A. Yu. Muzychka

Neutron spectroscopy of fullerite hydrogenated under high pressure; evidence for interstitial molecular hydrogen

Inelastic neutron scattering spectra of a hydrofullerite quenched after synthesis at 620 K under a hydrogen pressure of 0.6 GPa, and of the same sample after annealing at 300 K for 35 h, which reduced the hydrogen content by molecules per unit, were measured at 85 K. The quenched sample is shown to consist of molecules with and of interstitial molecular hydrogen. The interstitial molecular hydrogen left the sample during annealing at room temperature, whereas the molecules were stable at this temperature. The intramolecular and intermolecular vibrations of and in the fullerite are discussed in view of the measured spectra.

The reflection of very cold neutrons from diamond powder nanoparticles

We study the possibility of efficiently reflecting very cold neutrons (VCN) from powders of nanoparticles. In particular, we measured the scattering of VCN on powders of diamond nanoparticles as a function of powder sample thickness, neutron velocity and scattering angle. We observed extremely intense scattering of VCN even off thin powder samples. This agrees qualitatively with the model of independent nanoparticles at rest. We show that this intense scattering would allow us to use nanoparticle powders very efficiently as the very first reflectors for neutrons with energies within a complete VCN range up to 10-4eV.

Mechanism of small variations in energy of ultracold neutrons interacting with a surface

The cause of the small heating of ultracold neutrons (UCNs) by ∼10−7 eV with a probability of 10−8–10−5 per collision with a surface was investigated. Neutrons heated in this way will be called vaporized UCNs (VUCNs). It was established that a preliminary heating of a sample in vacuum up to a temperature of 500–600 K can increase small-heating probability PVUCN by a factor of at least ∼100 and 10 on a stainless steel and a copper surface, respectively. For the first time, an extremely vigorous small heating of UCNs was observed on a powder of diamond nanoparticles. In this case, both the VUCN spectrum and the temperature dependence of probability PVUCN were similar to those previously obtained for stainless steel, beryllium, and copper samples. On the surface of single crystal sapphire, neither the small heating of UCNs nor nanoparticles were found. All these facts indicate that VUCNs are likely produced by inelastic scattering of UCNs on weakly bound surface nanoparticles being in permanent thermal motion.

Identification of a new escape channel for UCN from traps

Abstract Ultra-cold neutrons (UCN) can be stored in a trap if their energy is lower than the trap wall potential. It is well known that the neutron density in a trap decreases due to neutron beta-decay, upscattering and absorption on surfaces but we have identified a complementary escape channel. This arises from a small increase in the energy of UCN during their interaction with a surface. Higher-energy neutrons can then escape into the bulk material or penetrate through the trap wall if it is thin enough.

Crystal field potential of NdCu2Si2: A comparison with CeCu2Si2

Abstract The results of an inelastic neutron scattering investigation of the crystal field potential of NdCu2Si2 are reported. We have observed three ground state and three excited state transitions which establish unambiguously the crystal field level scheme with an overall splitting of 11.45 meV. By performing a profile refinement of the measured spectra we have determined the Hamiltonian parameters for the tetragonal crystal field: B02 = (-3.1 ± 0.1) × 10-2 meV, B04 = (1.1 ± 0.1) × 10-3 meV, B44 = (1.33 ± 0.04) × 10-3 meV, B06 = (-3.17 ± 0.04) × 10-5 meV and B46 = (6.62 ± 0.02 × 10-4 meV. The parameters are analysed in terms of Newmans superposition model and compared to recent results on the isostructural heavy fermion compound CeCu2Si2.

Temperature dependence of inelastic ultracold-neutron scattering at low energy transfer

The temperature dependence of inelastic ultracold-neutron scattering on beryllium and copper surfaces at low energy transfers (about 10−7 eV) is investigated, and the results of this investigation are presented. The recorded flux of neutrons inelastically scattered by these surfaces at liquid-nitrogen temperature is less than that at room temperature by a factor of about two.

Quasi-specular reflection of cold neutrons from nano-dispersed media at above-critical angles

We predicted and observed for the first time the quasi-specular albedo of cold neutrons at small incidence angles from a powder of nanoparticles. This albedo (reflection) is due to multiple neutron small-angle scattering. The reflection angle as well as the half-width of angular distribution of reflected neutrons is approximately equal to the incidence angle. The measured reflection probability was equal to ~30% within the detector angular size that corresponds to 40 − 50% total calculated probability of quasi-specular reflection. Coherent scattering of ultracold (UCN), very cold (VCN) and cold (CN) neutrons on nanoparticles could be used (1), (2) in fundamental and applied low-energy neutron physics (3), (4), (5), (6). A theoretical analysis of such scattering could be found, for instance, in (7). In the first Born approximation, the scattering amplitude equals f θ = − 2mU0 ħ2 r sin qr qr 3 − cos qr qr 2 , q = 2ksin θ

Study of bound hydrogen in powders of diamond nanoparticles

In order to access feasibility of increasing albedo of very cold neutrons from powder of diamond nanoparticles, we studied hydrogen bound to surface of diamond nanoparticles, which causes unwanted losses of neutrons. We showed that one could decrease a fraction of hydrogen atoms from a ratio C7.4 ± 0.15H to a ratio C12.4 ± 0.2H by means of thermal treatment and outgasing of powder. Measurements of atomic excitation spectra of these samples, using a method of inelastic incoherent neutron scattering, indicate that residual hydrogen is chemically bound to carbon, while a removed fraction was composed of adsorbed water. The total cross section of scattering of neutrons with a wavelength of 4.4 Å on residual hydrogen atoms equals 108 ± 2 b; it weakly changes with temperature. Thus preliminary cleaning of powder from hydrogen and its moderate cooling do not improve considerably neutron albedo from powder of nano-diamonds. An alternative approach is isotopic replacement of hydrogen by deuterium.

A new escape channel for ultracold neutrons in traps

A surprising new escape channel for ultracold neutrons (UCNs) in traps was reported recently. It could be relevant to the long-standing puzzle of the “too high” loss rate of UCNs from traps, which has yet to be completely understood and eliminated. In the present work we positively identify the new phenomenon and investigate it in detail. The escape of UCNs from traps is due to rare events in which their energy undergoes a small increase (∼10−7 eV). The reason for such an energy gain and its impact on the physics of UCN storage is still to be investigated.

Experimental estimation of the possible subbarrier penetration of ultracold neutrons through vacuum-tight foils

The probability of subbarrier penetration of ultracold neutrons through 15 μm-thick vacuum-tight beryllium foil (boundary energy for beryllium is Elim Be=249 eV) was measured. It is equal to (−1.2±1.0) × 10−8 per collision of neutrons with energy lower than ∼160 neV.