Interaction of neutrons with nanoparticles

Two hypotheses concerning the interaction of neutrons with nanoparticles and having applications in the physics of ultracold neutrons (UCN) are considered. In 1997, it was found that, upon reflection from the sample surface or spectrometer walls, UCN change their energy by about 10^?7 eV with a probability of 10^?7–10^?5 per collision. The nature of this phenomenon is not clear at present. Probably, it is due to the inelastic coherent scattering of UCN on nanoparticles or nanostructures weakly attached at the surface, in a state of Brownian thermal motion. An analysis of experimental data on the basis of this model allows one to estimate the mass of such nanoparticles and nanostructures at 10⁷ a.u. The proposed hypothesis indicates a method for studying the dynamics of nanoparticles and nanostructures and, accordingly, their interactions with the surface or with one another, this method being selective in their sizes. In all experiments with UCN, the trap-wall temperature was much higher than a temperature of about 1 mK, which corresponds to the UCN energy. Therefore, UCN increased their energy. The surface density of weakly attached nanoparticles was low. If, however, the nanoparticle temperature is lower than the neutron temperature and if the nanoparticle density is high, the problem of interaction of neutrons with nanoparticles is inverted. In this case, the neutrons of initial velocity below 10² m/s can cool down, under certain conditions, owing to their scattering on ultracold heavy-water, deuterium, and oxygen nanoparticles to their temperature of about 1 mK, with the result that the UCN density increases by many orders of magnitude.     

Experimental study of quasi-elastic scattering of ultracold neutrons

Ultracold neutrons (UCN) are lost from traps if they are quasi-elastically scattered from the wall with an energy gain sufficient to exceed the Fermi potential for the wall. Possible mechanisms of a quasi-elastic energy transfer are, for instance, scattering from hydrogen diffusing in an impurity surface layer or on surface waves at a liquid wall. Using two different experimental methods at the UCN source of the Institut Laue-Langevin we have investigated both the energy-gain and the energy-loss side of quasi-elastic UCN scattering on Fomblin grease coated walls. For Fomblin oil and similar new types of oil we report up-scattering data as a function of temperature and energy transfer. These low-temperature oils may be used in an improved measurement of the neutron lifetime, which requires extremely low wall reflection losses. Received 13 March 2002 Published online 31 July 2002     

UCN anomalous losses and the UCN capture cross section on material defects

Experimental data shows anomalously large ultra cold neutrons (UCN) reflection losses and that the process of UCN reflection is not completely coherent. UCN anomalous losses under reflection cannot be explained in the context of neutron optics calculations. UCN losses by means of incoherent scattering on material defects are considered and cross-section values calculated. The UCN capture cross section on material defects is enhanced by a factor of 10⁴ due to localization of UCN around defects. This phenomenon can explain anomalous losses of UCN.     

Multiple scattering theory for slow neutrons (from thermal to ultracold)

The general theory of neutron scattering is presented, valid for the whole domain of slow neutrons from thermal to ultracold. Particular attention is given to multiple scattering which is the dominant process for ultracold neutrons (UCN). For thermal and cold neutrons, when the multiple scattering in the target can be neglected, the cross-section is reduced to the known value. A new expression for inelastic scattering cross-section for UCN is proposed. Dynamical processes in the target are taken into account and their influence on inelastic scattering of UCN is analyzed. Received 21 July 1999     

Bulk effects in the coherent inelastic scattering of ultracold neutrons

With the use of a theory developed earlier, bulk effects in ultracold neutron coherent inelastic scattering are considered both for solid and liquid target samples related to energy and momentum exchange with phonon and diffusion-like modes. For the neutron in a material trap, differential and integral probabilities for the energy transfer per bounce are presented in a simple analytic form which exhibits parameter dependence. As an example, the theoretical values for the ultracold-neutron loss rate from a storage bottle with Fomblin-coated walls and stainless-steel walls are evaluated. A possible contribution from incoherent inelastic scattering is discussed.     

A quantum wavepacket study of three-dimensional Ne—H+ 2 scattering

The state-to-state and state-to-all reaction probabilities have been calculated for three-dimensional reactive and inelastic scattering of Ne + H⁺ ₂ (v = 0, j = 0, 1, 2) at zero total angular momentum. The time-dependent Schrödinger equation is solved by means of Fourier grid and discrete variable representation techniques. The inelastic and reactive scattering probabilities for a broad range of energies are calculated. Although the system has an open reactive channel, the results show that a high fraction of the flux is reflected back due to endoergicity and potential barriers leading to inelastic scattering.     

Neutron lifetime and density of states of fluoropolymers at low temperatures

Present status of the measurements of the neutron lifetime is shortly reviewed. We report the inelastic neutron scattering measurement of the density of vibrational states G(ω) of two fluoropolymers, which are promising coating materials for the storage of ultracold neutrons (UCNs) in closed volumes covered with polymer film. From determined G(ω), we calculate the expected UCN loss coefficients.     

