J.A. Goldstone

A spectrometer for inelastic scattering using neutrons from 1 eV to 186 eV

Abstract A Filtered Beam Spectrometer (FBS) has been developed at the spallation neutron source at the Los Alamos National Laboratory. It has been demonstrated that this spectrometer can measure inelastic neutron scattering using neutrons from 1 eV to 186 eV. In the FBS, a filter having a strong and narrow resonance is used to define the energy of neutrons before (or after) scattering and time of flight is used to measure the energy after (or before) scattering. In this paper, the FBS is described and examples of data are given.

Lineshape analysis and filter difference method for a high intensity time-of-flight inelastic neutron scattering spectrometer

Abstract The use of polycrystalline filter analysers in inelastic neutron scattering allows relatively high count rates to be achieved. Its application to time-of-flight (TOF) techniques is important because the new pulsed neutron sources are expected to provide higher neutron fluxes than currently available. We present therefore a detailed lineshape analysis for inelastic spectra taken by a TOF filter analyser. By means of a convolution of the instrinsic line shape for the vibrational mode with that for instrumental parameters and the filter transmission function, the asymmetry caused by the latter can be modelled out by a fitting procedure. Most of the drawbacks of this approach are eliminated by taking a difference between two filter analysers with different cutoffs, which results in symmetrized, narrow experimental lineshapes. The analysis is extended to this case and the design along with some experimental results of a successfully operating prototype filter difference spectrometer at the Los Alamos pulsed spallation neutron source (WNR/PSR Facility) are presented.

Inelastic neutron scattering studies of some intramolecular hydrogen bonded complexes: A new correlation of γ(OHO) vs R (OO)

Inelastic neutron scattering spectra of several complexes {potassium hydrogen maleate, nickel dimethylglyoxime, and tripotassium cis‐dichloro (hydrogen disulfito) and tripotassium cis‐dichloro (hydrogen disulfito) platinate (II)} with strong intramolecular hydrogen bonds have been obtained and the out‐of‐plane bending modes γ(OHO) of the hydrogen bond assigned. Using previously published data, primarily for intermolecularly hydrogen bonded complexes, it is shown that as the oxygen–oxgyen distance {R(OO)} decreases γ(OHO) increases. Our new data do not follow this correlation since γ(OHO) is also observed to decrease as R(OO) decreases. It is suggested that this observation is due not to a fundamental difference in the behavior of inter‐ and intramolecular hydrogen bonds but rather to the different range of values of R(OO) found for each type. For the intermolecular systems R(OO) is greater than ∼ 2.44 A, while for the intramolecular complexes (including potassium monochloromaleate) R(OO) is less than this...

Tunneling states in methane at high pressures

Rotational tunneling states in phases II and III of solid CH4 were studied by inelastic neutron scattering techniques at 8 K and pressures up to 850 bar. The tunnel energies were found to depend strongly on pressure, while the rotational peak of the disordered molecules shifts little with pressure up to the II–III phase transition. In phase III, additional intensity was found at about 0.18 meV, which we interpret as tunneling states for those molecules that are disordered in phase II.

The inelastic neutron scattering spectrum of chromous acid at high energy transfers

The inelastic incoherent neutron scattering spectrum of chromous acid, at 77 K, is presented. It is dominated by the intense bending mode at 1254 cm−1 with some modes at lower frequencies showing indications of dispersion. The antisymmetric stretch νas(OHS) {‖1〉‐‖2〉} was assigned to a broad band centered at ∼2050 cm−1, significantly displaced for the IR assignment (1650 cm−1). The breadth of the band is due to the dispersion, and kinematic coupling, that is anticipated for this compound. These new data allows us to fit chromous acid more clearly into the general trend of hydrogen bonded compounds. Chromous acid compares very well in its overall INS spectrum with the isomorphous sodium bifluoride, except that the kinematic coupling between νas(OHO) and the symmetric stretch does not occur in this compound.

Debye-Waller factors in incoherent inelastic neutron scattering spectra of molecular polycrystals

The incoherent inelastic neutron scattering (IINS) spectrum of HCCo3(CO)9 has been obtained in order to test some present methods of calculating band intensities for polycrystalline samples. The fundamental and first overtones of the carbon-hydrogen bending mode were observed at 860 and 1712 cm-1 respectively. The C-H stretching mode was observed at 3041 cm-1 and this is the first report of the observation of such a mode in which there is no possibility of confusion with overtone and combination bands.

Optic modes in FeTiH1.14

The inelastic incoherent neutron scattering spectrum of nominally beta -FeTiH1.14 shows a broad peak due to optic modes near 100 meV. Its width increases strongly on heating from 10K to 300K. Small side peaks at 70 meV and 85 meV may be due to different phases of FeTiHx.

Point defect-dislocation interactions in copper following pulsed neutron and electron irradiations

Two aspects of point defect-dislocation interactions, peaking effect and kinetics following pulsed electron and neutron irradiations have been studied in high-purity polycrystalline copper. In all experiments, an initial very rapid rise in the internal friction and Youngs modulus was observed, followed by a slower continuous change. The rise of the internal friction shows the presence of the peaking effect. The magnitude of the rapid change in the modulus was proportional to the fluence. The initial rise was too fast for detailed kinetics analysis; however, the slower process was analysed. Results are interpreted in terms of a model wherein interstitials are rapidly deleted from the lattice and responsible for the initial rise, while the slower process is due to vacancy diffusion to dislocations. These data also show that interstitials are responsible for the peaking effect. Models proposed to explain the peaking effect are discussed in the light of these results.

Localized Vibrations of Hydrogen in Metals

We report inelastic neutron scattering experiments on H and D in β-TaH0.5 and Nb99Ti1H(D)1. On TaH0.5 higher harmonics of the hydrogen vibrations were found up to 400 meV. Qualitatively the observed anharmonicity agrees with recent ab initio frozen phonon calculations on NbH. Spectroscopic results on the impurity system NbTiH(D) allow for the first time a determination of the H-potential for a dilute metal-H- system. The observed anharmonicity is twice as large as in a corresponding hydride phase.

Localized Vibrations of H in Nb and Ta at High Concentration and Low Temperatures

Extensive measurements have been performed of the optic mode frequencies in the hydrides of Nb and Ta at concentrations between 70% and 95% using inelastic incoherent neutron scattering techniques at the WNR pulsed neutron source of the Los Alamos National Laboratory. The aim of this study was to gain new information on anharmonicities of the H potential as well as to relate observations of broading or splitting of the local mode peaks to details in the high concentration, low temperature portion of the NbHx phase diagram. The higher harmonic of the singlet excitation was observed in all cases at energies about 3–5% less than the harmonic value. For some cases a simultaneous excitation of the singlet and doublet is apparent, which provides additional information on the hydrogen potential when these vibrations are coupled by anharmonic interactions. A previously reported1 splitting in the optic mode peaks of NbH0.87, which occurs at temperatures below about 210K, as well as broadening of the optic mode peaks of NbH0.78 and NbD0.8 can be discussed using new information on the NbHX phase diagram in terms of coexisting phases with differing hydrogen sites.