Elastic Neutron Scattering Cross Sections Research Articles

Antiferromagnets with random vacancies and substitutional spins on the triangular lattice

We discuss theoretically static and dynamical properties of $XY$ and Heisenberg antiferromagnets on triangular lattice with random vacancies and substitutional spins. It is shown that the distortion of $120^\circ$ magnetic order produced by a single defect is described by electrostatic equations for a field of an electrically neutral complex of six charges located around the impurity. The first finite term in the multipole expansion of this field is the octupole moment which decays as $1/r^3$ with the distance $r$. The linearity of equations allows to describe analytically the distortion of the long-range magnetic order at a small concentration $c$ of defects. We obtain analytically renormalization of the elastic neutron scattering cross section and the magnon spectrum $\epsilon_{\bf k}$ in the leading order in $c$. We find that the scattering on impurities renormalizes weakly the bare spectrum $\epsilon_{\bf k}\propto k$ at $k\gg\sqrt c$. However the renormalization is substantial of the long-wavelength magnon spectrum at $k\ll\sqrt c$: $\epsilon_{\bf k}\propto \sqrt{c /\ln(1/k)}$ at $k\to0$ and there is a parametrically large region in which magnons with not too small momenta are overdamped and localized. This strong modification of the long-wavelength spectrum leads to the stabilization of the slightly distorted magnetic long-range order at $T<T_N\sim S^2J/\ln(1/c)$ and to the considerable change in the density of states and in the specific heat. The overdamped modes arise also in quasi-2D spin systems on a stacked triangular lattice.

Nuclear-recoil energy scale in CDMS II silicon dark-matter detectors

The Cryogenic Dark Matter Search (CDMS II) experiment aims to detect dark matter particles that elastically scatter from nuclei in semiconductor detectors. The resulting nuclear-recoil energy depositions are detected by ionization and phonon sensors. Neutrons produce a similar spectrum of low-energy nuclear recoils in such detectors, while most other backgrounds produce electron recoils. The absolute energy scale for nuclear recoils is necessary to interpret results correctly. The energy scale can be determined in CDMS II silicon detectors using neutrons incident from a broad-spectrum 252Cf source, taking advantage of a prominent resonance in the neutron elastic scattering cross section of silicon at a recoil (neutron) energy near 20 (182)keV. Results indicate that the phonon collection efficiency for nuclear recoils is 4.8−0.9+0.7% lower than for electron recoils of the same energy. Comparisons of the ionization signals for nuclear recoils to those measured previously by other groups at higher electric fields indicate that the ionization collection efficiency for CDMS II silicon detectors operated at ∼4V/cm is consistent with 100% for nuclear recoils below 20keV and gradually decreases for larger energies to ∼75% at 100keV. The impact of these measurements on previously published CDMS II silicon results is small.

Reactivity Impact of 2H and 16O Elastic Scattering Nuclear Data on Critical Systems with Heavy Water

The accuracy of deuterium nuclear data is important for reactor physics simulations of heavy water (D2O) reactors. The elastic neutron scattering cross section data at thermal energies, σs,th, have been observed to have noticeable impact on the reactivity values in simulations of critical systems involving D2O. We discuss how the uncertainties in the thermal scattering cross sections of 2H(n,n)2H and 16O(n,n)16O propagate to the uncertainty of the calculated neutron multiplication factor, keff, in thermal critical assemblies with heavy water neutron moderator/reflector. The method of trial evaluated nuclear data files, in which specific cross sections are individually perturbed, is used to calculate the sensitivity coefficients of keff to the microscopic nuclear data, such as σs(E) characterized by σs,th. Large reactivity differences of up to ≃ 5–10 mk (500–1000 pcm) were observed using 2H and 16O data files with different elastic scattering data in MCNP5 simulations of the LANL HEU heavy-water solution thermal critical experiments included in the ICSBEP handbook.