Backscattered electron spectra from graphite

The backscattered electron spectra from graphite sample were studied both experimentally and theoretically at impact energies between 500 and 5000 eV. The angle of the incident electron beam was 50° and the detection angle was 0° with respect to the surface normal, respectively. Monte Carlo (MC) simulations were performed based on the Classical Transport Theory (CTT) model to mimic the experimental spectra. In our simulations, both elastic and inelastic scattering of primary electrons and secondary electron emission from graphite are taken into account. There is found satisfactory agreement between measured and calculated electron spectra.     

Temperature dependence in atom–surface scattering

It is shown that a straightforward measure of the temperature dependence of energy resolved atom–surface scattering spectra measured under classical conditions can be related to the strength of the surface corrugation. Using classical perturbation theory combined with a Langevin bath formalism for describing energy transfer, explicit expressions for the scattering probabilities are obtained for both two-dimensional, in-plane scattering and full three-dimensional scattering. For strong surface corrugations results expressed as analytic closed-form equations for the scattering probability are derived which demonstrate that the temperature dependence of the scattering probability weakens with increasing corrugation strength. The relationship to the inelastic rainbow is briefly discussed.     

Brillouin and Umklapp scattering in polybutadiene: comparison of neutron and x-ray scattering

We report a comparison of high resolution inelastic x-ray Brillouin scattering to coherent inelastic neutron scattering for amorphous deuterated polybutadiene, done for one temperature in the glass phase and another one in the melt. The x-ray scattering proves to be by far the better technique for such a polymer within its present resolution bounds. The neutron scattering allows one to extend these measurements to a much better resolution, showing an additional quasielastic signal in the melt. The results suggest x-ray measurements at higher momentum transfer, to see whether they are complementary to neutrons.     

New type of low temperature source of Ultra-cold neutrons and production of continous beams of UCN

We discuss a new type of source for Ultra-cold neutrons (UCN) in which the UCN are produced in a thin film on the walls of a cryogenic container. The UCN build up to a significant density inside the container, and the build-up time can be adjusted without effecting the UCN density. Applications to the production of intense, continous beams of UCN for scattering experiments are emphasized. The new source is well suited for installation inside the moderator of an intense neutron source.     

Comparative analysis of characteristic electron energy loss spectra and inelastic scattering cross-section spectra of Fe

The inelastic electron scattering cross section spectra of Fe have been calculated based on experimental spectra of characteristic reflection electron energy loss as dependences of the product of the inelastic mean free path by the differential inelastic electron scattering cross section on the electron energy loss. It has been shown that the inelastic electron scattering cross-section spectra have certain advantages over the electron energy loss spectra in the analysis of the interaction of electrons with substance. The peaks of energy loss in the spectra of characteristic electron energy loss and inelastic electron scattering cross sections have been determined from the integral and differential spectra. It has been shown that the energy of the bulk plasmon is practically independent of the energy of primary electrons in the characteristic electron energy loss spectra and monotonically increases with increasing energy of primary electrons in the inelastic electron scattering cross-section spectra. The variation in the maximum energy of the inelastic electron scattering cross-section spectra is caused by the redistribution of intensities over the peaks of losses due to various excitations. The inelastic electron scattering cross-section spectra have been analyzed using the decomposition of the spectra into peaks of the energy loss. This method has been used for the quantitative estimation of the contributions from different energy loss processes to the inelastic electron scattering cross-section spectra of Fe and for the determination of the nature of the energy loss peaks.     

Monte Carlo simulation of quasi-elastic scattering and above-barrier neutrons in the neutron lifetime experiment MAMBO I

Motivated by the strong disagreement of a recent result for the neutron lifetime with the previous world average value we report results of a Monte Carlo simulation of the neutron lifetime experiment MAMBO I, which was carried out some 20 years ago. In addition to all experimental parameters and procedures known to us, the analysis included quasi-elastic neutron scattering on the surface of liquid fomblin oil wall coatings of the UCN storage vessel, and above-barrier neutrons. The original analysis, leading to the published result of 887.6 ± 3 s, did not take into account these effects. For an exemplary set of model parameters we find a negative correction of 6.0 seconds, which demonstrates that these hitherto neglected effects may be very important also in the analysis of other neutron lifetime experiments using UCN storage vessels with fomblin oil coating close to room temperature.     

Relative contribution of various factors to the formation of the energy spectrum of fast, medium-energy, charged particles and ions transmitted through a thin target with fluctuations of the target thickness

The characteristics of the energy spectra of kiloelectron-volt protons transmitted through a free-standing foil are investigated theoretically and experimentally as functions of the angle of incidence of the beam on the target. Analytical expressions for the average characteristics of the transmitted-particle energy spectrum are determined for the case of small-angle scattering. The combined influence of various factors affecting the formation of the energy spectra is taken into account: systematic stopping of particles in the medium, fluctuations of the particle energy losses in inelastic collisions, bending of the particle trajectories due to multiple elastic scattering, and fluctuations of the target thickness. It is shown that the contributions of these factors to the width of the transmitted-particle energy spectrum depend differently on the angle of incidence of the beam on the target surface. On the basis of this differentiation it is inferred from the experimental dependence of the width of the energy spectra of kiloelectron-volt protons transmitted through a free-standing foil on the angle of incidence of the beam that fluctuations of the particle energy losses in inelastic collisions are the predominant factor in the formation of the proton energy spectra. Zh. Tekh. Fiz. 67, 81–93 (May 1997)     

