Neutron scattering and neutron emission cross sections on 27Al, 93Nb and 209Bi at 7 MeV

Elastic and inelastic neutron scattering and neutron emission at 7 MeV incident energy has been studied for monoisotopic samples of ²⁷Al, ⁹³Nb and ²⁰⁹Bi. Time-of-flight spectra were taken at nine angles between 30° and 140° using the beam-swinger spectrometer. The efficiency of the neutron detectors was maximized using a dynamic bias; low background was obtained throughout the neutron energy detected which correspond to about 6 MeV excitation energy. The data were converted to energy spectra and corrected for sample attenuation, finite geometry and multiple scattering. For unresolved states the data were averaged over 0.5 MeV energy bins. Calculations for elastic and inelastic transitions and for compound and precompound processes are presented.     

High-spin states in 166Lu

It is shown that the discrepancy between the results obtained for different neutron energy ranges, when neutron polarizability is derived from the neutron scattering data, can be removed if one assumes that at the fast neutron scattering a strong-interaction long-range potential of Van der Waals (～ r ^?6 ) or Casimir-Polder (～ r ^?7 ) is observed. This strong-interaction long-range potential has possibly some experimental confirmation in the elastic p-p scattering.     

Resonance neutron capture in 56Fe

The neutron capture cross section of ⁵⁶Fe has been measured with 0.2–0.3% energy resolution from 2.5 keV up to the inelastic neutron scattering threshold. Results are compared with recent total cross-section data and average parameters are derived for s-, p- and d-wave resonances. The low correlation coefficient observed between the s-wave reduced neutron and radiative widths is consistent with the minor contribution of the valence capture mechanism as calculated in the framework of the optical model. Broad E1 and M1 doorway states for s-, p- and d-wave resonances are postulated to explain the cross-section data and γ-ray spectra up to 1 MeV.     

Interactions of neutrons with239Pu in the energy range between 1.5 and 5.5 MeV

Neutron scattering angular distributions of²³⁹Pu were measured at seven primary neutron energies between 1.5 and 5.5 MeV. Absolute differential scattering cross-section results are plotted and tabulated. Cross section calculations using a central optical model were made and the results were compared with the experimental values. Moreover, nuclear temperatures of the fission and the inelastic neutron evaporation processes were extracted from the measured spectra.     

Angular-distribution neutron-emission spectra of niobium following bombardment by fast neutrons

Neutron-emission spectra at ten angles between 20 and 160 degrees and incident neutron energies of 5.9, 7.1 and 8.4 MeV were measured relative to the neutron field emitted after spontaneous fission of ²⁵²Cf. The angular distribution of inelastically scattered neutrons appears to be essentially isotropic where it can be separated from the dominant elastic scattering peak. Above an excitation energy of 4 MeV the spectra are well described by a Maxwellian temperature distribution.     

Form of the long-range potential observed in fast-neutron scattering by heavy nuclei

It is shown that the discrepancy between the results obtained for different neutron-energy ranges, when the neutron polarizability is derived from data on neutron scattering, can be removed if one assumes that a strong-interaction long-range potential of van der Waals (～r ^?6) or of Casimir and Polder (～r ^?7) is observed in fast-neutron scattering. This strong-interaction long-range potential possibly has some experimental confirmation in elastic pp scattering.     

Fast neutron scattering from rubidium

A study was made of the (n, n), (n, n′), and (n, n′ γ) reactions which occur when a natural rubidium scattering sample is exposed to mono-energetic neutron fluxes of energies ranging from 120 to 1910 keV. Total cross sections, elastic scattering angular distributions and excitation functions for inelastic scattering were measured with a neutron time-of-flight spectrometer. Results are compared with Hauser-Feshbach theory. A 40 cc Ge(Li) detector was used in the time-gated mode to measure γ-rays from the (n, n′ γ) reaction. The latter measurements were used to refine and extend the energy level schemes derived from neutron spectroscopy. Several new levels were discovered in the low-lying (below 1900 keV) energy level spectra of⁸⁵Rb and⁸⁷Rb. Gamma decay schemes and branching ratios were determined for the low-lying levels of⁸⁵Rb and⁸⁷Rb.     

Multiple scattering of low-energy rare-gas ions at solid surfaces

We have studied the multiple scattering of He⁺, Ne⁺, and Ar⁺ from a TaC(001) surface in the energy region of the order of 1 keV. The experimental data revealed large differences between the energy spectra of these ions. The spectral peak corresponding to quasi-double scattering is clearly observed for Ar⁺ but not observed for Ne⁺. The multiple scattering effect appearing in the energy spectra of rare-gas ions is discussed on the basis of electron exchange between these ions and solid surfaces.     

