Spin filters and supermirrors: a comparison study of two methods of high-precision neutron polarisation analysis

The precise knowledge of the neutron polarisation is needed in tests of the electroweak Standard Model using angular correlations in polarised neutron beta decay. We performed an experimental comparison study of two different methods of polarisation analysis of a cold neutron beam which are based on spin-dependent reflection, respectively, transmission. The compared devices are a supermirror analyser used in prior neutron decay studies and an opaque transmission spin filter of either polarised ³He or polarised protons. The results of the neutron polarisation measured with the supermirror analyser and with the spin filter coincided in three different experiments within 0.1–0.2%.     

Investigation of fuel cells using scanning neutron imaging and a focusing neutron guide

We present two different methods to increase the size of available neutron beams in order to allow for the investigation of large objects. Application of these methods is demonstrated for radiographic imaging of fuel cells. The first approach is a scanning procedure based on the coordinated translation of detector and sample through the beam. Further advancement was achieved by installing a focusing neutron guide, which offers an expanded neutron beam size after diverging from a focused point source.     

Simulation and preliminary experimental results for an active neutron counter using a neutron generator for a fissile material accounting

An active neutron coincidence counter using a neutron generator as an interrogation source has been suggested. Because of the high energy of the interrogation neutron source, 2.5 MeV, the induced fission rate is strongly affected by the moderator design. MCNPX simulation has been performed to evaluate the performance achieved with these moderators. The side- and bottom-moderator are significantly important to thermalize neutrons to induce fission. Based on the simulation results, the moderators are designed to be adapted to the experimental system. Their preliminary performance has been tested by using natural uranium oxide powder samples. For a sample of up to 3.5 kg, which contains 21.7 g of ²³⁵U, 2.64 cps/g-²³⁵U coincidence events have been measured. Mean background error was 9.57 cps and the resultant coincidence error was 13.8 cps. The experimental result shows the current status of an active counting using a neutron generator which still has some challenges to overcome. However, the controllability of an interrogation source makes this system more applicable for a variety of combinations with other non-destructive methods like a passive coincidence counting especially under a harsh environment such as a hot cell. More precise experimental setup and tests with higher enriched samples will be followed to develop a system to apply it to an active measurement for the safeguards of a spent fuel treatment process.     

Acceptance diagram calculations of the performance of neutron removal mirrors

We have used the method of acceptance diagrams to compute the performance of low energy neutron removal mirrors, or “deflectors”, placed within a parallel neutron guide. Such devices are typically used to remove long wavelength neutrons from cold neutron beams. With appropriate coatings they may also be used as low energy neutron polarizers, ideally transmitting one spin state and reflecting the other spin state out of the beam. Within the small angle approximation, ignoring absorption, and representing reflectivities using unit step functions (either 0% or 100%, depending on the angle of incidence and the critical angle), the transmission probability reduces to a function of 3 ratios among 4 angles: the inclination angle of the deflector and the critical angles (which are proportional to neutron wavelength) of the upstream entrance guide, the deflector, and the guide within which the deflector is placed. The results of the acceptance diagram calculations, and of complementary ray-tracing calculations using realistic reflectivity profiles for the deflector, should benefit scientists and engineers involved in the design of neutron scattering instruments that potentially incorporate neutron deflectors.     

UCN Production With a Single Crystal of Ortho-Deuterium

The present paper reports on the preliminary experimental results concerning a new concept of ultracold neutron production with a single crystal converter of ortho-deuterium lying in the ground rotational state at the low temperature of about 10 K, which should make it possible to utilize a guided cold neutron beam instead of irradiating the converter material in the inside of high radiation fields. The successful observation of the clear Bragg scattering pattern from the single crystal converter and the reasonable results from the first experimental trial of the ultracold neutron production with the single crystal are shown.     

Sensitivity of peak positions to flight-path parameters in a deep-inelastic scattering neutron TOF spectrometer

The effects of small changes in flight-path parameters (primary and secondary flight paths, detector angles), and of displacement of the sample along the beam axis away from its ideal position, are examined for an inelastic time-of-flight (TOF) neutron spectrometer, emphasising the deep-inelastic regime. The aim was to develop a rational basis for deciding what measured shifts in the positions of spectral peaks could be regarded as reliable in the light of the uncertainties in the calibrated flight-path parameters. Uncertainty in the length of the primary or secondary flight path has the least effect on the positions of the peaks of H, D and He, which are dominated by the accuracy of the calibration of the detector angles. This aspect of the calibration of a TOF spectrometer therefore demands close attention to achieve reliable outcomes where the position of the peaks is of significant scientific interest and is discussed in detail. The corresponding sensitivities of the position of peak of the Compton profile, J(y), to flight-path parameters and sample position are also examined, focusing on the comparability across experiments of results for H, D and He. We show that positioning the sample to within a few mm of the ideal position is required to ensure good comparability between experiments if data from detectors at high forward angles are to be reliably interpreted.     