Reactivity Impact of 16O Thermal Elastic-Scattering Nuclear Data for Some Numerical and Critical Benchmark Systems

The thermal neutron elastic-scattering cross-section data for 16O used in various modern evaluated nuclear data libraries were reviewed and found to be generally too high compared with the best available experimental measurements. Some of the proposed revisions to the ENDF/B-VII.0 16O data library and recent results from the TENDL system increase this discrepancy further. The reactivity impact of revising the 16O data downward to be consistent with the best measurements was tested using the JENDL-3.3 16O cross-section values and was found to be very small in MCNP5 simulations of the UO2 and reactor-recycle MOX fuel cases of the American Nuclear Society Doppler defect numerical benchmark. However, large reactivity differences of up to [approximately]14 mk (1400 pcm) were observed using 16O data files from several evaluated nuclear data libraries in MCNP5 simulations of the Los Alamos National Laboratory highly enriched uranium (HEU) heavy water solution thermal critical experiments, which were performed in the 1950s. The latter result suggests that new measurements using HEU in a heavy water-moderated critical facility, such as the ZED-2 zero-power reactor at the Chalk River Laboratories, might help to resolve the discrepancy between the 16O thermal elastic-scattering cross-section values and thereby reduce or better define its uncertainty, although additional assessment work would be needed to confirm this.

Covariance Matrix of a Double-Differential Doppler-Broadened Elastic Scattering Cross Section

Legendre moments of a double-differential Doppler-broadened elastic neutron scattering cross section on 238U are computed near the 6.67 eV resonance at temperature T = 103 K up to angular order 14. A covariance matrix of these Legendre moments is computed as a functional of the covariance matrix of the elastic scattering cross section. A variance of double-differential Doppler-broadened elastic scattering cross section is computed from the covariance of Legendre moments.

Neutron Elastic Scattering Cross-Section Measurements at 175 MeV

An off-line data analysis of an experimental campaign on elastic neutron cross-section measurements at 175 MeV for iron and bismuth performed by the SCANDAL set-up has started. Energy calibration has shown expected values of energy resolution and MCNPX simulations have provided suggestions for further analysis.

Measurement of neutron elastic scattering cross sections for carbon at 134 MeV

Abstract Angular differential neutron elastic scattering cross sections have been measured by using a 7 Li(p,n) quasi-monoenergetic neutron beam. The neutron beam whose peak energy is 134 MeV was incident upon a natural carbon sample, and the scattering neutrons were measured by liquid organic scintillators with the time-of-flight (TOF) technique. The results were compared with the existing experimental data measured by a recoil-proton telescope, and the evaluated nuclear data of JENDL/HE-2007 and ENDF/B-VII.0.

MCNPX simulations of the SCANDAL setup for measurement of neutron scattering cross section at 175 MeV

The Scattered Nucleon Detection Assembly (SCANDAL) setup at The Svedberg Laboratory has been used to produce neutron elastic scattering cross section data at 175 MeV for bismuth and iron. This work presents MCNPX simulations of the experimental setup and aims to describe processes and data important for the upcoming off-line data analysis. In the experiment, neutrons scattered off the target are converted to protons, which are stopped in scintillator crystals. The results include a description of the proton spectra dependence on the neutron-to-proton conversion angle, suggesting a cut at a conversion angle of 15.2°. Calculation of the hit position gates indicates high proton leakage from the crystals. A study of the converter describes the role of its chemical composition and also the role of other plastic scintillators on the proton spectra. The neutron-to-proton conversion efficiency of the converter simulated by MCNPX is 5.1×10−4 and corresponds to theoretical predictions.

Neutron Scattering from an Asymmetric Rotor: 197Au

Differential neutron elastic-scattering cross sections of 197Au are measured from ≈4.5 to 10.0 MeV at incident-neutron energy intervals of ≈0.5 MeV. These results are combined with previous lower-energy work by the author and associates to form a neutron-scattering database extending from ≈0.3 to 10.0 MeV. A few elastic-scattering distributions and total cross sections from the literature are added to it, and the composite is interpreted in terms of optical-statistical, dispersion, and coupled-channels models. The results are compared with models in the literature and with relevant portions of the ENDF/B-VI nuclear data file. A collective rotational model for the prediction of neutron interactions in this mass-energy region is suggested.

