Noel Guardala

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I have been doing accelerator-based physics researcher for 37 years. I was a research physicist for the Navy for 27 years and since Sept. 2017 have been a research professor in the Dept. of Physics at GWU in Washington, D.C.

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Papers by Noel Guardala

Energetic photo-, Auger, and Compton electrons produced by synchrotron radiation

Bulletin of the American Physical Society, May 1, 1993

Energetic (3-12 keV) photo-, Auger, and Compton-recoil electrons have been measured at the x-ray ... more Energetic (3-12 keV) photo-, Auger, and Compton-recoil electrons have been measured at the x-ray ring of the National Synchrotron Light Source (NSLS). Focussed, filtered broad-band and monochromatic photon beams of synchrotron radiation have been used to study inner-shell photoionization and subsequent radiative Auger decay processes, and Compton-electron scattering. Noble gas and solid films were used as targets. The electrons were analyzed by a semi-hemispherical electrostatic spectrometer with an energy resolution. The electrons were detected at 0{degrees}, 90{degrees}, and 180{degrees} with respect to the photon beam direction. Further detailed study by means of high-resolution electron spectroscopy and electron-photoion coincidence technique is in progress. The recent experimental results will be presented.

Heavy ion beam backscattering spectroscopy of GaAs/BaF[sub 2]/GaAs heterostructures

The fourteenth international conference on the application of accelerators in research and industry, 1997

Heavy Ion Backscattering Spectroscopy (HIBS) using 12.4 MeV Carbon ion beams was used to examine ... more 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

Inner-shell photoelectron spectroscopy using X-ray synchrotron radiation

Bulletin of the American Physical Society, Apr 1, 1993

High energy (3-12 keV) photoelectrons have been measured at the x-ray ring of the National Synchr... more High energy (3-12 keV) photoelectrons have been measured at the x-ray ring of the National Synchrotron Light Source (NSLS). Broad-band and monochromatic photon beams of synchrotron radiation have been used to study inner-shell photoionization and subsequent Auger processes. Ne and Ar gas jets and Fe, Cu, and Ag solid films were used as targets. The electrons were analyzed by a semi-hemispherical electrostatic spectrometer at the energy resolution of {approximately}3%. The electrons were detected at 0{degrees}, 90{degrees}, and 180{degrees} with respect to the photon beam direction. The 90{degrees} photoelectrons produced from the gas targets well reflected the predicted photon energy distribution, suggesting a way of determining in situ incoming photon flux distribution of {open_quotes}white{close_quotes} x-ray. No appreciable intensity of photoelectrons was detected at 0{degrees} from the Ar gas targets as expected. However, strong intensity of the 0{degrees} photoelectrons was detected from the solid targets. Recent results will be presented.

Discrimination of photon from proton irradiation using glow curve feature extraction and vector analysis

Radiation Protection Dosimetry, Apr 13, 2006

Two types of thermoluminescence dosemeters (TLDs), the Harshaw LiF:Mg,Ti (TLD-100) and CaF(2):Tm ... more Two types of thermoluminescence dosemeters (TLDs), the Harshaw LiF:Mg,Ti (TLD-100) and CaF(2):Tm (TLD-300) were investigated for their glow curve response to separate photon and proton irradiations. The TLDs were exposed to gamma irradiation from a (137)Cs source and proton irradiation using a positive ion accelerator. The glow curve peak structure for each individual TLD exposure was deconvolved to obtain peak height, width, and position. Simulated mixed-field glow curves were obtained by superposition of the experimentally obtained single field exposures. Feature vectors were composed of two kinds of features: those from deconvolution and those taken in the neighbourhood of several glow curve peaks. The inner product of the feature vectors was used to discriminate among the pure photon, pure proton and simulated mixed-field irradiations. In the pure cases, identification of radiation types is both straightforward and effective. Mixed-field discrimination did not succeed using deconvolution features, but the peak-neighbourhood features proved to discriminate reliably.