Determination of the parahydrogen fraction in a liquid hydrogen target using energy-dependent slow neutron transmission

The NPDGamma collaboration is performing a measurement of the very small parity-violating asymmetry in the angular distribution of the 2.2 MeV γ-rays from the capture of polarized cold neutrons on protons (A_γ). The estimated size of A_γ is 5×10⁻⁸, and the measured asymmetry is proportional to the neutron polarization upon capture. Since the interaction of polarized neutrons with one of the two hydrogen molecular states (orthohydrogen) can lead to neutron spin-flip scattering, it is essential that the hydrogen in the target is mostly in the molecular state that will not depolarize the neutrons (≥99.8% parahydrogen). For that purpose, in the first stage of the NPDGamma experiment at the Los Alamos Neutron Science Center (LANSCE), we operated a 16-l liquid hydrogen target, which was filled in two different occasions. The parahydrogen fraction in the target was accurately determined in situ by relative neutron transmission measurements. The result of these measurements indicate that the fraction of parahydrogen in equilibrium was 0.9998±0.0002 in the first data taking run and 0.9956±0.0002 in the second. We describe the parahydrogen monitor system, relevant aspects of the hydrogen target, and the procedure to determine the fraction of parahydrogen in the target. Also assuming thermal equilibrium of the target, we extract the scattering cross-section for neutrons on parahydrogen.     

Determination of the scattering lengths of gallium isotopes by neutron interferometry with PNO-apparatus in JRR-3M

For carrying out experiments in the field of the so-called precise neutron optics (PNO), we have implemented special multi-purpose apparatus called the “PNO-apparatus” at JRR-3M. Making use of an Si triple-Laue (LLL) neutron interferometer with the PNO-apparatus, we successfully determined the coherent neutron scattering lengths of gallium isotopes, ⁶⁹Ga and ⁷¹Ga. The results are 8.053±0.013 fm for ⁶⁹Ga and 6.170±0.011 fm for ⁷¹Ga, respectively.     

Development of a neutron tomography system using a low flux reactor

A neutron tomography instrument was designed and developed at the Royal Military College (RMC) of Canada with Queen's University to enhance these institutions' non-destructive evaluation capabilities. The neutron imaging system was built around a Safe Low-Power C(K)ritical Experiment (SLOWPOKE-2) nuclear research reactor. The low power and physical geometry of the reactor required that a novel design be developed to facilitate tomography. A unique rotisserie style rotary stage and clamping apparatus was developed. Furthermore, the low flux at the image plane (3×10⁴ n cm⁻² s⁻¹), necessitated that the image acquisition and reconstruction processes be optimized. Tomographs of numerous samples were obtained using the new tomography instrument at RMC.     

Calibration of the electron volt spectrometer, a deep inelastic neutron scattering spectrometer at the ISIS pulsed neutron spallation source

The electron Volt Spectrometer (eVS) is an inverse geometry filter difference spectrometer that has been optimised to measure the single atom properties of condensed matter systems using a technique known as Neutron Compton Scattering (NCS) or Deep Inelastic Neutron Scattering (DINS). The spectrometer utilises the high flux of epithermal neutrons that are produced by the ISIS neutron spallation source enabling the direct measurement of atomic momentum distributions and ground state kinetic energies. In this paper the procedure that is used to calibrate the spectrometer is described. This includes details of the method used to determine detector positions and neutron flight path lengths as well as the determination of the instrument resolution. Examples of measurements on 3 different samples are shown, ZrH₂, ⁴He and Sn which show the self-consistency of the calibration procedure.     

