Thermal neutron cross sections of amino acids from average contributions of functional groups

Development of a multi-layer high-efficiency GEM-based neutron detector for spallation sources

Neutron detection is nowadays mostly based on 3He gas detectors, but its shortage and the continuous upgrades of the neutron facilities require new devices to perform experiments with maximum performances. This work presents a new detector based on the Gas Electron Multiplier (GEM) combined with several boron layers. This detector combines the features of GEM technology with the properties of boron as a neutron converter and the device is produced to sustain high neutron fluxes with high detection efficiency. The detector has been characterised at the ISIS Pulsed Neutron and Muon Source (UK). Based on the analysis of our results, the detector has shown a good response to thermal and epithermal neutrons reaching a detection efficiency of 16% at 1.8 Å (25 meV). The good detection efficiency (even increasable with the addition of further boron GEM foils) and the good time resolution, make the detector a unique device for the neutron techniques. In particular, its use can easily be envisaged in techniques involving neutron transmission measurements, that require high fluxes impinging on the detectors, with the added bonus of a 2D-resolved capability due to the padded anode.

BNPLA: borated plastic for 3D-printing of thermal and cold neutron shielding

3D printing technologies such as fused filament fabrication (FFF) offer great opportunities to enable the fabrication of complex geometries without access to a workshop or knowledge of machining. By adding filler materials to the raw filaments used for FFF, the material properties of the plastic can be adapted. With the addition of neutron absorbing particles, filaments can be created that enable 3D printing of neutron shielding with arbitrary geometry. Two materials for FFF are presented with different mixing ratios of hexagonal Boron nitride (h-BN) and Polylactic acid (PLA). BNPLA25 with 25 %wt h-BN and BNPLA35 with 35 %wt h-BN are compared to the commercially available Addbor N25 material. To qualify the applicability of BNPLA25 and BNPLA35 as shielding material for neutron instrumentation, such as neutron imaging, we investigated the overall neutron attenuation, the influence of non-optimized print settings, as well as characterized the incoherent neutron scattering and the microstructure using neutron imaging, and time-of-flight small-angle-neutron-scattering. Finally, the tensile strength of the material was determined in standardized tensile tests. The measured neutron attenuation shows excellent agreement with analytical calculations, thus validating both the material composition and the calculation method. Approximately 6 mm (8 mm) BNPLA35 are needed for 1 × 10 - 3 transmission of a cold (thermal) neutron beam. Lack of extrusion due to suboptimal print settings can be compensated by increased thickness, clearly visible defects can be mitigated by 11-18% increase in thickness. Incoherent scattering is shown to be strongly reduced compared to pure PLA. The tensile strength of the material is shown not to be impacted by the h-BN filler. The good agreement between the measured attenuation and calculation, combined with the adoption of safety factor enables the quick and easy development as well as the performance estimation of shielding components. BNPLA is uniquely suited for 3D printing neutron shielding because of the combination of non-abrasive h-BN particles in standard PLA, which results in a filament that can be printed with almost any off-the-shelf printer and virtually no prior experience in 3D printing. This mitigates the slightly lower attenuation observed as compared to filaments containingB 4 C , which is highly abrasive and requires extensive additive manufacturing experience.

Non-invasive detection of hazardous materials with a thermal-to-epithermal neutron station: a feasibility study towards practical application

The growing scale of the devastation that even a single terrorist attack can cause requires more effective methods for the detection of hazardous materials. In particular, there are no solutions for effectively monitoring threats at sea, both for the off-shore infrastructure and ports. Currently, state-of-the-art detection methods determine the density distribution and the shapes of tested subjects but only allow for a limited degree of substance identification. This work aims to present a feasibility study of the possible usage of several methods available on the thermal-to-epithermal neutron station, VESUVIO, at the ISIS neutron and muon spallation source, UK, for the detection of hazardous materials. To this end, we present the results of a series of experiments performed concurrently employing neutron transmission and Compton scattering using melamine, a commonly used explosive surrogate, in order to determine its signal characteristics and limits of detection and quantitation. The experiments are supported by first-principles modelling, providing detailed scrutiny of the material structure and the nuclear dynamics behind the neutron scattering observables.