A polarised SUSANS facility to study magnetic systems

Using a right-angled magnetic air prism, we have achieved a separation of ～10 arcsec between ～2 arcsec wide up and down-spin peaks of 5.4 Å neutrons. The polarised neutron option has thus been introduced into the SUSANS instrument. Strongly spindependent SUSANS spectra have been observed over ±1.3 × 10^?4 Å^?1 range for several magnetic alloy samples. Spatial pair-distribution functions for the up and down-spins as well as the nuclear and magnetic scattering length density distributions in the micrometer domain, have been deduced from these spectra.     

Spectrum of vibrational frequencies of crystalline tungsten at temperatures of 293 and 2400 K

The slow neutron inelastic scattering spectra for a refractory (T _melt = 3680 K) Group VI transition metal of the Periodic Table, namely, tungsten, were measured for the first time in the range from room temperature to 2400 K. Measurements of the neutron scattering spectra of tungsten were performed on a DIN-2PI time-of-flight spectrometer installed at the IBR-2 reactor (Dubna, Russia). The sample was heated in a TS3000 K high-temperature thermostat. The spectrum of vibrational frequencies of the crystal lattice of tungsten at temperatures of 293 and 2400 K was determined from the measured neutron scattering spectra by the iterative method. A softening of the frequency spectrum of tungsten was observed with increasing temperature. This was explained by the increasing role of vibrational anharmonicity effects at high temperatures. The experimental results were compared with model calculations of the frequency spectrum of tungsten.     

The deuteron breakup reaction induced by protons of 65, 85 and 100 MeV

Coplanar energy sharing spectra for p + d breakup at 65, 85 and 100 MeV proton bombarding energies were measured using the University of Maryland sectored isochronous cyclotron, by measuring the energies of either two protons or one proton and one neutron in coincidence. The detector angles were chosen to enhance either the p-p or p-n quasifree scattering, or the p-n final state interaction. The energy dependence of the peak cross section at equal symmetric quasifree scattering angle pairs was extracted for the ²H(p, 2p)n and ²H(p, pn)p reactions. Quasifree angular distributions were obtained for the reaction ²H(p, 2p)n at 65 MeV and for the reaction ²H(p, pn)p at 65, 85 and 100 MeV. The plane wave impulse approximation theory can only qualitatively reproduce the shape of the quasifree scattering peak in the energy sharing spectra and the shape of the p-p quasifree angular distribution. The discrepancies observed between the plane wave impulse approximation theory and the experimental data imply that the presence of the spectator particle (i.e., the multiple scattering effects) has a strong influence on the magnitude and the shape of the experimental results. Multiple scattering calculations were carried out in the three-body model of Aaron, Amado and Yam except that the S-wave separable two-body amplitudes were modified to fit two-nucleon elastic scattering data at higher energies. Comparisons of the results of these multiple scattering calculations to the experimental data show excellent quantitative agreement throughout the energy range and the angular region of this experiment, except for a few cases in which this model is inherently insufficient; namely, regions in which the Coulomb interaction is important, or regions for which a Hulthén wave function is inaccurate and the off-shell effects are not properly taken into account.     

Magnetic excitation spectra in BaFe2As2: a two-particle approach within a combination of the density functional theory and the dynamical mean-field theory method

We study the magnetic excitation spectra in the paramagnetic state of BaFe(2)As(2) from the ab initio perspective. Both the one-particle and the magnetic two-particle excitation spectra are determined within the combination of the density functional theory and the dynamical mean-field theory method. This method reproduces all the experimentally observed features in inelastic neutron scattering and relates them to both the one-particle excitations and the collective modes. The magnetic excitation dispersion is well accounted for by our theoretical calculation in the paramagnetic state without any broken symmetry; hence, nematic order is not needed to explain the inelastic neutron scattering experimental data.     

The reaction mechanism of the 7Li(n,tα)n reaction at En = 14.4 MeV

The correlation spectra of the three-body break-up of the n+⁷Li system have been studied at a neutron energy of 14.4 MeV. The analysis of the data reveals the predominant role of sequential processes. The Phillips-Griffy-Biedenharn theory has been applied to fit the data. The contribution of quasi-free scattering has also been observed and analysed in the frame of the PWIA.     

Small-angle neutron scattering and the Mössbauer effect in nitrogen austenite

Small-angle neutron scattering (SANS) in Fe-19%Cr-19%Mn-0.9%N and Fe-18%Cr-10%Mn-16%Ni-0.5%N (wt %) high-nitrogen austenites is investigated. The neutron diffraction patterns and Mössbauer spectra are measured and analyzed. The SANS curves are approximated in the framework of the Guinier and Porod models. The nature of small-angle neutron scattering in nitrogen austenite is explained in terms of microinhomogeneities whose structure depends on the alloying with nickel. The mean size and shape of inhomogeneities are evaluated. The effect of low-temperature annealing of the Fe-19%Cr-19%Mn-0.9%N austenite on small-angle neutron scattering and the parameters of the Mössbauer spectra is analyzed.     