Evaluation of water distribution in a small operating fuel cell using neutron color image intensifier

Neutron radiography is one of the useful tools for visualizing water behavior in operating fuel cells. In order to observe the detailed information about the water distribution in membrane electrode assembly (MEA) and gas diffusion layer (GDL) in fuel cells, a high performance neutron imaging system is required. A neutron color image intensifier (NCII) is a high spatial resolution and high sensitivity neutron image detector. We have developed an imaging system using an NCII for visualizing the behavior of water in fuel cells. The pixel size of the imaging system is around 4.7 μm in the small view field. By using this system, water distribution of a small sized fuel cell was observed continuously every 20 s at the Thermal Neutron Radiography Facility (TNRF). In the results, the water area appears from the GDL and MEA regions, and expanded to the cathode side channel with time. However, the voltage was gradually reduced with time, and steeply dropped. It is considered that the reduction and the drop of voltage were caused by a blockage of gas flow due to accumulation of water in the GDL and the gas flow channel in the cathode side.     

Experimental studies of the effect of the ortho/para ratio on the neutronic performance of a liquid hydrogen moderator for a pulsed neutron source

Liquid hydrogen is a realistic cold moderator material for high-power spallation neutron sources. The neutronic performance of a hydrogen moderator depends on the ortho/para ratio of hydrogen, and thus experimental data are needed that will clarify the ortho/para ratio effects on neutronic performance. In this study, we measured the neutronic performance of a liquid hydrogen moderator at several para hydrogen concentrations.Our experiment was performed at the Hokkaido University 45 meV electron linac facility. The neutron energy spectra were measured by the time-of-flight method. Pulse shapes were measured by the Bragg scattering of a mica crystal.The neutron energy spectra change within 20%, depending on the para hydrogen concentrations. With increasing para hydrogen concentration, the pulse peak intensity increases and the pulse width becomes narrower. Furthermore, for a decoupled moderator, the pulse decay becomes faster with increasing para hydrogen concentration. From a viewpoint of the figure of merit (FOM=I/FWHM²) the para hydrogen moderator showed almost the same performance as that of the solid methane moderator, which is considered to be a high performance moderator for pulsed neutron source.     

Flat-response neutron detector using spatial distribution of thermal neutrons in a moderator

We propose a novel neutron detector that realized flat-response using information of a spatial distribution of thermal neutrons in a moderator. The proposed detector consists of a ³He position sensitive proportional counter (PSPC) and a cylindrical moderator surrounding the ³He PSPC. The cylindrical detector is irradiated by neutrons along the cylinder axis. The spatial response of the ³He PSPC is used to correct the detector response into flat-response. We adopt a weighting method to achieve flat-response, in which detected neutrons weighted depending on their detected positions are accumulated as the detector response. Through Monte Carlo simulation studies, we confirm that the flat-response neutron detector can be realized by correcting the response of the proposed detector using the weights determined by a multiple least square method (MLSM). Additionally, fundamental property of the ³He PSPC is experimentally investigated to check applicability to the proposed flat-response neutron detector. We conclude that we should take account of the end effect when determining the weights and correcting the detector response.     

Development of multi-moderator neutron spectrometer using a pair of Li and Li glass scintillators

A multi-moderator spectrometer using a pair of ⁶Li and ⁷Li glass scintillators has been developed. This new type of neutron spectrometer can measure the neutron spectrum in a mixed field of neutrons, charged particles and gamma-rays. The particle identification capability was investigated in neutron–gamma-ray and neutron–proton mixed fields and the neutron response functions of the spectrometer were obtained by calculations and experiments up to 200 MeV. This spectrometer has been applied to measure neutron spectrum in a neutron–proton mixed field, produced by bombarding a Be target by 70 MeV protons from the cyclotron.     

Development of a fast neutron radiography converter using wavelength-shifting fibers

A fluorescent converter for fast neutron radiography (FNR) comprising a scintillator and hydrogen-rich resin has been developed and applied to electronic imaging. The rate of the reaction between fast neutrons and the converter is increased by thickening the converter, but its opaqueness attenuates emitted light photons before they reach its surface. To improve the luminosity of a fluorescent converter for FNR, a novel type of converter was designed in which wavelength-shifting fibers were adopted to transport radiated light to the observation end face. The performance of the converter was compared with that of a polypropylene-based fluorescent converter in an experiment conducted at the fast-neutron-source reactor YAYOI in the University of Tokyo.     

