and implementation of additives which impart superior performance and mechanical properties to PSD-capable plastics as compared to commercially-available plastics and performance comparable to PSD-capable liquids. The second section of this talk will focus on a class of small-molecule organic scintillators based on modified indoles and oligophenylenes which form amorphous glasses as PSD-capable neutron scintillation materials. Though indoles and oligophenylenes have been known for many decades, their PSD properties have not been investigated and their scintillation properties only scantily investigated. Well-developed synthetic methodologies have permitted the synthesis of a library of structural analogs of these compounds as well as the investigation of their scintillation properties. The emission wavelengths of many indoles are in the sensitive region of common photomultiplier tubes, making them appropriate to be used as scintillators in either pure or doped form. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. This work has been supported by the U.S. Department of Energy Office of Nonproliferation Research and Development (NA-22) and by the Defense Threat Reduction Agency (DTRA).

6Li-containing organic scintillators have been prepared and characterized as a new type of transparent, single-phase materials with pulse shape discrimination (PSD) properties for simultaneous detection of thermal and fast neutrons discriminated from gamma radiation. Tests conducted with recently developed PSD-capable plastic scintillators showed that incorporation of 6Li into the aromatic matrix with fast-neutron/gamma discrimination properties offers the additional sensitivity to thermal neutrons, substantially increasing efficiency and the energy range of neutron detection. Comparative analyses of 6Li-loaded plastic, liquid and single crystal organic scintillators provide evidence that, in addition to neutron/gamma discrimination, these novel materials have the ability for discrimination between the signatures of fast and thermal neutrons.

Large, 10 cm-size, single crystals of trans- stilbene were grown for the first time from solution by a temperature reduction technique. Their scintillation performance for fast neutron detection depended on the purity of the initial starting material. A light-yield-inhibitor-free starting material was specially synthesized and crystals from this material were compared to those grown from commercially available powders of trans -stilbene. The use of pure material made possible the growth of the large size crystals (>400 g) with excellent neutron–gamma discrimination performance, which are needed for use as a radiation detection material.

A possibility of manufacturing plastic scintillators with efficient neutron/gamma pulse shape discrimination (PSD) is demonstrated using a system of a polyvinyltoluene (PVT) polymer matrix loaded with a scintillating dye, 2,5-diphenyloxazole (PPO). Similarities and differences of conditions leading to the rise of PSD in liquid and solid organic scintillators are discussed based on the classical model of excited state interaction and delayed light formation. First characterization results are presented to show that PSD in plastic scintillators can be of the similar magnitude or even higher than in standard commercial liquid scintillators.

Neutron/gamma pulse shape discrimination (PSD) utilized for detection of high-energy neutrons with organic scintillators was investigated using a model system of mixed diphenylacetylene-stilbene single crystals of different compositions. The results of the studies, which include experimental tools of crystal growth and characterization combined with computer simulation, showed that the presence of impurities with lower bandgap energies can be a major factor influencing PSD properties of organic materials. Depending on the concentration, an impurity may suppress or increase the rate of excited triplet state interaction leading, respectively, to a complete disappearance or enhancement of PSD, consistent with a percolation threshold. The results are applied to produce novel materials with controlled decay characteristics. Single crystals with a large fraction of delayed light and enhanced PSD have been grown for high energy neutron detection, while crystals with suppressed delayed light were produced for use as low-afterglow scintillators for energetic neutron detection in time-of-flight experiments.

Single crystals of a pure hydrocarbon, 1,3,5-triphenylbenzene, with properties suitable for high-energy neutron detection were grown from toluene solutions using slow evaporation and temperature reduction methods with growth rates up to 10 mm/day. Application of the rapid growth technique developed earlier for growth of large water-soluble crystals shows that crystals of aromatic compounds can be successfully grown from solutions in large volumes required for their use as scintillator materials for radiation detection.

