Search Results (13 total)

We present a detailed comparison of the measured characteristics of Thomson backscattered x rays produced at the Picosecond Laser-Electron Interaction for the Dynamic Evaluation of Structures facility at Lawrence Livermore National Laboratory to predicted results from a newly developed, fully three-dimensional time and frequency-domain code. Based on the relativistic differential cross section, this code has the capability to calculate time and space dependent spectra of the x-ray photons produced from linear Thomson scattering for both bandwidth-limited and chirped incident laser pulses. Spectral broadening of the scattered x-ray pulse resulting from the incident laser bandwidth, perpendicular wave vector components in the laser focus, and the transverse and longitudinal phase spaces of the electron beam are included. Electron beam energy, energy spread, and transverse phase space measurements of the electron beam at the interaction point are presented, and the corresponding predicted x-ray characteristics are determined. In addition, time-integrated measurements of the x rays produced from the interaction are presented and shown to agree well with the simulations.

¹⁹F NMR at 46.6 MHz has been studied for single crystals of CaF₂ doped with NdF₃, DyF₃, HoF₃, and TmF₃ for doping concentrations of 0.5, 1.0, and 2.0 mol%. ¹⁹F NMR at 84.67 MHz has been done for crystals doped with NdF₃ and EuF₃. Data on the main satellite lines, due to lattice fluorides having one nearest-neighbor rare-earth ion, show that the anisotropic portion of the NMR shift is entirely due to the direct dipolar interaction for all ions except Nd³⁺ and Eu³⁺. Analysis of the isotropic portion of the shift and the nondipolar portion of the anisotropic shift reveals that the spin-transfer mechanism is primarily the polarization mechanism in the first half of the rare-earth series but that the covalent mechanism becomes predominant in the second half of the series. Evidence for extensive clustering is found in all systems studied in the form of resonance lines that must arise from lattice fluorides with two nearest-neighbor rareearth ions. A possible form for the cluster is presented that will explain all the NMR results. T₁ measurements on the main ¹⁹F line were done as a function of orientation for crystals doped with NdF₃ and EuF₃. In both cases, large variations with orientation were found. It is shown that present theories will account for this variation.

¹⁹F NMR studies of single crystals of CaF₂ doped with 2, 1, and 0.5 mole percent of both ErF₃ and YbF₃ have been carried out at room temperature. In ErF₃-doped crystals, ¹⁹F resonances have been identified for lattice fluorides having both one and two Er³⁺ ions in nearest-neighbor cation sites and for an interstitial fluoride with two Er³⁺ ions in nearest-neighbor sites. In YbF₃-doped crystals, resonances have also been identified for lattice fluorides having both one and two Yb³⁺ ions in nearest-neighbor cation sites plus a fluoride with two nearby Yb³⁺ ions not in normal cation sites. Analysis of the results indicate that all the rare-earth ions are associated in some type of cluster at all concentrations studied and that the structure of these clusters does not agree with any model proposed in the literature. Comparison of these results with an earlier study of doped CdF₂ crystals shows that the same dopant produces a greater distortion of the cubic fluoride ion lattice in CaF₂ than in CdF₂.

The Mössbauer effect associated with the 35.6-keV transition of Te¹²⁵ has been investigated in the host materials ZnTe, MnTe, βFeTe, Te, TeO₂, Na₁TeO₃, TeF₄, TeI₄, Cu, and NaIO₃. Quadrupole splitting is not observed in host materials of cubic symmetry, i.e.,ZnTe and Cu. The host materials of noncubic symmetry all exhibit quadrupole splitting. The quadrupole splitting in Te at 4.8°K is (0.90±0.03) × 10^-6 eV (0.76 ± 0.02 cm/sec). The temperature dependence of the quadrupole splitting in Te between 4.8 and7 7.9°K implies a molecular torsional frequency of ≈6×10¹² sec^-1. I¹²⁵ incorporated into a Cu matrix as a Mössbauer source gives a single resonance line, while I¹²⁵ in a NaIO₃ source gives four resonance lines. The latter spectrum is interpreted on the hypothesis that Te¹²⁵ is produced in two observable charge states following the decay of I¹²⁵ in NaIO³. The two charge states may result from K x-ray and Auger transitions following the I¹²⁵ decay. In this model the higher charge state decays by electron capture with a half-life of ≈1.6×10^-9 sec. The recoilless fraction for the Cu source at 82°K is ≈0.2. In NaIO₃ it is 0.14 (82°K) and 0.28 (4.8°K). This corresponds to an apparent NaIO₃ Debye temperature of ≈155°K.

Nonelastic neutron cross sections have been measured for Li⁶, Li⁷, U²³⁵, U²³⁸, and Pu²³⁹ at 8.1, 11.9, and 14.1 MeV by means of the sphere transmission technique.

Elastic scattering of protons from Cu⁶³ and Cu⁶⁵ has been observed for several energies in the range 7 to 12 Mev. When plotted as the ratio-to-Rutherford, the isotopic differential cross sections exhibit a shift which is two to three times larger than would be expected if the nuclear radius were governed by the A^{1 / 3} law.

Inelastic scattering and (p, α) cross sections were measured to contribute to our knowledge of the reaction cross sections and to an unambiguous optical-model analysis.

Neutron nonelastic cross sections for carbon have been measured over the energy range from 5.8 to 12.9 Mev. The main features of the carbon nonelastic cross section as a function of neutron energy are a gradual rise from threshold (4.8 Mev) up to 7 Mev, a steep rise from 7 to 8 Mev, and an approximately constant value from 8 to 14 Mev. In the vicinity of sharp resonances in the total cross section, good cross-section determinations are difficult to obtain using present-day sphere transmission techniques.

Neutron activation cross sections have been measured at 25 kev for 31 isotopes. An Sb-Be photoneutron source was used, and thermal activations served to calibrate the beta- and gamma-detector efficiencies. The cross sections were measured relative to iodine. A comparison was made between measured cross sections and predictions based on known low-energy resonance parameters.

Neutron nonelastic collision cross sections for eleven elements have been measured in the energy range 21-29 Mev. Corrections were applied to the data by means of a UNIVAC calculation. Since no experimental neutron angular distributions are available in this energy region, optical-model calculations were used for the correction problem. In general the cross sections at 25 Mev are 10% to 20% lower than at 14 Mev.

Neutron nonelastic cross sections for seven elements have been measured over the range from 7 to 14 Mev. The cross sections are almost constant in this range, showing that variations in the total cross sections are due primarily to variations in the elastic cross sections.

Nonelastic neutron cross sections for 23 elements have been measured at 14.2 Mev by means of the sphere transmission technique. Corrections have been applied to the data, principally for elastic energy loss, multiple scattering, and finite detector size. The results are compared with predictions of the optical model.