Time-of-flight neutron imaging at a continuous source: Proof of principle using a scintillator CCD imaging detector

Energy resolved imaging has recently gained attention for the potential of spatially resolved texture, crystallographic phase and strain investigations. Especially a time-of-flight (TOF) approach that takes maximum advantage of the new generation of pulsed spallation neutron sources is currently in the focus of investigations. Here, we present results of corresponding TOF measurements recorded at the continuous source of Helmholtz Center Berlin. The critical component for TOF imaging is however the availability of a high resolution imaging detector capable of the required time resolution. Here, a gated time-integrating detector without the corresponding continuous time resolution has been used and the measurements therefore have to be interpreted as proof-of-principle experiments as will be discussed. Measurements of different series of samples revealing structural differences related to their crystalline structure will be presented as well as a strain measurement on a dieless drawn wire.     

First demonstration of neutron resonance absorption imaging using a high-speed video camera in J-PARC

The neutron resonance absorption imaging technique with a high-speed video camera was successfully demonstrated at the beam line NOBORU, J-PARC. Pulsed neutrons were observed through several kinds of metal foils as a function of neutron time-of-flight by utilizing a high-speed neutron radiography system. A set of time-dependent images was obtained for each neutron pulse, and more than a thousand sets of images were recorded in total. The images with the same time frame were summed after the measurement. Then the authors obtained a set of images having enhanced contrast of sample foils around the resonance absorption energies of cobalt (132 eV), cadmium (28 eV), tantalum (4.3 and 10 eV), gold (4.9 eV) and indium (1.5 eV).     

Detection unit optimization of a neutron searching detector using Monte Carlo Simulations

The NSD is a portable Neutron Searching Detector developed at Rotem Industries Ltd. with a high efficiency for counting fast and thermal neutrons employing improved gamma rejection. The NSD detection-unit consists of two ³He detectors installed within a polypropylene moderator. The latest international standards for detection of illicit trafficking of radioactive materials require high sensitivity, relatively small dimensions, and light mass. In order for it to meet these standards, the NSD detection-unit was optimized using Monte Carlo N-Particle transport code (MCNP). The moderator mass and dimensions were reduced without deterioration, even improving the instrument's sensitivity. The purposed moderator improvements covered in this paper work well for traditional hand-held neutron search detectors based on ³He tubes as well as for new neutron detection technologies due to the severe worldwide shortage of ³He.Three geometrical moderator configurations were examined using the MCNP code—a rectangular box, a circular cylinder, and an elliptical base cylinder. The optimization results showed that both the rectangular box moderator and the elliptical base cylinder moderators achieve the appropriate sensitivity required by the standards with about 30% reduced mass. A prototype was fabricated with the rectangular box moderator configuration, and its response was successfully validated by comparing empirical measurements against the results of the MCNP code.Performance examination of the optimal detection unit prototype was made regarding the latest international standards. The results showed a 17% improvement in detection limit for radioactive materials along with a 14% to 17% increased neutron detection response, while keeping the false alarm rate below the required threshold, and maintaining a 26% mass reduction.     

Quantitative determination of higher harmonic content in the soft x-ray spectra of toroidal grating monochromator using a reflection multilayer

Use of a grating monochromator causes a problem of higher harmonic contaminations in a synchrotron beamline operating in the soft x ray/vacuum ultraviolet region. Generally gratings are used to experimentally determine the higher harmonic contaminations. In this method, the relative contribution of contaminant wavelengths is measured with respect to the first harmonic wavelength (desired wavelength). The quantitative fit of grating spectra is rather complex, and therefore qualitative analysis is carried out. Analysis of multilayer reflectivity data has become rather simple with recent advances in the theoretical modeling. Therefore we propose to use a multilayer mirror and analyze its reflectivity data for quantitative determination of harmonic contamination in a soft x ray beamline. In the present study we used a Mo/Si multilayer of d=97 ? to quantify the spectral purity of 600 lines/mm toroidal grating at the reflectivity beamline of Indus-1 450 MeV synchrotron source. The measured reflectivity spectra at each wavelength is analyzed and the actual contribution of higher harmonics in the incident beam is obtained. Details of methodology and results are discussed.     

Polarization of a neutron beam using a superconductor

Based on the London theory of superconductivity and using the quasistatic approximation, we have calculated the interaction energy between a superconducting plane and a dipole which is moving toward or away from the plane. Using the decoupling approximation, we have investigated the displacement and the angle flipping of a neutron, when the neutron moves toward or away from the superconducting plane. The theoretical model can, in principle, be applied to construct an apparatus to polarize the beam from a thermal neutron reactor.