Optical quality lithium salicylate (LiSal) single crystals have been grown from water and methanol solutions for thermal neutron detection studies. The results of structural studies showed that LiSal crystallizes from water as a stable modification different from the monohydrate LiSal·H2O reported previously. Growth from methanol results in the formation of unstable crystals of LiSal·MeOH that quickly degrade in air due to the loss of methanol. Initial radiation detection tests showed a good neutron/gamma separation in water-grown modification making LiSal in its single crystal form a potential material for thermal neutron detection.

Efficient, readily-available, low-cost, high-energy neutron detectors can play a central role in detecting illicit nuclear weapons since neutrons are a strong indication for the presence of fissile material such as Plutonium and Highly-Enriched Uranium. The main challenge in detecting fast neutrons consists in the discrimination of the signal from the gamma radiation background. At present, the only well-investigated organic crystal scintillator for fast neutron detection, in a n/gamma mixed field, is stilbene, which while offering good pulse shape discrimination, is not widely used because of its limited availability and high cost.

The application of static high pressure provides a means to precisely control and investigate many fundamental and unique properties of nanoparticles. CdSe is a model quantum-dot system, the behavior of which under high pressure has been extensively studied; however, the effect of nonuniform stresses on this system has not been fully appreciated. Photoluminescence data obtained from CdSe quantum-dot solids in different stress environments varying from purely uniform to highly nonuniform are presented. Small deviations from a uniform stress distribution profoundly affect the electronic properties of this system. In nonuniform stress environments, a pronounced flattening of the photoluminescence enegy is observed above 3 GPa. The observations are validated with theoretical calculations obtained using an all-atom semiempirical pseudopotential technique. This effect must be considered when investigating other potentially pressure-mediated phenomena.

Si nanocrystals 1−10 nm in size highly resistant to oxidation were prepared by thermal (680 °C) or gold-induced (450−600 °C) decomposition of tetramethylsilane and tetraethylsilane using trioctylamine as an initial solvent. Transmission electron microscopy analysis of samples obtained in the presence of gold showed that Si nanocrystals form via solid-phase epitaxial attachment of Si to the gold crystal lattice. The results of computational modeling performed using first principles density functional theory calculations show that the enhanced stability of nanocrystals to oxidation is due to the presence of N or N-containing groups on the surface of nanocrystals.

We present a novel notion of binding site local similarity based on the analysis of complete protein environments of ligand fragments. Comparison of a query protein binding site (target) against the 3D structure of another protein (analog) in complex with a ligand enables ligand fragments from the analog complex to be transferred to positions in the target site, so that the complete protein environments of the fragment and its image are similar. The revealed environments are similarity regions and the fragments transferred to the target site are considered as binding patterns. The set of such binding patterns derived from a database of analog complexes forms a cloud-like structure (fragment cloud), which is a powerful tool for computational drug design. It has been shown on independent test sets that the combined use of a traditional energy-based score together with the cloud-based score responsible for the quality of embedding of a ligand into the fragment cloud improves the self-docking and screening results dramatically. The usage of a fragment cloud as a source of positioned molecular fragments fitting the binding protein environment has been validated by reproduction of experimental ligand optimization results. Proteins 2007. © 2007 Wiley-Liss, Inc.

Ge nanoparticles with sizes from 1 to 15 nm were synthesized by thermal decomposition of the germane precursors trichlorogermane, tetraethylgermane, and tetrabutylgermane. Experiments were performed in the organic solvents trioctylamine, squalene, and octacosane, which have boiling points in the temperature range from 380 to 429 °C. The use of high-boiling-point solvents extended the temperature range of the reactions, thus enabling the experiments to be conducted with organogermane precursors which have high decomposition points above 400 °C. Characterization by X-ray diffraction and transmission electron microscopy showed that, depending on the reaction temperature, the final decomposition products consist of amorphous Ge nanoparticles, nanocrystals, or nanowires that grow only in the 〈110〉 direction.