Surface chemical state populations in the molecular beam epitaxy deposition of BaF2 on GaAs by x‐ray photoelectron spectroscopy and heavy‐ion backscattering spectroscopy

Journal of vacuum science & technology, 1996

X-ray photoelectron spectroscopy (XPS) and heavy-ion backscattering spectroscopy (HIBS) were used... more X-ray photoelectron spectroscopy (XPS) and heavy-ion backscattering spectroscopy (HIBS) were used to investigate the chemical states of barium atoms in submonolayer BaF2 depositions on GaAs (100) substrates. XPS measurements showed barium atoms to have two different chemical states at the BaF2/GaAs interface. The relative abundance of these two states was also determined by XPS. The number of barium atoms in each state was found by normalizing integrated XPS peak intensities to HIBS measurements of the total number of barium atoms on the surface. The results were used to formulate a model of a two-stage deposition mechanism whereby BaF2 first reacts with gallium at the surface and dissociates, releasing gaseous GaF. This reaction is apparently self-limiting, resulting in a barium ‘‘template’’ layer that enables subsequent BaF2 molecules to form an epitaxial (100)-oriented film.

Optimization of Bismuth Nanowire Arrays by Electrochemical Deposition

MRS Proceedings, 2000

ABSTRACTBismuth nanowires for thermoelectric applications have been made by electrochemical depos... more ABSTRACTBismuth nanowires for thermoelectric applications have been made by electrochemical deposition from an aqueous Bi solution into nanoporous mica substrates, and Anopore (Al2O3) filters. The nanoporous substrates are created by acid etching damage tracks that are produced in the mica using heavy-ion irradiation. In this work, further improvements in the fabrication process are made by investigating the electrochemical growth at the 1-d to 2-d transition region at the nanochannel-substrate surface interface as a function of deposition time and electrochemistry. Issues related to doping of the Bi wires with Te along with the problem of electrochemical growth of contacts to the nanowires as compared to vacuum deposition of contacts will be discussed.

Feasibility of a Neutron Detector-dosemeter based on Single-event Upsets in Dynamic Random-access Memories

Radiation Protection Dosimetry, Aug 1, 2002

The feasibility was investigated of a solid-state neutron detector/dosemeter based on single-even... more The feasibility was investigated of a solid-state neutron detector/dosemeter based on single-event upset (SEU) effects in dynamic random-access memories (DRAMs), commonly used in computer memories. Such a device, which uses a neutron converter material to produce a charged particle capable of causing an upset, would be light-weight, low-power, and could be read simply by polling the memory for bit flips. It would have significant advantages over standard solid-state neutron dosemeters which require off-line processing for track etching and analysis. Previous efforts at developing an SEU neutron detector/dosemeter have suffered from poor response, which can be greatly enhanced by selecting a modern high-density DRAM chip for SEU sensitivity and by using a thin 10B film as a converter. Past attempts to use 10B were not successful because the average alpha particle energy was insufficient to penetrate to the sensitive region of the memory. This can be overcome by removing the surface passivation layer before depositing the 10B film or by implanting 10B directly into the chip. Previous experimental data show a 10(3) increase in neutron sensitivity by chips containing borosilicate glass, which could be used in an SEU detector. The results are presented of simulations showing that the absolute efficiency of an SEU neutron dosemeter can be increased by at least a factor of 1000 over earlier designs.

Progress towards a LaBr3-based associated particle imaging test bed for contraband detection and bulk materials analysis

Chemical, Biological, Radiological, Nuclear, and Explosives (CBRNE) Sensing XVII, 2016

An array of nine 811 cm3 LaBr3:Ce crystals coupled to photomultiplier tubes is used to detect γ r... more An array of nine 811 cm3 LaBr3:Ce crystals coupled to photomultiplier tubes is used to detect γ rays induced from materials by neutrons emitted from a Deuterium-Tritium neutron generator. The accompanying digital data acquisition system has been developed to understand operational limits for remote detection of explosive contraband and analysis of material composition. Results are presented demonstrating current system performance, with the eventual goal of detecting a small (less than 5%) change in the composition of a material. Improvement expected over existing analog data collection systems are described along with discussion of the enhancements.

Measurement of Compton scattered electrons using monochromatic X-rays

AIP Conference Proceedings, 2001

The technique of zero degree electron spectroscopy, first developed in experiments involving ion-... more 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

Glow Curve Analysis Applied to the Discrimination of X Ray Versus Proton Irradiation

Radiation Protection Dosimetry, 2002

Three types of thermoluminescence dosemeters (TLDs): LiF:Mg,Ti (TLD-100), CaF 2 :Tm (TLD-300), an... more Three types of thermoluminescence dosemeters (TLDs): LiF:Mg,Ti (TLD-100), CaF 2 :Tm (TLD-300), and ␣-Al 2 O 3 :C (TLD-500), were investigated for their glow curve response to separate X ray and proton irradiations. The glow curve structure for each individual TLD's exposure to the X ray and proton irradiations was analysed and compared. Distinguishable differences between the glow curve structure characteristic of each type of radiation were observed. The proton TLD-100 glow curve has revealed a complex high-temperature peak structure that was used for the proton/X ray discrimination algorithm. Proton irradiation of TLD-300 resulted in an apparent switch in the relative heights of peaks 3 and 5 as compared to X ray. In TLD-500, proton irradiation produced a more subtle difference in the glow curve with an increase in the ratio between high-and low-temperature peaks. Results demonstrate promising differences in glow curve structure present allowing for discrimination between X ray and proton radiation field exposures.

