We give the results of a study using Monte Carlo ion interaction codes to simulate and optimize elastic recoil detection analysis for {sup 3}He buildup in tritide films. Two different codes were used. The primary tool was MCERD, written especially for simulating ion beam analysis using optimizations and enhancements for greatly increasing the probabilities for the creation and the detection of recoil atoms. MPTRIM, an implementation of the TRIMRC code for a massively parallel computer, was also used for comparison and for determination of absolute yield. This study was undertaken because of a need for high-resolution depth profiling of 3He and near-surface light impurities (e.g. oxygen) in metal hydride films containing tritium.
Mechanisms of H release from Mg-doped, p-type GaN were investigated in vacuum, in N{sub 2} and O{sub 2} gases, and in electron-cyclotron-resonance N{sub 2} plasmas. Replacing grown-in protium with deuterium (D) and employing sensitive nuclear-reaction analysis allowed the retained concentration to be followed quantitatively over two decades during isothermal heating, illuminating the kinetics of controlling processes. Oxidation attending the O{sub 2} exposures was monitored through nuclear-reaction analysis of {sup 18}O. N{sub 2} gas at atmospheric pressure increases the rate of D release appreciably relative to vacuum. The acceleration produced by O{sub 2} gas is much greater, but is diminished in later stages of the release by oxidation. The N{sub 2} plasma employed in these studies had no resolvable effect. We argue that surface desorption is rate controlling in the D release, and that it occurs by D-D recombination and the formation of N-D and O-D species. Our results are quantitatively consistent with a theoretical model wherein the bulk solution is in equilibrium with surface states from which desorption occurs by processes that are both first and second order in surface coverage.
Proposed next-step devices for development of fusion energy present a major increase in the energy content and duration of plasmas far beyond those encountered in existing machines. This increases the importance of controlling interactions between the fusion plasma and first-wall materials. These interactions change the wall materials and strongly affect the core plasma conditions. Two critical processes are the erosion of materials by the plasma, and the redeposition of eroded material along with hydrogen isotopes from the plasma. These impact reactor design through the lifetime of plasma-facing components and the inventory of tritium retained inside the vessel. Ion beam analysis has been widely used to investigate these complex plasma-material interactions in most of the large fusion plasma experiments. The design and choice of plasma-facing materials for next-step machines rely on knowledge obtained from these studies. This paper reviews the use of ion beam analysis for fusion energy research, and shows how these studies have helped to guide the design and selection of materials for a next-step machine.
This report documents the strategies for verification and validation of the codes LSP and ICARUS used for simulating the operation of the neutron tubes used in all modern nuclear weapons. The codes will be used to assist in the design of next generation neutron generators and help resolve manufacturing issues for current and future production of neutron devices. Customers for the software are identified, tube phenomena are identified and ranked, software quality strategies are given, and the validation plan is set forth.
This LDRD is aimed to place Sandia at the forefront of GaN-based technologies. Two important themes of this LDRD are: (1) The demonstration of novel GaN-based devices which have not yet been much explored and yet are coherent with Sandia's and DOE's mission objectives. UV optoelectronic and piezoelectric devices are just two examples. (2) To demonstrate front-end monolithic integration of GaN with Si-based microelectronics. Key issues pertinent to the successful completion of this LDRD have been identified to be (1) The growth and defect control of AlGaN and GaN, and (2) strain relief during/after the heteroepitaxy of GaN on Si and the separation/transfer of GaN layers to different wafer templates.
The diffusion, uptake, and release of H in p-type GaN are modeled employing state energies from density-function theory and compared with measurements of deuterium uptake and release using nuclear-reaction analysis. Good semiquantitative agreement is found when account is taken of a surface permeation barrier.
Formation energies and vibrational frequencies for H in wurtzite GaN were calculated from density functional theory and used to predict equilibrium state occupancies and solid solubilities for p-type, intrinsic, and n-type material. The solubility of deuterium (D) was measured at 600--800 C as a function of D{sub 2} pressure and doping and compared with theory. Agreement was obtained by reducing the H formation energies 0.2 eV from ab-initio theoretical values. The predicted stretch-mode frequency for H bound to the Mg acceptor lies 5% above an observed infrared absorption attributed to this complex. It is concluded that currently recognized H states and physical processes account for the equilibrium behavior of H examined in this work.