Jack L. Price

Growth and interfacial chemistry of insulating (100) barium fluoride on gallium arsenide

The epitaxial growth of insulating BaF2 films on (100) and (111)‐oriented GaAs substrates has been investigated. (100)‐oriented BaF2 was successfully deposited on (100) GaAs at temperatures as low as room temperature, in contrast to a previous report. This was accomplished by first establishing a chemically reacted template layer a few monolayers thick at the BaF2/GaAs interface. These films consistently exhibited epitaxial reflection high‐energy electron diffraction patterns with three‐dimensional growth modes for a wide range of incident BaF2 flux rates. The epitaxial quality of the (100) films was, however, temperature dependent. A film deposited on a (111) wafer at 600 °C was (111) oriented and showed two‐dimensional growth. X‐ray photoelectron spectroscopy studies of the interface chemistry indicate the existence of a Ba state other than that of the BaF2, authenticating the relevance of the template layer. The (100) BaF2 films are insulating, with a breakdown field of ∼1×106 V/cm.

High dose measurements using thermoluminescence of CaSO4:Dy

Abstract Thermoluminescence dosimetry has been very successful in monitoring the personnel level doses due to high sensitivity and reusability. However, these dosimeters saturate at high doses involved in radiation processing. Present investigations show that the range of high dose measurements can be increased by an order of magnitude by increasing the concentration of dysprosium in CaSO 4 :Dy. A further increase in high dose measurements is possible by considering the ratio of two high temperature peaks. As the ratio of two peaks is an intrinsic property of the material, it is expected that the initial calibration of these dosimeters may not be required. This may be advantageous at very high doses where calibration of the dosimeters is quite problematic. Use of thermoluminescence peaks higher than 300°C also make this technique appropriate for high dose measurements at high temperatures.

Observation of radiation effects on three-dimensional optical random-access-memory materials for use in radiation dosimetry

Abstract The first experimental investigation has been performed of radiation effects on three-dimensional optical random-access-memory materials. Thin films of poly(methyl methacrylate) doped with spirobenzopyran were irradiated with uniform fluxes of protons, α-particles and 12 C +3 ions, at fixed energies per nucleon from 0.5 to 2.5 MeV and fluences from 10 10 to 10 14 cm −2 . The exposed films were examined under a confocal laser scanning microscopy system which is capable of optically sectioning the materials. The irradiation resulted in a permanent change in the materials from a nonfluorescent form to a form which is fluorescent under both 488 and 514 nm excitation. Profiles were measured of fluorescent intensity versus depth, and of intensity versus dose. It was found that both the particle energy and the dose can be obtained from measuring the width of the depth profile and the fluorescent intensity. These properties are very promising for dosimetry applications since they allow calculation of an accurate dose equivalent.

Determination of growth mechanisms of MBE grown BaF2 on Si(100) by target angle dependence of RBS yields

Abstract The angular dependence of 2.0 MeV 4He1+ Rutherford backscattering spectroscopy (RBS) yields have been used to determine the growth mechanism of epitaxial BaF2 films grown on Si(100) substrates by molecular beam epitaxy (MBE). RBS yields from uniformly thick layers are characterized by a (cos φ)−1 dependence, where φ is the angle between the incident beam and the target normal. Deviations from this relationship have been attributed to layers which are composed of islands, rather than films of uniform thickness. A series of BaF2 films of increasing deposition time were examined in this way. The results of this analysis show that the BaF2 at first grows in small islands which eventually coalesce into a uniform epitaxial layer.

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.

On-Demand High Energy Density Materials

*† ‡ Excited states of nuclei can store 5 orders of magnitude more energy than that stored in chemical bonds. Conventional nuclear batteries, using radio-isotopes, cannot be turned-on on demand. A switchable nuclear battery can be formed from long-lived (i.e. 100 years) excited spin-states in the nucleus. The most efficient techniques to trigger the excited isomeric spin states of these nuclei, into decaying modes of radio-isotopes, must be determined. Triggering cross sections are measured, so engineering development can be pursued. We are comparing the triggering characteristics (i.e. Photon energy and flux required) of several candidate isomeric materials. Burn-up measurements have been performed using 177mLu to determine triggering cross sections. A broadband gamma source of 300keV endpoint bremsstrahlung irradiated the target material. The long-lived isomeric states are switched to short-lived radioisotope decaying states. An upperbound for the integrated cross section of 177mLu was measured to be ≤10 -25 cm -1 . Optimizing the efficiency of a nuclear battery would require that the decay modes (and decay products α, β, and γ) of these high-energy-density materials be matched to direct energy conversion(DEC) materials. The choice of material, packaging geometry, thickness of material layer, number of layers, will necessarily depend on the specific radioisotope output product. Samples of SiC have been irradiated with gammas from a 300 kev bremstrahlung gamma source and betas from 90 Sr. Efficiency as high as 2% has been measured from the SiC PIN device (under irradiation from the 90 Sr). GaN and diamond materials will also be compared in the future. DEC using SiC would appear to be promising material for the applications of small, long-lived (10-year) batteries, once parametric studies in thickness, packaging, and materials are carried out.