Inelastic phonon interactions at solid–graphite interfaces

The presented model predicts thermal boundary conductance at interfaces where one material comprising the junction is characterized by high elastic anisotropy. In contrast to previous approaches, the current methodology accounts for contributions from inelastic scattering through consideration of multiple-phonon interactions. Inelastic contributions become significant as the temperature, as well as the degree of acoustic mismatch between the materials, increases. Inclusion of the inelastic interactions is necessary for a variety of interfacial systems including the metal–graphite boundary examined here. Improvement is shown over existing approaches that address only elastic scattering as both three- and four-phonon interactions significantly augment the transport.     

Simultaneous measurement of lithium and fluorine momentum in (7)LiF

We present momentum widths and mean kinetic energies of lithium and fluorine in (7)LiF, as determined simultaneously from deep inelastic neutron scattering (DINS) measurements. Experimental data across a temperature range from 4 to 300?K are presented, and these results compared to those calculated using a quasi-harmonic density-functional approach. In all cases, measured momentum widths are seen to be within approximately 5% of those calculated, despite the very low scattering cross sections of both (7)Li and (19)F. This is the first determination and comparison with theory of such simultaneous measurements for nuclei of mass >?4?amu, and demonstrates the implementation of the DINS method in its current form as a mass-selective neutron spectroscopy.     

Closed-form quantal description of inelastic heavy ion scattering

The amplitude for inelastic heavy ion scattering, given by the distorted-wave theory for excitation of low-lying collective states, is evaluated in closed form. Use is made of the Austern-Blair relation and of other approximations appropriate for strongly absorptive interaction to express the inelastic partial-wave amplitude entirely in terms of the elastic S-matrix elements in the initial and final channels. The resulting formulae display explicitly the various contributions to the transition amplitude, whose superposition gives rise to the variety of interference patterns observable in the angular distributions and excitation functions of inelastic heavy ion scattering. It is shown that, as for elastic scattering, the dominant mechanism in inelastic heavy ion collisions near and above the Coulomb barrier is diffractive scattering of Fresnel type.     

Temperature effect for an inelastic neutrino scattering cross section

The influence of temperature on inelastic neutrino scattering on hot nuclei is studied with the ⁵⁴Fe nucleus as an example. The strength distribution of charge-neutral Gamov-Teller transitions in ⁵⁴Fe at finite temperature is calculated within the framework of a random phase approximation using the thermo field dynamics formalism. It is shown that for neutrino energies lower than the energy of the Gamov-Teller resonance, the inelastic scattering cross section depends substantially on temperature.     

Dynamics of hydrogen-bonded water networks under high pressure: Neutron scattering and computer simulation

Abstract

Molecular dynamics simulation of proton-ordered high pressure ice modifications II and IX was performed. Dynamics of both isotope varieties, H₂O and D₂O, was simulated. Rectangular simulation box of ice II contained 576 and that of ice IX 768 molecules. The average kinetic energy corresponded to 82 and 201 K for ice II and to 87 and 203 K for ice IX. One-phonon densities of states were calculated via Fourier transformation of velocity autocorrelation functions and compared with those found experimentally from inelastic incoherent neutron scattering. This characteristic was calculated for all the molecules, as well as for the molecules of a particular crystallographic type. Both simulated ice modifications contain molecules of two different structural types. Dynamic characteristics of molecules of different types are slightly different. Splitting of the librational peak at about 60–70meV observed in the ice II experimental spectrum is mainly due to such difference. In the case of ice II simulated spectra reproduce experimental ones quite reasonably in the whole range of energies, while in the case of ice IX agreement with the experiment is worse.     

Measurement of cross sections for inelastic cold-neutron scattering in metals and polymers by the method of (<Emphasis Type="Italic">n, γ</Emphasis>) analysis

The results obtained by measuring the cross sections for the inelastic scattering of very cold neutrons for a number of metals and polymers by the method of a neutron-irradiation analysis are presented. The method is based on simultaneously measuring events of inelastic scattering and neutron capture in the sample under investigation via recording gamma radiation with a semiconductor germanium detector. Neutron capture by a nucleus of the sample is accompanied by the prompt radiation of gamma rays having a known spectrum. Upon inelastic scattering, a neutron acquires thermal energy. Upon leaving the sample, this neutron is absorbed in a special converter that contains the isotope ¹⁰B. The capture of the neutron by a ¹⁰B nucleus is followed by the emission of a 477-keV gamma ray. The probabilities of capture and inelastic scattering are proportional to the respective neutron-interaction cross sections, and the ratio of the recorded detector counts corresponding to events of the two types does not depend on the spectrum of the incident flux of very cold neutrons or on the trajectory of neutron motion in the sample. The sought inelastic-scattering cross section at a fixed sample temperature is calculated by using this ratio and the known cross section for neutron capture by the sample isotope having a known gamma-radiation spectrum.