Publications

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Herein we report the progress towards an organic glass scintillator with fast and thermal neutron sensitivity providing “triple” pulse shape discrimination (PSD) through the inclusion of a boron-incorporated aromatic molecule. The commercially available molecule 2-(p-tolyl)-1,3,2-dioxaborinane (TDB) can be readily synthesized in one step using inexpensive materials and incorporated into the organic glass scintillator at 20% by weight or 0.25% 10B by mass. In addition, we demonstrate that TDB can be easily scaled up and formulated into organic glass scintillator samples to produce a thermal neutron capture signal with a light yield equivalent to 120.4 ± 3.7 keVee, which is the highest value reported in the literature to date.

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The Single-Volume Scatter Camera (SVSC) approach to kinematic neutron imaging, in which an incident neutron’s direction is reconstructed via multiple neutron-proton scattering events, potentially offers much greater efficiency and portability than current systems. In our first design of an Optically-Segmented (OS) SVSC, the detector consists of an 8×8 array of 5×5×200 mm3 bars of EJ-204 scintillator wrapped in Teflon tape, optically coupled with SensL J-series 6 x 6 mm Silicon Photomultiplier (SiPM) arrays, all inside an aluminum frame that serves as a dark box. The SiPMs are read out using custom (multi-GSPS) waveform sampling electronics. In this work, construction, characterization, and electronics updates are reported. The position, time, and energy resolutions of individual bars were obtained by measuring different scintillators with different reflectors. This work was carried out in parallel at the University of Hawaii and at Sandia National Laboratories and resulted in the preliminary design of the camera. Monte-Carlo simulations using the Geant4 toolkit were carried out for individual scintillator bars, as well as the array setup. A custom analysis using ROOT libraries in C++ simulated the SiPM response from Geant4 photon hits. This analysis framework is under development and will allow for seamless comparisons between experimental and simulated data.

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In the last 20 years, biodiesel consumption in the United States has rapidly increased to ∼2 billion gallons per year as a renewable supplement to fossil fuel. However, further expansion of biodiesel use is currently limited in part by poor cold weather performance, which prevents year-round blending and necessitates blend walls ≤5% v/v. In order to provide a diesel fuel blendstock with improved cold weather performance (cloud point, pour point, and cold filter plug point), while at the same time maintaining other required fuel performance specifications, several biodiesel redox analogues were synthesized and tested. The best performing candidate fuels from this class showed improvement in the derived cetane number (29.3% shorter ignition delay), lower heating value (+4.7 MJ/kg), relative sooting tendency (-7.4 YSI/MJ), and cloud point (15 °C lower) when compared to a B100 biodiesel composed of an identical fatty acid profile. It was observed as a general trend that the reduced form of biodiesel, fatty alkyl ethers (FAEs), shows performance improvements in all fuel property metrics. The suite of improved properties provided by FAEs gives biodiesel producers the opportunity to diversify their portfolio of products derived from lipid and alcohol feedstocks to include long-chain alkyl ethers, a biodiesel alternative with particular applicability for winter weather conditions across the US.

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We demonstrated production of a superior performance biodiesel referred to here as fatty acid fusel alcohol esters (FAFE) – by reacting fusel alcohols (isobutanol, 3-methyl-1-butanol, and (S)-(-)-2-methyl-1-butanol) with oil (glyceryl trioleate) using lipase from Aspergillus oryzae. Reaction conditions corresponding to a molar ratio of 5:1 (fusel alcohols to oil), enzyme loading of 2% w/w, reaction temperature of 35 °C, shaking speed of 250 rpm, and reaction time of 24 h achieved >97% conversion to FAFE. Further, FAFE obtained from reacting a fusel alcohol mixture with corn oil were evaluated for use as a fuel for diesel engines. FAFE mixtures showed superior combustion and cold-flow properties, with the derived cetane numbers up to 4.8 points higher, cloud points up to -6 °C lower, and the heat of combustion up to 2.1% higher than the corresponding FAME samples, depending on the fusel mixture used. This represents a significant improvement for all three metrics, which are typically anti-correlated. Finally, FAFE provides a new opportunity for expanded usage of biodiesel by addressing feedstock limitations, fuel performance, and low temperature tolerance.

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The light output, time resolution, pulse shape discrimination (PSD), neutron light output, and interaction position reconstruction of melt-cast small-molecule organic glass bar scintillators were measured. The trans-stilbene organic scintillator detects fast neutrons and gamma rays with high efficiency and exhibits excellent PSD, but the manufacturing process is slow and expensive and its light output in response to neutrons is anisotropic. Small-molecule organic glass bars offer an easy-to-implement and cost-effective solution to these problems. These properties were characterized to evaluate the efficacy of constructing a compact, low-voltage neutron and gamma-ray imaging system using organic glass bars coupled to silicon photomultiplier arrays. A complete facility for melt-casting organic glass scintillators was setup at the University of Michigan. 6×6×50 mm3 glass bars were produced and the properties listed above were characterized. The first neutron image using organic glass was produced in simple backprojection.

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A series of fluorescent silyl-fluorene molecules were synthesized and studied with respect to their photophysical properties and response toward ionizing neutron and gamma-ray radiation. Optically transparent and stable organic glasses were prepared from these materials using a bulk melt-casting procedure. The prepared organic glass monoliths provided fluorescence quantum yields and radiation detection properties exceeding the highest-performing benchmark materials such as solution-grown trans-stilbene crystals. Co-melts based on blends of two different glass-forming compounds were prepared with the goal of enhancing the stability of the amorphous state. Accelerated aging experiments on co-melt mixtures ranging from 0% to 100% of each component indicated improved resistance to recrystallization in the glass blends, able to remain fully amorphous for >1 month at 60 °C. Secondary dopants comprising singlet fluorophores or iridium organometallic compounds provided further improved detection efficiency, as evaluated by light yield and neutron/gamma particle discrimination measurements. Optimized singlet and triplet doping levels were determined to be 0.05 wt % 1,4-bis(2-methylstyryl)benzene singlet fluorophore and 0.28 wt % Ir3+, respectively.

In this work we report a new class of organic-based scintillators that combines several of the desirable attributes of existing crystalline, liquid, and plastic organic scintillators. The prepared materials may be isolated in single crystalline form or melt-cast to produce highly transparent glasses that have been shown to provide high light yields of up to 16,000 photons/MeVee, as evaluated against EJ-200 plastic scintillators and solution-grown trans-stilbene crystals. The prepared organic glasses exhibit neutron/gamma pulse-shape discrimination (PSD) and are compatible with wavelength shifters to reduce optical self-absorption effects that are intrinsic to pure materials such as crystalline organics. The combination of high scintillation efficiency, PSD capabilities, and facile scale-up via melt-casting distinguishes this new class of amorphous materials from existing alternatives.