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Intense sources of very cold neutrons (VCNs) would be beneficial for various neutron scattering techniques and low-energy particle physics experiments. Binary clathrate hydrates hosting deuterated tetrahydrofuran (THF-d) and dioxygen show promise as potential moderators for such sources due to a rich spectrum of localized low-energy excitations of the encaged guest molecules. In this article, we present a reliable manufacturing technique for such hydrates. Neutron diffraction data confirm their clathrate structure as type II (CS-II), determine their purity, and cage occupancy. Furthermore, we present data on the thermal expansivity of THF-d– and THF-d–O2clathrates, drawing attention to them as an interesting case study for the complex structure and dynamics of this class of material.

It is proposed that nanosized graphene aggregation could facilitate coherent neutron scattering under particle size conditions similar to nanodiamonds to enhance neutron intensity below cold neutrons. Using the RIKEN accelerator-driven compact neutron source and iMATERIA at J-PARC, we performed neutron measurement experiments, total neutron cross-section and small-angle neutron scattering on nanosized graphene aggregation. For the first time, the measured data revealed that nanosized graphene aggregation increased the total neutron cross-sections and small-angle scattering in the cold neutron energy region. This is most likely due to coherent scattering, resulting in higher neutron intensities, similar to nanodiamonds.

In recent years, neutron radiography and tomography have been applied at different beam lines at Los Alamos Neutron Science Center (LANSCE), covering a very wide neutron energy range. The field of energy-resolved neutron imaging with epi-thermal neutrons, utilizing neutron absorption resonances for contrast as well as quantitative density measurements, was pioneered at the Target 1 (Lujan center), Flight Path 5 beam line and continues to be refined. Applications include: imaging of metallic and ceramic nuclear fuels, fission gas measurements, tomography of fossils and studies of dopants in scintillators. The technique provides the ability to characterize materials opaque to thermal neutrons and to utilize neutron resonance analysis codes to quantify isotopes to within 0.1 atom %. The latter also allows measuring fuel enrichment levels or the pressure of fission gas remotely. More recently, the cold neutron spectrum at the ASTERIX beam line, also located at Target 1, was used to demonstrate phase contrast imaging with pulsed neutrons. This extends the capabilities for imaging of thin and transparent materials at LANSCE. In contrast, high-energy neutron imaging at LANSCE, using unmoderated fast spallation neutrons from Target 4 [Weapons Neutron Research (WNR) facility] has been developed for applications in imaging of dense, thick objects. Using fast (ns), time-of-flight imaging, enables testing and developing imaging at specific, selected MeV neutron energies. The 4FP-60R beam line has been reconfigured with increased shielding and new, larger collimation dedicated to fast neutron imaging. The exploration of ways in which pulsed neutron beams and the time-of-flight method can provide additional benefits is continuing. We will describe the facilities and instruments, present application examples and recent results of all these efforts at LANSCE.

The ultra-cold neutron (UCN) source being commissioned at North Carolina State University’s PULSTAR reactor is uniquely optimized for UCN production in the former graphite-filled thermal column outside of the reactor pool. The source utilizes a remote moderation design, which is particularly well suited to the PULSTAR reactor because of its high thermal and epithermal neutron leakage from the core face. This large non-equilibrium flux from the core is efficiently transported to the UCN source through the specially designed beam port in order to optimize UCN production at any given reactor power. The increased distance to the source from the core also greatly limits the heat load on the cryogenic system. A MCNP (Monte Carlo N-Particle) model of this system was developed and is in good agreement with gold foil activation measurements using a test configuration as well as with the real UCN source’s heavy water moderator. These results established a firm baseline for estimates of the cold neutron flux available for UCN production and prove that remote moderation in a thermal column port is a valuable option for future designs of cryogenic UCN sources.

A novel concept of cold neutron source employing chessboard or staircase assemblies of high-aspect-ratio rectangular para-hydrogen moderators with well-developed and practically fully illuminated surfaces of the individual moderators is proposed. An analytic approach for calculating the brightness of para-hydrogen moderators is introduced. Because the brightness gain originates from a near-surface effect resulting from the prevailing single-collision process during thermal-to-cold neutron conversion, high-aspect-ratio rectangular cold moderators offer a significant increase, up to a factor of 10, in cold neutron brightness compared to a voluminous moderator. The obtained results are in excellent agreement with MCNP calculations. The chessboard or staircase assemblies of such moderators facilitate the generation of wide neutron beams with simultaneously higher brightness and intensity compared to a para-hydrogen-based cold neutron source made of a single moderator (either flat or voluminous) of the same cross-section. Analytic model calculations indicate that gains of up to approximately 2.5 in both brightness and intensity can be achieved compared to a source made of a single moderator of the same width. However, these gains are affected by details of the moderator–reflector assembly and should be estimated through dedicated Monte Carlo simulations, which can only be conducted for a particular neutron source and are beyond the scope of this general study. The gain reduction in our study, from a higher value to 2.5, is mostly caused by these two factors: the limited volume of the high-density thermal neutron region surrounding the reactor core or spallation target, which restricts the total length of the moderator assembly, and the finite width of moderator walls. The relatively large length of moderator assemblies results in a significant increase in pulse duration at short pulse neutron sources, making their straightforward use very problematic, though some applications are not excluded. The concept of “low-dimensionality” in moderators is explored, demonstrating that achieving a substantial increase in brightness necessitates moderators to be low-dimensional both geometrically, implying a high aspect ratio, and physically, requiring the moderator’s smallest dimension to be smaller than the characteristic scale of moderator medium (about the mean free path for thermal neutrons). This explains why additional compression of the moderator along the longest direction, effectively giving it a tube-like shape, does not result in a significant brightness increase comparable to the flattening of the moderator.