Quantum motion of oxygen and hydrogen in water: Atomic and total kinetic energy across melting from neutron scattering measurements

We provide a concurrent measurement of the hydrogen and oxygen nuclear kinetic energies in the water molecule across melting at 270 K in the solid phase and 276 K in the liquid phase. Experimental values are obtained by analyzing the neutron Compton profiles of each atomic species in a deep inelastic neutron scattering experiment. The concurrent measurement of the atom kinetic energy of both hydrogen and oxygen allows the estimate of the total kinetic energy per molecule due to the motion of nuclei, specifically 35.3 ± 0.8 and 34.8 ± 0.8 kJ/mol for the solid and liquid phases, respectively. Such a small difference supports results from ab initio simulations and phenomenological models from the literature on the mechanism of competing quantum effects across the phase change. Despite the experimental uncertainties, the results are consistent with the trend from state-of-the-art computer simulations, whereby the atom and molecule kinetic energies in the liquid phase would be slightly lower than in the solid phase. Moreover, the small change of nuclear kinetic energy across melting can be used to simplify the calculation of neutron-related environmental dose in complex locations, such as high altitude or polar neutron radiation research stations where liquid water and ice are both present: for neutron energies between hundreds of meV and tens of keV, the total scattering cross section per molecule in the two phases can be considered the same, with the macroscopic cross section only depending upon the density changes of water near the melting point.

Changes in the hydrogen nuclear kinetic energy across the several phases of methylammonium lead tribromide

We present an experimental investigation of methylammonium lead tribromide single crystals in the orthorhombic, tetragonal, and cubic phases based on inelastic and deep inelastic neutron scattering experiments. We show how the average hydrogen nuclear kinetic energy, mainly affected by zero-point vibrational energies, shows differences larger compared to the changes simply related to temperature effects when moving from one phase to another. In particular, the Gaussian contribution to the average nuclear kinetic energy is larger in the tetragonal phase compared to the cubic and orthorhombic ones. Moreover, we find that the vibrational densities of states of MAPbBr3 single crystals in the orthorhombic phase are compatible with previously reported results on powder samples, and that the only vibrational modes that show slightly different frequencies compared to MAPbI3 are those in the energy range between 100 and 300 cm-1, related to librational/rotational modes. As these shifts are of about 10 cm-1, and do not affect any higher-energy vibrational mode, we conclude that the zero-point energies and average nuclear kinetic energies in the two-hybrid organic/inorganic perovskites are expected to be approximately the same within a harmonic framework.

The correction of Inelastic Neutron Scattering data of organic samples using the Average Functional Group Approximation

The use of the Average Functional Group Approximation for self-shielding corrections at inelastic neutron spectrometers is discussed. By taking triptindane as a case study, we use the above-mentioned approximation to simulate a synthetic dynamic structure factor as measured on an indirect-geometry spectrometer, as well as the related total scattering cross section as a function of incident neutron energy and sample temperature, and the transmission spectra depending on the sample thickness. These quantities, obtained in a consistent way from the Average Functional Group Approximation, are used to calculate the energy-dependent self-shielding correction affecting the sample under investigation. The impact on the intensities of low-energy vibrational modes is discussed, showing that at typical experimental conditions the sample-dependent attenuation factor is about 15% higher compared to the correction at higher energies.

Glass Transition in Rice Pasta as Observed by Combined Neutron Scattering and Time-Domain NMR

Experimental protocols aiming at the characterisation of glass transition often suffer from ambiguity. The ambition of the present study is to describe the glass transition in a complex, micro heterogeneous system, the dry rice pasta, in a most unambiguous manner, minimising the influence of technique-specific bias. To this end, we apply an unprecedented combination of experimental techniques. Apart from the usually used NMR and DSC, we employ, in a concurrent manner, neutron transmission, diffraction, and Compton scattering. This enables us to investigate the glass transition over a range of spatio-temporal scales that stretches over seven orders of magnitude. The results obtained by neutron diffraction and DSC reveal that dry rice pasta is almost entirely amorphous. Moreover, the glass transition is evidenced by neutron transmission and diffraction data and manifested as a significant decrease of the average sample number density in the temperature range between 40 and 60 °C. At the microscopic level, our NMR, neutron transmission and Compton scattering results provide evidence of changes in the secondary structure of the starch within the dry rice pasta accompanying the glass transition, whereby the long-range order provided by the polymer structure within the starch present in the dry rice pasta is partially lost.