Measurement of neutron elastic scattering cross sections forC12,Ca40, andPb208at energies from65to225MeV

Differential neutron elastic scattering cross sections for $^{12}\mathrm{C}$, $^{40}\mathrm{Ca}$, and $^{208}\mathrm{Pb}$ have been measured using the continuum neutron source at the Los Alamos Neutron Science Center. The mean incident neutron kinetic energies ranged from $65\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}225\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$. The large acceptance drift chamber based $(n,p)$ recoil detector telescope spanned a center-of-mass angular range of 7\ifmmode^\circ\else\textdegree\fi{} to 23\ifmmode^\circ\else\textdegree\fi{}. The measured cross sections are compared with calculations obtained with different optical model potentials, including predictions based on microscopic theory and phenomenology.

Experiments on Neutron Scattering and Fission Neutron Spectra

Experiments on the 1) neutron elastic scattering for 55, 65 and 75 MeV neutrons, and 2) prompt fission neutron spectrum for MeV neutrons are described. The neutron elastic scattering cross sections have been measured for C, Si, Fe, Zr, and Pb at the 7Li(p, n) quasi-monoenergetic neutron source facility at TIARA, JAERI using the TOF method. Data were obtained at twenty-five laboratory angles between 2.6° and 53° that could clarify the angular distributions and angle-integrated values. Fission spectrum data were obtained for 233U, 238U and 232Th at 0.6, 1.9 and 4.1 MeV. The data at 1.9 and 4.1 MeV were given in absolute scale by taking the 238U data as the standard to clarify the problem of data files.

The interaction of fast neutrons with hafnium

Elemental hafnium neutron total cross sections are measured from ≈0.75 to 4.5 MeV in steps of ≈40 keV. Differential neutron elastic-scattering cross sections are measured from ≈4.5 to 10.0 MeV in ≈0.5 MeV steps and at 40 scattering angles distributed between ≈17 and 160°. These data are combined with those found in the literature to construct a comprehensive experimental data base which is interpreted in terms of coupled-channels models. Physical characteristics of the resulting potentials are discussed. Comparisons are made with ENDF/B-6 (MAT 7200). A detailed report of this work is given by Smith (Smith, A., 2001. Argonne National Laboratory Report ANL/NDM-153).

Measurements of Elastic Scattering Cross Sections of Carbon, Silicon, Iron, Zirconium and Lead for 75 MeV Neutrons

We performed measurements of neutron elastic scattering cross sections of carbon, silicon, iron, zirconium and lead at 75 MeV using a7Li(p,n) quasi-monoenergetic neutron source at TIARA of JAERI. Neutron spectra at 25 laboratory angles between 2.6°and 53.0° were measured by a time-of-flight method (TOF) using five liquid scintillation detectors. The data were corrected for the inelastic scattering neutrons and sample size effects. The experimental data were compared with the neutron cross section libraries (DLC119/HIL086, LA 150) and systematics used in cascade/transport codes (HETC-KFA2, NMTC/JAERI). The DLC119 data and sytematics show large discrepancy from the present data, while the LA 150 data are in fair agreement.

Fast neutrons incident on titanium

Differential neutron elastic-scattering cross sections of elemental titanium were measured from 4.5 to 10.0 MeV in incident-energy increments of and at forty or more scattering angles distributed between and degrees. Concurrently, differential neutron inelastic-scattering cross sections were measured for observed excitations of , , , , , and . All of the observed inelastic neutron groups were composites of contributions from several isotopes and/or levels. The experimental results were used to develop energy-average optical, statistical and coupled-channels models.

Neutron scattering from elemental silver

Differential neutron elastic-scattering cross sections of elemental silver were measured from 1.5 → 10 MeV at scattering angles distributed between ≈17° and 160°. Cross sections for ten inelastically scattered neutron groups were determined, corresponding to observed excitations of 328 ± 13, 419 ± 50, 748 ± 25, 908 ± 26, 1150 ± 38, 1286 ± 25, 1507 ± 20, 1623 ± 30, 1835 ± 20, and 1944 ± 26 keV, all of which were composites of contributions from the two isotopes 107Ag and 109Ag. The experimental results were interpreted in terms of the spherical-optical and rotational and vibrational coupled-channels models.

