Crawford J. MacCallum

Grain Boundary and Lattice Diffusion in Polycrystalline Bodies

Diffusion in a polycrystalline body is discussed under the assumption that two interrelated mechanisms consisting of diffusion through the grains and in the boundaries around the grains are dominant in different penetration regions. The general behavior of the average concentration‐distribution of diffusant is described for the separate action of each mechanism, and the effects of their combined action are inferred. A realistic diffusion model for a polycrystalline body and a suitable empirical function to describe the loss of diffusant from the boundaries to the grains form the basis for these computations.Results are specifically applied to the analysis of the usual sectioning technique for measuring the ratio of grain boundary to lattice diffusion constants in a polycrystalline body. It is shown that, in the penetration range most commonly covered in polycrystalline diffusion experiments, the log of the average concentration varies as the 6/5 power of penetration depth contrary to the linear law report...

Bremsstrahlung energy spectra from electrons of kinetic energy 1 keV ≤ T1 ≤ 2000 keV incident on neutral atoms 2 ≤ Z ≤ 92

Abstract A tabulation is presented of theoretical predictions for the electron bremsstrahlung energy spectrum from neutral atoms as a function of atomic number Z, incident electron kinetic energy T1, and fraction of energy radiated k T 1 . Tabulated values were obtained by interpolation from a smaller set of calculated data points. These calculated points were obtained by describing bremsstrahlung as a single-electron transition in a relativistic self-consistent screened potential and, after expansions in partial waves, by performing a numerical evaluation for radial wavefunctions and matrix elements.

Photo‐Compton currents in unbounded media

A flux of photons passing through a condensed medium drives with it a flux of recoil electrons, continually generated by Compton and photoelectric processes and continually brought to rest by the stopping power of the medium. We formulate an analytical description of the magnitude and angular distribution of this electron flux in unbounded media. Numerical results for net, forward, and back‐directed current densities are presented for a large set of elements and compounds and for photon energies from 10 keV to 20 MeV.

Prospects for gamma-ray line observations of individual supernovae

The gamma-ray line emission from individual type I and type II supernovae are studied using numerical simulations and photon propagation codes to predict the flux levels and line shapes. For both types, the gamma ray lines with the highest flux from an individual event are the 0.847 and 1.238 MeV lines from the Ni-56 to Co-56 to Fe-56 decay chain. For type I supernovae, the 0.847 MeV line peaks at about 70 days after event onset. The historical record indicates an approximate discovery rate of once in 10 years for balloon-borne instruments, once in two to three years for the Gamma-Ray Observatory, and once in one to two years for a proposed space mission. The 0.847 MeV line flux from type II supernovae peaks at about 600 days after event onset at a low level which restricts observations to the events in the Galaxy and its nearest neighbors. The expected line shape is narrower than for type I supernovae. 84 references.

Prospects for observations of nucleosynthetic gamma-ray lines and continuum from SN 1987A

Expected flux levels for nucleosynthetic gamma-ray line and continuum emissions from SN 1987A are calculated for several models. The dominant line emission is from freshly synthesized Ni-56 and its decay daughters, and the continuum is from Compton scattering of line photons. For a 15 solar mass Type II model, the light curve for the 0.847 MeV gamma-ray line peaks in September 1988 at 3 x 10 to the -4th photons/sq cm/s. This is detectable only by new, ultrasensitive balloon-borne spectrometers. For models with substantial mass loss from their envelopes, the peak is in early 1988 at about 0.01 photons/sq cm/s, which is detectable at high significance levels by all current instruments. The continuum emission in the 0.05-0.5 MeV band peaks about 100 days before the 0.847 MeV line peak. This Compton precursor is observable at about the same significance level as the lines.

ESR Studies of γ‐Irradiated η‐Octadecyl Disulfide

Octadecyl disulfide irradiated at — 196°C in the absence of oxygen exhibits radicals reflecting breakage at the sulfur group to form RS·radicals as well as breakage in the alkyl group. During storage of the samples at from 0° to 40°C an increase in RS·concentration is observed while reduction of radicals in the alkyl group takes place. After initial rapid reactions occurring immediately after warm‐up, a lower rate is observed for the increase in RS·content and decay of the radicals associated with the alkyl group. Both of these slower processes appear to occur as a result of a common reaction with an activation energy of about 13 kcal/mole. It is believed that this reaction involves the combination of a disulfide link with a radical in the alkyl group and occurs as a result of molecular diffusion in the crystalline material. Calculations based on a diffusion model yield theoretical curves which can be made to fit the experimental results within the limits of experimental error.

Photo‐Compton currents emitted from a surface

Weymouth’s method for calculating electron transport in a half‐space is generalized and used to obtain algebraic expressions for the magnitude and angular distribution of the photo‐Compton current emitted from a surface irradiated by photons of any energy between 10 keV and 10 MeV. Algebraic expressions for the energy deposition at the surface and for the energy distribution of the emitted electrons are also obtained. Comparisons with Monte Carlo calculations and with experiment are presented.

MOS-Transistor Radiation Detectors and X-Ray Dose-Enhancement Effects

Sandia National Laboratory (SNL) CMOS IC dose detectors and 3N161 MOS Transistors were evaluated as pulsed X-radiation dosimeters and used as monitors to measure doseenhancement effects. Measurements were made in the photon environments from the HydraMITE II, SPR III, MBS and PITHON radiation sources. The dosimeter evaluation data suggest that the 3N161 MOS transistors are useful dosimeters for measuring flash X-ray-induced doses in the oxide layers of modern metal-oxide-semiconductor (MOS) integrated circuits. However, doseenhancement calculations indicate that Monte Carlo codes, using 1-D geometries and calculated source spectra, consistently overpredict measured dose-enhancement ratios by factors as large as two.

The Development of a Segmented N-Type Germanium Detector, and Its Application to Astronomical Gamma-Ray Spectroscopy

Extensive calculations and simulations have shown that the instrumental background in a coaxial germanium gamma-ray detector flown at balloon altitudes or in space, can be substantially reduced by segmenting the outer contact. The contact is divided into horizontal strips around the side of the detector, giving it many characteristics similar to those of a stack of planar detectors. By choosing different segment coincidence requirements in different energy ranges, one can obtain a factor of ~ 2 increase in sensitivity to spectral lines between 70 keV and 1 MeV, compared with an unsegmented detector. The reverse electrode configuration (using n-type germanium), with the p contact outside, is preferred for this application due to its thin dead layer and resistance to radiation damage in space. A small two-segment n-type coaxial detector has been developed and tested. To our knowledge, this is the first n-type germanium coaxial detector to be successfully made with a segmented cathode. Plans are to use the experience gained from the development of this prototype detector to contruct a large multi-segment device.

Gamma-ray lines from the galactic center

The detection and identification of gamma-ray lines from the galactic center direction are reviewed the principal line of interest is the 511 keV line from electron-positron annihilation. Attention is given to line features detected at 476 plus or minus 24 keV, at 530 plus or minus 11 keV and at higher energies. It is noted that only the 511 keV line shows a statistically significant difference between target and background count rates, with the effect being independent of the direction in which the data were taken. Strong but not conclusive evidence for a three-photon continuum is found in the data, and it is suggested that the only galactic center gamma-ray line that has been conclusively detected is the 511 keV positron annihilation line.