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

Abstract

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.

Time-of-flight Fourier UCN spectrometer

“…Specifically, the gravity UCN spectrometry [7][8][9][10][11][12][13][14] and the spectrometry based on the use of quantum spectrometric devices (various interference filters) [11,12,[15][16][17] have received widespread use.…”

The reflection of very cold neutrons from diamond powder nanoparticles

“…One might also exploit the phenomenon of scattering off nanoparticles to study the dynamics of nanoparticles at a surface and in the material bulk [10][11][12]. It might also even be used to cool VCN down to the UCN energy range [13][14][15].…”

Study of levitating nanoparticles using ultracold neutrons

“…The first measurements were followed by other studies [19][20][21][22][23][24][25][26][27][28][29] carried out by several research groups, which essentially confirmed the initial observation but suggested controversial estimations of the absolute VUCN production rates. This controversy is explained by several factors: (a) measurements of such a kind, involving low-probability effects, require careful control of many systematic effects, which could imitate or hide the VUCN production; (b) the rate of VUCN detection depends strongly on the ranges (windows) of initial and final energies of UCN, which have not been under control in some experiments, (c) as we will see from analyses of the spectra presented in this work, very high energy resolution is needed; it has been provided so far only in the Big Gravitational Spectrometer (BGS) spectrometer [26,29]. Thus, the probability of VUCN production per UCN-wall collision might be largely mis-estimated in other experiments; (d) the VUCN production probability depends strongly on the procedure of a sample (surface) treatment or preparation, as has been demonstrated, for instance, in [26].…”

“…To summarize the status of all these studies, trap walls (samples) of two kinds were used in these experiments: liquid and solid walls (samples). While the origin of small heating of UCN on liquid surfaces is still under discussion [12,[19][20][21][30][31][32][33] (see also [34,35], which rule out some hypotheses), the origin of small heating of UCN on solid surfaces is usually attributed to weakly bound nanoparticles on the surface [12,29,36]; note that an increase in the scattering cross-section due to clustering of atoms has already been noted in [33]. Here we will compare the theoretical estimations within our model to all precision experimental data available on inelastic scattering of UCN on nanoparticles bound to solid surfaces [29,36].…”

“…While the origin of small heating of UCN on liquid surfaces is still under discussion [12,[19][20][21][30][31][32][33] (see also [34,35], which rule out some hypotheses), the origin of small heating of UCN on solid surfaces is usually attributed to weakly bound nanoparticles on the surface [12,29,36]; note that an increase in the scattering cross-section due to clustering of atoms has already been noted in [33]. Here we will compare the theoretical estimations within our model to all precision experimental data available on inelastic scattering of UCN on nanoparticles bound to solid surfaces [29,36]. The experimental data include the following measurements: (i) the probability of inelastic scattering and the spectrum of UCN scattered on detonation diamond nanoparticles above a copper surface as a function of temperature (liquid nitrogen and ambient temperature); (ii) the probability of inelastic scattering and the spectra of UCN scattered on nanoparticles formed due to thermal diffusion on metallic surfaces (copper, stainless steel, beryllium) as a function of temperature (liquid nitrogen and ambient temperature), also as a function of the temperature of the surface preceding treatment; and (iii) absence of small-energy-transfer inelastic scattering of UCN on the sapphire monocrystal surface, which could be considered as a 'zero test' of the validity of our model.…”