Neutron scattering and models: chromium

Neutron elastic-scattering cross sections of elemental chromium are measured from 4.5 - 10 MeV in steps of MeV and at scattering angles distributed between and . Concurrently cross sections for the inelastic neutron-excitation of the yrast (1.434 MeV) level in are determined. The experimental results are combined with lower-energy values previously reported from this group and with results from the literature to form an extensive scattering database which is interpreted in the context of spherical-optical and coupled-channels models. The physical implications of the measurements and models are discussed including; deformation, coupling, dispersive and asymmetry effects.

Fast-neutron scattering from vibrational palladium nuclei

Differential neutron elastic-scattering cross sections are measured from ≈ 1.5 to 10 MeV in sufficient energy and angle detail to define the energy-average behavior. Concurrently, neutron inelastic-scattering cross sections are measured from ≈ 1.5 to 8 MeV. Inelastically-scattered neutron groups are observed corresponding to excitations of: 306 ± 14, 411 ± 47, ≈ 494, 791 ± 20, 924 ± 20, 1156 ± 24, 1358 ± 35, 1554 ± 47 and 1706 ± 59 keV, with additional tentative groups at 1938 and 2059 keV. Particular attention is given to the inelastic excitation of the 2 + yrast states of the even isotopes. This broad data base is examined in the context of optical-statistical and coupled-channels models. The resulting model parameters are consistent with systematic trends in this vibrational mass region and provide a suitable vehicle for many applications.

Fast-neutron total and scattering cross sections of 58Ni and nuclear models

Neutron total cross sections of 58Ni are measured from approximately=1 to >10 MeV using white-source techniques. Differential neutron elastic scattering cross sections are measured from approximately=4.5 to 10 MeV at approximately=0.5 MeV intervals with >or=75 differential values per distribution. Differential neutron inelastic scattering cross sections are measured, corresponding to fourteen levels with excitations up to approximately=4.8 MeV. The measured results, combined with relevant data available in the literature, are interpreted and the physical implications discussed in the contexts of optical-statistical, dispersive-optical and coupled-channels models.

Neutron scattering from elemental indium: Optical model and bound-state potential.

Neutron elastic-scattering cross sections of indium are measured from 4.5 to 10 MeV at intervals of \ensuremath{\approxeq}500 keV. Seventy or more differential values are obtained at each incident energy, distributed between \ensuremath{\approxeq}18\ifmmode^\circ\else\textdegree\fi{} and 160\ifmmode^\circ\else\textdegree\fi{}. These are combined with lower-energy data previously obtained at this laboratory, and with 11- and 14-MeV results from the literature, to form a comprehensive elastic-scattering database extending from \ensuremath{\approxeq}1.5 to 14 MeV. These data are interpreted in terms of a conventional spherical optical model. The resulting potential is extrapolated to the bound-state regime. It is shown that in the middle of the 50--82 neutron shell, the potential derived from the scattering results adequately describes the binding energies of particle states, but does not do well for hole states. The latter shortcoming is attributed to hole states having occupational probabilities sufficiently different from unity so that the exclusion principle becomes a factor, to rearrangement of the neutron core, and to the fact that the shell-model potential was assumed to have an energy-independent geometry. The systematic behavior of the real optical potential is discussed, and it is shown that the isovector strength deduced from neutron scattering is consistent with the nucleon-nucleon scattering data when a mass dependence of the radius is used.

The Interaction of Fast Neutrons with Beryllium

Neutron total cross sections of elemental beryllium are measured from 1 to 10 MeV with good precision. Differential neutron elastic scattering cross sections are measured from 4.5 to MeV at intervals of {approx}0.5 MeV and at {approx}100 angular steps distributed between {approx}18 and 160 deg at each incident energy. Concurrently, differential cross sections for the emission of a discrete inelastic neutron group corresponding to an excited level at (2.43 {plus minus} 0.06) MeV are determined over the same incident energy and angular range. Angle-integrated elastic scattering cross sections are deduced from the observed differential values to accuracies of {approx}2l5%, and angle-integrated inelastic scattering cross sections to accuracies of {approx}10%. The experimental results are compared with values given in ENDF/B-V, with attention to descrepancies and implications. Qualitative reaction mechanisms are suggested.