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With the aim of providing experimental verification of the validity of the neutron resonance spectroscopy method for measuring the moments of the phonon frequency spectra, we have determined some of the phonon spectral parameters for W and Ta in the bcc phase of these metals by measuring the Doppler broadening of neutron resonances as a function of temperature. We find the mean phonon energy (first moment of the phonon frequency spectrum) of W to be 〈 hν 〉=20.2±0.4 meV and the second moment to be 〈( hν ) 2 〉=(4.9±0.2)×10 −4 eV 2 . The Boltzmann-weighted moments from all the measurable resonances of tungsten are quite consistent with a Debye model with a Debye temperature of 315±4 K. For Ta, 〈 hν 〉=14.3±0.1 meV, 〈( hν ) 2 〉=(2.2±0.2)×10 −4 eV 2 . The fitted Debye model temperature for the tantalum data is 217±2 K. In both cases the first and second moments agree well with those determined from neutron coherent inelastic scattering. Results with reasonable accuracy were extracted from resonances up to a few tens of eV. These results give high confidence that neutron resonance Doppler broadening spectroscopy is a viable and useful method for determining the characteristics of phonon frequency spectra.

Solid state information from coherent neutron scattering can be extracted from transmission experiments on polycrystalline materials. The transmission technique has been applied to dynamic structural and phase change investigations and will be developed to cover the fields of strain mapping and structural tomography.

We have determined some of the phonon spectral parameters for Pu and Ga in the {open_quotes}stabilized{close_quotes} {delta} phase (fcc) of the Pu-3.6thinspat.thinsp{percent}thinspGa alloy by measuring the Doppler broadening of neutron resonances as a function of temperature. We find the mean phonon energy (first moment of the phonon spectrum) of Pu to be {l_angle}h{nu}{r_angle}=8.22{plus_minus}0.12thinspmeV and the second moment to be {l_angle}(h{nu}){sup 2}{r_angle}=(7.2{plus_minus}0.3){times}10{sup {minus}5}thinspeV{sup 2}. These moments and the value of the ({minus}1)st moment determined from Debye-Waller factors from neutron powder diffraction are consistent with a Debye model with a Debye temperature of 127.2{plus_minus}1.7thinspK. For Ga in the Pu-Ga alloy, {l_angle}h{nu}{r_angle}=16.3{plus_minus}1.4thinspmeV, which is slightly larger than the m{sup 1/2}-weighted value of 15.3 meV expected from the value for Pu (however, the measured value cannot be distinguished from the latter value within experimental error). These results show that Pu-3.6thinspat.thinsp{percent}thinspGa behaves very much like a Debye solid at ambient pressures and low temperature and that the Ga impurity experiences approximately the same (or a slightly stiffer) force field compared with the Pu it replaces. {copyright} {ital 1998} {ital The American Physical Society}

The quantal expression for Doppler broadening of neutron resonances by creation and annihilation of phonons in a crystal is developed into expansions in terms of moments of the phonon frequency spectrum, one incorporating the recoilles line, the other explicity excluding this term. Such expressions are useful for calculating the details of resonance line-shapes when some knowledge of the phonon spectrum is available or, conversely if it is wished to use measurements of resonance Doppler broadening to extract information on the phonon spectrum of the target material (specifically, that portion of the spectrum centred on the atomic species in which the resonance occurs). The possible sensitivity of the method is tested by simulating sets of neutron transmission data for low energy resonances in the cross sections of a number of different nuclides and doing a least-squares analysis on these to determine resonance parameters simultaneously with phonon moments. It is found that phonon moments can be determined independently of the resonance parameters and with very useful precision for at least the first three moments.

The first successful application of a new method to investigate phase transformations in real time, like the decomposition of austenite into bainite in grey iron, is described. During the ongoing transformation, transmission spectra of thermal neutrons, which contain Bragg edges corresponding to the crystal structure of the transforming phases, are recorded. By evaluating the height of these Bragg edges, which is a measure of the volume fraction of the phase, at different transformation times, the transformation can be followed in-situ in a time resolved manner. The method is compared to other previously used methods (micrographs, dilatometry, diffraction techniques); also a summary and an outlook are given.

Low-energy neutron transmission diffraction has been successfully demonstrated to be a powerful experimental method to investigate changes in crystal structure development down to the millisecond range. Bragg edge diffraction measurements thus can be the basis for future experiments like strain radiography, determination of the strain-rate dependent elastic properties of engineering materials, temporal development of phase changes, structural properties of materials under extreme environmental conditions like high pressure or temperature states. At high instantaneous neutron bursts current-mode detection can be used, which enables both transient and stroboscopic measurements. Progress has been made in rapid data acquisition and analysis as well as temperature measurement by resonance doping.

The neutron intensity transmitted through polycrystalline material exhibits sudden well defined increases whenever the neutron wavelength ceases to fulfil the Bragg coherent scattering condition for a specific subset of lattice planes. Since in the transmitted beam all diffracted neutrons are absent, the so-called Bragg edges build up very rapidly, because diffraction of neutrons into the full solid angle contributes to their presence. High peak-intensity neutron bursts may be detected using the current-mode technique, opening up the possibility to do both transient and stroboscopic measurements. This new method is relevant to different research areas, like transient phase transformation, melting and solidification, dynamic stress influence or materials under extreme environmental conditions. Progress has been made in rapid data acquisition and analysis as well as temperature measurement by resonance doping.

Statistical information on the phonon frequency spectrum of materials can be measured by neutron transmission techniques if they contain nuclei with low energy resonances, narrow enough to be Doppler-broadened, in their neutron cross sections. The authors have carried out some measurements using this technique for materials of the lanthanum barium cuprate class, La{sub 2{minus}x}Ba{sub x}CuO{sub 4}. Two samples with slightly different concentrations of oxygen, one being superconductive, the other not, were examined. Pure lanthanum cuprate was also measured. Lanthanum, barium and copper all have relatively low energy narrow resonances. Thus it should be possible to detect differences in the phonons carried by different kinds of atom in the lattice. Neutron cross section measurements have been made with high energy resolution and statistical precision on the 59m flight path of LANSCE, the pulsed spallation neutron source at Los Alamos National Laboratory. Measurements on all three materials were made over a range of temperatures from 15K to 300K, with small steps through the critical temperature region near 27K. No significant changes in the mean phonon energy of the lanthanum atoms were observed near the critical temperature of the super-conducting material. It appears however that the mean phonon energy of lanthanum in themore » superconductor is considerably higher than that in the non-superconductors. The samples used in this series of experiments were too thin in barium and copper to determine anything significant about their phonon spectra.« less

Time resolved phase transition and strain experiments have been performed on the millisecond time scale using a Bragg-edge transmission technique that has been developed at the Los Alamos National Laboratory. The precision with which lattice parameters can be determined from edge positions is sufficient to perform high-resolution strain measurements in uniaxial stress.

A summary is given of characteristics of x-ray sources used by Los Alamos National Laboratory to calibrate various x-ray diagnostic packages and components. Included are D.C. sources in electron impact and fluorescence modes, a pulsed laser source for soft x rays with 100 ps time resolution, Febetron pulsed electron impact sources, and both EUV and x-ray synchrotron beamlines.