Neutron response of a laser-heated thermoluminescence dosimetry system

Abstract A laser-heated thermoluminescence dosimetry (LHTLD) system was tested for albedo response to mono-energetic neutrons ranging in energy from 200 keV to 2.8 MeV. For neutron response measurements, pairs of magnesium borate (Mg 10/11 B 4 O 7 :Tm) and lithium fluoride ( 6/7 LiF:Mg,Ti) phosphor chips were used. In general, the LHTLD system based on Mg 10/11 B 4 O 7 :Tm shows response similar to the LiF system, that is a decreasing response ratio (R=dose read/dose applied) with increasing neutron energy approaching a response of 0.07 for higher energies. In addition, the neutron responses consistently indicates a higher response for the LiF system than the LHTLD system by about 22%, possibly due in part to the internal shielding of non-albedo thermal neutrons in the LHTLD badge.

Improved Photoconductivity of ZnO by Ion Beam Bombardment

In this study, single crystal ZnO films are grown by pulsed laser deposition and ion beam processing is used alter the resistivity of the films. A 3 He ion beam was chosen with a specific energy to transmutate oxygen into nitrogen. Analytical ion beam techniques were used to monitor the transmutation process, and changes in film characteristics were monitored by making resistance, photoconductivity and luminescence measurements before and after ion beam processing. The amount of nitrogen produced by this method was estimated to be ∼ 10 14 cm −3 , and was too low to be observable as a p-type dopant due to compensation by the naturally n-type material. However, the ion beam processed films improved dramatically in resistivity, defect luminescence was reduced and photoconductivity increases consistent with the improvements with resistivity were observed. These improvements were attributed to ion beam annealing of the crystal resulting in a reduction of point defects. In some films, blistering of the surface occurred and was attributed to the formation of gas bubbles which causes delamination of the film from the substrate.

Selectivity and efficiency of pyrene attachment to alkanes induced by broadband X-rays

Bombardment of pyrene-doped n-heneicosane (C21H44) in its orthorhombic solid phase with <1.3> MeV broadband X-rays results in the formation of both mono- and di-heneicosylpyrenes, whereas the same dose in liquid cyclohexane yields only monosubstituted pyrene. In both cases, the reaction efficiency decreases as pyrene concentration is increased from 10-5 to 10-2 M. Qualitatively, the overall attachment efficiency is higher in orthorhombic n-heneicosane than in liquid cyclohexane, but the selectivity of attachment is greater in cyclohexane. Differences between these results and those from irradiations of the same samples with eV range photons are discussed.

Heavy ion backscattering spectroscopy (HIBS) analysis of gallium arsenide diffusion into barium fluoride layers grown by molecular beam epitaxy

Abstract Heavy Ion Backscattering Spectroscopy (HIBS) using 12 MeV 12C ions was used to examine GaAs (1 0 0) and GaAs (1 1 1) layers grown by Molecular Beam Epitaxy (MBE) on BaF2 (1 0 0) and BaF2 (1 1 1) layers, respectively. HIBS was the only available technique able to verify the Ga As ratio in these samples because X-ray Photoelectron Spectroscopy (XPS) is too surface sensitive for this task and the lighter ions used in Rutherford Backscattering Spectroscopy (RBS) cannot resolve the Ga and As signals. A modified RBS analysis program was used to analyze the HIBS spectra. Using the stopping power data of Ziegler et al., the HIBS analysis overestimated the thickness of a BaF2 layer by about 14% when compared to RBS analysis of the same layer. HIBS was able to resolve the Ga and As peaks, as well as the two isotopes of Ga. This enhanced mass resolution made the determination of the GaAs layer stoichiometry possible, and introduced spectral features that aided in fitting HIBS simulation spectra to the data points. HIBS analysis of GaAs/BaF2/Si heteroepitaxies also showed diffusion of Ga and As into BaF2. This diffusion is temperature dependent and therefore subject to control. The information gathered from the HIBS analysis was used to confirm information obtained by in situ MBE diagnostic techniques, and in the case of the diffusion studies, provided information that was not available by those surface sensitive techniques.