D.J. Land

An overview of the ion-beam analysis laboratory at White Oak

Abstract The Naval Surface Warfare Center (NSWC) at White Oak is completing a major upgrading of its positive-ion accelerator facilities dedicated to the study of ion-atom collisions and ion-beam analysis of materials. At the heart of the facility is a new National Electrostatics Corporation 3 MV tandem accelerator which replaced a 2.5 MV Van de Graaff. There are two ion sources available which will enable the accelerator to produce an ion from nearly any element in the periodic table. Mass-energy control is defined by a 90 ° analyzing magnet. A downstream switching magnet allows for the establishment of five separate beam lines. Currently three beam lines are in operation: an RBS and PIXE analysis system, an NRA analysis system, and an experimental line for measuring the stopping powers of ions in gases. In addition to numerous PCs for data manipulation, a VAX computer system is being upgraded to perform computer-controlled acquisition and analysis of spectral data. Details on the upgrade will be presented as well as recent results of ion-beam analysis.

Application of a time-dependent, distorted initial-state wave function in calculations of K-shell ionization

Initial- and final-state distorted wave functions, which in the semiclassical approximation depend upon projectile position and, hence, also upon time, are considered in the context of K-shell ionization. The use of such time-dependent wave functions can lead to the occurrence of nonorthogonal basis states. Issues associated with constructing the scattering amplitude in the presence of such states are identified and discussed. The incorporation of a time-dependent, distorted initial-state wave function along with a time-dependent binding energy is shown to give rise at low velocities to a large enhancement in the total K-shell ionization cross section over what is obtained with a time-independent wave function. The system studied is that of 3He incident on Ti in the energy region 0.2 to 1.6 MeV/u. This enhancement effectively cancels a corresponding large reduction which is found to occur from the use of a time-dependent vs a time-independent binding energy.

Contrasts in the application of distortion in calculations of the K-shell ionization cross section

Abstract Two models which describe the proton-induced ionization from the K shell of an atom are contrasted in terms of the differing forms of distortion employed. Results of calculations of both total and differential cross section are seen to be surprisingly close despite the large disparity between the models. Some consequences are discussed, particularly with regard to the use of pseudostates to represent continuum states.

Stopping powers for 400 to 2400 keV N ions in He and Ar

Abstract Stopping powers for 400 to 2400 keV N ions in He and Ar gas targets were measured using a time-of-flight technique. The stopping power was directly determined from changes in ion time-of-flight over a fixed distance after passing through a differentially-pumped, windowless gas cell. Start and stop pulses, derived from secondary electrons produced by passing the ion through thin C foils and detected by microchannel plates, are used to trigger a TAC spectrometer. The measured stopping power values are analyzed as a function of their linearity with projectile velocity and are compared to values compiled by Ziegler et al. and to the predictions of the Firsov-Land-Brennan (FLB) theory. In general, the measured values agree with FLB but are consistently lower than those of Ziegler et al. Comparison of results obtained with 14N and 15N and with N+ and N2+ projectiles are presented. Details of the measurement and detection systems are also given.

Measurement of Compton scattered electrons using monochromatic X-rays

The technique of zero degree electron spectroscopy, first developed in experiments involving ion-atom collisions, has been applied to study the distribution of inelastically scattered electrons to the K-shell ionization limit. Focused x-rays, generated at the National Synchrotron Light Source (NSLS), were used to produce ejected electrons from a thin 20 nm carbon foil at both zero and 180 degrees. Electrons were energy analyzed using a spherical sector electrostatic analyzer coupled to a charge multiplier detector. At zero degrees, both Compton and K-shell photoelectrons are emitted. The emission of Compton-recoil electrons is suppressed at the highest electron energies, where the infrared divergence is greatest. Comparison of the zero and 180 degree spectra could provide experimental evidence of the infrared divergence, due to the large enhancement of inelastically-scattered recoil electrons observed at zero degrees.

Heavy ion beam backscattering spectroscopy of GaAs/BaF2/GaAs heterostructures

Heavy Ion Backscattering Spectroscopy (HIBS) using 12.4 MeV Carbon ion beams was used to examine GaAs/BaF2/GaAs heterostructures deposited on GaAs substrates by Molecular Beam Epitaxy (MBE). The ability of the heavier ions to separate the backscattered signals of Arsenic and Gallium, as well as the individual isotopes of Gallium, was used to verify the 50/50 stoichiometry of the MBE grown GaAs layers. This information is not readily available using traditional insitu MBE diagnostic techniques. HIBS spectra were analyzed using a modification of an existing Rutherford Backscattering Spectroscopy (RBS) analysis program. The HIBS spectra were also instrumental in identifying interdiffusion of BaF2 and GaAs layers taking place during MBE growth. This information was used to modify the MBE growth process to achieve sharper interfaces between the BaF2 and GaAs layers.

Compton-electron spectroscopy using high-energy synchrotron X-rays

Abstract Compton-recoil electron spectra have been measured using high-energy (46 and 56 keV) synchrotron X-rays produced at the National Synchrotron Light Source. Thin foils of C and A1 (

RBS and XPS analyses of phosphor packages for laser-heat thermoluminescence dosimetry

Phosphor packages for a laser-heat thermoluminescence radiation-dosimetry system have been analyzed with Rutherford Backscattering Spectrometry and X-ray Photoelectron Spectroscopy. Samples consist of 20--50 {mu}m diameter powder grains of CaSO{sub 4}:Tm and LiF:(Mg,Ti) phosphor embedded in a transparent silicone matrix about 60 {mu}m thick. Our principal finding with regard to layer morphology indicates an inhomogeneous outer layer of areal density at least {similar to}300 {mu}g/cm{sup 2} depleted of phosphor and contaminated with boron.