The study of scalar field theories like the chameleon field model is of increasing interest due to the Universe’s accelerated expansion, which is believed to be caused in part by dark energy. These fields can elude experimental bounds set on them in high-density environments since they interact with matter in a density-dependent way. This paper analyzes the effect of chameleon fields on the quantum gravitational states of ultra-cold neutrons (UCNs) in qBounce experiments with mirrors. We discuss the deformation of the neutron wave function due to chameleon interactions and quantum systems in potential wells from gravitational forces and chameleon fields. Unlike other works that aim to put bounds on the chameleon field parameters, this work focuses on the quantum mechanics of the chameleonic neutron. The results deepen our understanding of the interplay between quantum states and modified gravity, as well as fundamental physics experiments carried out in the laboratory.

Neutron–antineutron oscillation (nnbar-osc) is a baryon number-violating process and a sensitive probe for physics beyond the standard model. Ultra-cold neutrons (UCNs) are attractive for nnbar-osc searches because of their long storage time, but earlier analyses indicated that phase shifts on wall reflection differ for neutrons and antineutrons, leading to severe decoherence and a loss of sensitivity. Herein, we revisit this problem by numerically solving the time-dependent Schrödinger equation for the two-component n/nbar wave function, explicitly including wall interactions. We show that decoherence can be strongly suppressed by selecting a wall material whose neutron and antineutron optical potentials are nearly equal. Using coherent scattering length data and estimates for antineutrons, we identify a Ni–Al alloy composition that matches the potentials within a few percent while providing a high absolute value, enabling long UCN storage. With such a bottle and an improved UCN source, the sensitivity could reach an oscillation period τnnbar of the order 1010 s, covering most of the range predicted with certain grand unified models. This approach revives the feasibility of high-sensitivity nnbar-osc searches using stored UCNs and offers a clear path to probe baryon number violation far beyond existing limits.

More than 15 years ago, the study of nanodiamond (ND) powders as a material for designing reflectors of very cold neutrons (VCNs) and cold neutrons (CNs) began. Such reflectors can significantly increase the efficiency of using such neutrons and expand the scope of their application for solving applied and fundamental problems. This review considers the principle of operation of VCN and CN reflectors based on ND powders and their advantages. Information is presented on the performed experimental and theoretical studies of the effect of the size, structure, and composition of NDs on the efficiency of reflectors. Methods of chemical and mechanical treatments of powders in order to modify their chemical composition and structure are discussed. The aim is to avoid, or at least to decrease, the neutron inelastic scatterers and absorbers (mainly hydrogen atoms but also metallic impurities and nitrogen) as well as to enhance coherent elastic scattering (to destroy ND clusters and sp2 carbon shells on the ND surface that result from the preparation of NDs). Issues requiring further study are identified. They include deeper purification of NDs from impurities that can be activated in high radiation fluxes, the stability of NDs in high radiation fluxes, and upscaling methods for producing larger quantities of ND powders. Possible ways of solving these problems are proposed.

We report a novel neutron interferometry scheme aimed at probing the potential existence of mirror neutrons, which have been proposed as viable dark matter candidates. Our theoretical analysis demonstrates that if mirror neutrons exist, ordinary neutrons would acquire a measurable geometric phase as a result of their mixing with these mirror counterparts [1].

In December 1997 the Osiris beamline at the ISIS facility, UK, recorded its first neutron spectrum. The instrument enjoyed a first stint as a cold neutron diffractometer before the spectroscopic capabilities were fully commissioned. Osiris soon became a workhorse quasielastic spectrometer as well as a highly successful low-energy spectrometer. The status of the instrument is recognized by the user community with high-impact publications ranging from energy materials over life science to quantum matter. To enhance the existing capabilities a silicon analyzer is under construction. The primary spectrometer will be upgraded with a new supermirror guide providing a factor 10 to 14 increased flux in combination with a new hydrogen moderator. Beyond these developments further improvements of the energy resolution with a combination of a fast pulse shaping chopper and using a direct backscattering geometry are being investigated.