Noel A. Guardala

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.

An intense, compact fourth-generation positron source based on using a 2 MeV proton accelerator

A method of producing an intense source of e+’s that uses the internal pair production decay of the 6.05 MeV, 0+ first excited state of O16 is described. The first excited state decays overwhelmingly by this process with the competing decays of double gamma-ray emission and atomic electron internal conversion contributing about 1 decay in 104. The most effective way to produce this excited state of 16O in terms of compactness, power consumption, and general ease of operation is to use the strong proton-induced resonance on F19 at 1.89 MeV. This resonance is the most favorable low-energy proton resonance in F19 for populating the 6.05 MeV, 0+ state. We predict low-energy e+ beams with intensity of 107–108/s by using off-the-shelf accelerator technology producing a 2 MeV proton beam with average currents of 1 to 5 mA.

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.

Detection of Hidden Explosives Using Resonant Gamma Rays From In‐Flight Annihilation of Fast Positrons

Gamma rays with tunable energies can be produced by the in‐flight annihilation of fast positrons. The kinetic energy of the positron beam determines the gamma‐ray energy of annihilation photons emitted in a narrow cone in the forward direction. These photons can be used for nuclear resonant fluorescence determination of explosive materials rich in 14N selecting gamma rays with energies that match excited states in 14N and then observing the emitted nuclear fluorescence.

Innovative method of using in-flight annihilation of fast positrons to detect explosives

We have developed an innovative method using radiation produced by in flight annihilation of energetic positrons to detect hidden explosives and other illegal substances. The system uses either radioisotope or compact accelerator based methods to generate a high energy positron beam. The high energy positrons annihilate in flight producing a tunable, narrow spectrum beam of high energy photons. The photon energy, which is determined by the positron energy, can be chosen to be resonant with elements of the explosive or other target. The concentration of the target material determines the intensity of the return signal. Standard gamma radiation detection techniques are used to detect the emitted gamma rays. