Walter J. Trela

Neutron Doppler broadening studies of tantalum and tungsten metal

Abstract 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.

Vibronic coupling and other many‐body effects in the 4σ−1g photoionization channel of CO2

Vibrational branching ratios and photoelectron angular distributions were measured for 4σ−1g photoionization of CO2 in the energy range 20–28 eV. Of particular interest are three vibrational components of the resulting CO+2 C 2Σ+g state—the allowed (000) and (100) bands and the forbidden (101) band. The wavelength dependence of the beta parameter for the forbidden band deviated significantly from that of the two allowed bands, showing instead a strong resemblance to that of the B 2Σ+u state. This behavior suggests that vibronic coupling to the B 2Σ+u state is responsible for the appearance of the forbidden (101) band in the C 2Σ+g state photoelectron spectrum. We also observe evidence for other many‐body effects—shape‐resonance‐induced continuum–continuum coupling and doubly excited autoionizing resonances—in the present data.

Investigation of the austenite-bainite transformation in gray iron using real time neutron transmission

Abstract In globulitic gray iron (3.6 wt.% C, 2.4 wt.% Si and 0.45 wt.% Mn) the decomposition of austenite into bainite is investigated in real time using a new method based on neutron scattering. During the phase transformation transmission spectra of thermal neutrons are continuously recorded containing specific Bragg edges. Thus, the transformation is followed in-situ time resolved by evaluating the relative (micro) structural fractions of the transforming phases from the heights of the corresponding Bragg edges. The progression of the transformation is described and the results are discussed. The observed late time deviations from the Avrami law are explained by residual stresses of the second kind (homogeneous microstresses), which occur during the transformation because of the different specific volumes of the phases.

Real time neutron transmission investigation of the austenite-bainite transformation in grey iron

Abstract 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.

Los Alamos X-Ray Characterization Facilities For Plasma Diagnostics

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.

Resonance neutron methods for determining statistical properties of phonon spectra

Abstract 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.

BRAGG-EDGE TRANSMISSION AS AN ADDITIONAL TOOL FOR STRAIN MEASUREMENTS

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.

Progress in single-shot neutron transmission diffraction

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.

Single-shot neutron transmission diffraction

Abstract 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.