G. V. Nekhaev

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.

About interpretation of experiments on small increase in energy of UCN in traps

Abstract A new surprising escape channel for ultracold neutrons (UCN) in traps was identified recently. We suppose that the additional UCN loss results from rare events of small increase in their energy (∼10 −7 eV). The only clearly pronounced alternative interpretation of our experiments assumes a temporary adhesion of few UCN to trap walls. We show that this hypothesis contradicts to our experimental data.

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.

AN INVESTIGATION INTO THE ORIGIN OF SMALL ENERGY CHANGES (~ 10-7eV) OF ULTRACOLD NEUTRONS IN TRAPS

We studied the phenomenon of relatively small changes in the energy of ultracold neutrons (UCN) (when compared to thermal motion energy) when these are reflected on a surface. The changes observed involved both increases in UCN energy (their heating) and decreases (cooling) of the order of ~ 10-7 eV. The probability values of this process on various surfaces ranged between 10-8 and 10-5 per one collision; the probability of such a small heating was many times larger than that of such a small cooling. We measured the spectra of such heated neutrons and the dependence of small heating probability on the temperature of sample out-gazing. We found that out-gazing of samples in vacuum at a temperature of 500–600 K could increase the small heating probability on stainless steel surface by a factor of ~ 100; and on copper surface by a factor of ~ 10. We observed, for the first time, extremely intensive small heating of UCN on powder of diamond nanoparticles. Neither small heating of UCN, nor nanoparticles could be found on a sapphire single crystal surface. This set of experimental data indicates that the inelastic scattering of UCN on weakly bound nanoparticles at a surface in a state of thermal motion is responsible for the process investigated.

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.

UCN Source at an External Beam of Thermal Neutrons

We propose a new method for production of ultracold neutrons (UCNs) in superfluid helium. The principal idea consists in installing a helium UCN source into an external beam of thermal or cold neutrons and in surrounding this source with a solid methane moderator/reflector cooled down to ~4 K. The moderator plays the role of an external source of cold neutrons needed to produce UCNs. The flux of accumulated neutrons could exceed the flux of incident neutrons due to their numerous reflections from methane; also the source size could be significantly larger than the incident beam diameter. We provide preliminary calculations of cooling of neutrons. These calculations show that such a source being installed at an intense source of thermal or cold neutrons like the ILL or PIK reactor or the ESS spallation source could provide the UCN density 105 cm−3, the production rate 107 UCN/s−1. Main advantages of such an UCN source include its low radiative and thermal load, relatively low cost, and convenient accessibility for any maintenance. We have carried out an experiment on cooling of thermal neutrons in a methane cavity. The data confirm the results of our calculations of the spectrum and flux of neutrons in the methane cavity.

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.

Neutron transportation in a closed vessel

Results of the experiments on measurement of ultracold neutron (UCN) storage time in moving vessels are reported. A theory for change of the UCN spectrum in the vessel swinging on a long thread like a pendulum is presented. It is found that the average kinetic energy of the UCN increases proportionally to the first derivative of the acceleration but only during those quarters of a period in which the absolute magnitude of acceleration increases. The results of measurement and theoretical consideration of UCN storage time in a vessel struck by a hammer are also given.