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

Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 1991

The Naval Surface Warfare Center (NSWC) at White Oak is cornpfeting a major upgrading of its posi... more The Naval Surface Warfare Center (NSWC) at White Oak is cornpfeting a major upgrading of its positive-ion accelerator facilities dedicated to the study of ion-atom collisions and ion-beam andysis of materials. At the heart of the facility is a new National Electrostatics Corporation 3 MV tandem accelerator which repiaced 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 establislnnent 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, 8 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.

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Determination of radiation exposure history of common materials and computer hardware by using atomic (and magnetic force) microscopy

Applied Physics Letters, 2003

Defects produced by ionizing radiation are smaller than a micrometer and are unobservable in an o... more Defects produced by ionizing radiation are smaller than a micrometer and are unobservable in an optical microscope. An atomic force microscope was utilized to reveal their counts and structure in common materials like mica, silicon, organic solids, polymers, sugar, quartz, and calcite. A magnetic force microscope has shown the damage of radiation on computer hard disks. The present work shows that exposure to radioactive material leaves a permanent record, which can be read for dosimetric or forensic purposes by using atomic force microcopy on common objects or a magnetic force microscope on magnetic media.

Investigation of 3D ORAM materials as radiation detectors/dosimeters

1999 IEEE Nuclear Science Symposium. Conference Record. 1999 Nuclear Science Symposium and Medical Imaging Conference (Cat. No.99CH37019)

Abstract The effects of exposure of 3D GRAM materials to protons, alphas and 12 C ions have been ... more Abstract The effects of exposure of 3D GRAM materials to protons, alphas and 12 C ions have been investigated in the energy range of 0.5 to 2.5 MeV per nucleon. A permanent change occurs in the properties of the materials due to irradiation. The exposed materials are observed to fluoresce under a confocal laser scanning microscope. The intensity and depth of the fluorescence is dependent on the type and energy of the particles to which the materials were exposed. These properties should prove useful for charged particle and ...

Relativistic electron- and proton-impact ionization of highly stripped heavy ions determined from projectile-electron loss inH2and He

Physical Review A, Mar 1, 1993

We show that electron and proton impact ionization cross sections for highly stripped heavy ions ... more We show that electron and proton impact ionization cross sections for highly stripped heavy ions can be deducedJrom the projectile electron loss cross sections determined by collisions with a H2 and a He target. _We measure electron loss for 100-and 380-MeV/u Au 52+, and 405-Me V /u U86+ in H2 and He targets, and extract the electron and proton impact ionization cross sections. Our results are compared with calculations.

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Electron-electron interaction in projectile electron loss

Physical Review A, Aug 1, 1991

Page 1. PHYSICAL REVIEW A VOLUME 44, NUMBER 3 1 AUGUST 1991 ... H.-P. Hiilskotter Department ofPh... more

Evidence for electron-electron interaction in projectileK-shell ionization

Physical Review Letters, Oct 30, 1989

We consider in our work single crystal thin films of Bi 2 Se 3 , grown by molecular beam epitaxy,... more We consider in our work single crystal thin films of Bi 2 Se 3 , grown by molecular beam epitaxy, both with and without Pb doping. Angle-resolved photoemission data demonstrate topological surface states with a Fermi level lying inside the bulk band gap in the Pb-doped films. Transport data show weak localization behavior, as expected for a thin film in the two-dimensional limit (when the thickness is smaller than the inelastic mean free path), but a detailed analysis within the standard theoretical framework of diffusive transport shows that the temperature and magnetic field dependences of resistance cannot be reconciled in a theory that neglects inter-electron interactions. We demonstrate that an excellent account of quantum corrections to conductivity is achieved when both disorder and interaction are taken into account. These results clearly demonstrate that it is crucial to include electron-electron interaction for a comprehensive understanding of diffusive transport in topological insulators. While both the ordinary bulk and the topological surface states presumably participate in transport, our analysis does not allow a clear separation of the two contributions.

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