Diffraction limit of the theory of multiple small-angle neutron scattering by a dense system of scatterers

Multiple small-angle neutron scattering by a high-density system of inhomogeneities has been considered. A combined approach to the analysis of multiple small-angle neutron scattering has been proposed on the basis of the synthesis of the Zernike–Prince and Moliére formulas. This approach has been compared to the existing multiple small-angle neutron scattering theory based on the eikonal approximation. This comparison has shown that the results in the diffraction limit coincide, whereas differences exist in the refraction limit because the latter theory includes correlations between successive scattering events. It has been shown analytically that the existence of correlations in the spatial position of scatterers results in an increase in the number of unscattered neutrons. Thus, the narrowing of spectra of multiple small-angle neutron scattering observed experimentally and in numerical simulation has been explained.     

Hydrostatic pressure and temperature dependence study of d-camphor and dl-borneole by neutron scattering

Abstract

Neutron scattering investigations have been performed down to helium temperatures under different hydrostatic pressures up to 330 MPa. Neutron diffraction (ND) and incoherent inelastic neutron scattering (IINS) spectra showed clearly the existence of a phase transition in d-camphor, which was not observed in dl-borneole. The ND spectra point to a possible cubic structure of the dl-borneole crystal down to helium temperatures. Pressure and temperature dependence of the IINS spectra assumed a glass-like transition from the dynamical to static orientational disorder of molecules in this crystal.     

A Gorizont neutron reflectometer—small-angle spectrometer based on the IN-06 pulsed neutron source

The Gorizont time-of-flight neutron reflectometer—small-angle spectrometer has been designed, manufactured, and assembled to operate with the IN-0 6 pulsed neutron source. The instrument has a vertical neutron scattering plane enabling research on liquid surfaces and interfaces. Model calculations of neutron spectra, beam profiles, and spectrometer resolutions have been performed via the Monte Carlo method. The spectrometer operates at wavelengths from 1.5 to 9 Å and ensures measurements in the momentum-transfer range of 0.003—1.5 Å^-1.     

Comparison of unfolding approaches for monoenergetic and continuous fast-neutron energy spectra

Proton-recoil detectors offer the possibility to unfold fast-neutron energy spectra of various sources. However, quantifying the confidence of the unfolding methodology is a complex task. In this paper, we present a comparative analysis of the maximum-likelihood, expectation-maximization (MLEM) method and one-step-late (OSL) method for neutron energy spectra unfolding. The analysis is performed on Monte Carlo simulated data for several monoenergetic neutron sources and continuous-in-energy ²⁵²Cf, ²⁴¹Am–Be and ²⁴¹Am–Li neutron sources. The results obtained for the monoenergetic neutron spectra show that both unfolding methods provide results that are in good agreement with the reference data. Very good agreement between the unfolded and the reference data is achieved for ²⁵²Cf, ²⁴¹Am–Be, and ²⁴¹Am–Li neutron spectra by using the OSL method. In the paper it is demonstrated that the MLEM and OSL methods can be applied to accurately unfold the simulated pulse-height distributions for organic liquid scintillation detectors. Comparative analysis between the two unfolding methods has shown that the OSL method has superior unfolding performance than the MLEM method.     

Acoustic dissipation and density of states in liquid, supercooled, and glassy glycerol

Combined Brillouin spectra collected at visible, ultraviolet, and x-ray frequencies are used to reconstruct the imaginary part of the acoustic compliance J' over a wide frequency range between 0.5 GHz and 5 THz. For liquid, supercooled, and glassy glycerol, J' is found to be linearly dependent on the tagged-particle susceptibility measured by incoherent neutron scattering up to ?1 THz, giving evidence of a clear relation between acoustic power dissipation and density of states. A simple but general formalism is presented to quantitatively explain this relation, thus clarifying the connection between the quasielastic component observed in neutron scattering experiments and the fast relaxation dynamics probed by Brillouin scattering.     

Excitation and neutron decay of xenon studied by forward-angle scattering of high-energy 17O ions

Excitation energy spectra and neutron decay of xenon have been measured in extreme forward angle scattering of 250A and 400A MeV ¹⁷O ions, using one quadrant of the CELSIUS storage ring as a magnetic spectrometer. The observed excitation energy spectrum, ranging from 12 to 36 MeV, has been compared with Coulomb cross section calculations. Neutron and missing energy spectra are compared with statistical decay calculations using the code CASCADE. In addition to statistical decay from giant resonances and other continuum excitations, a large fraction of forward peaked fast neutrons was observed throughout the entire excitation energy region. A possible reaction mechanism behind these fast neutrons is discussed.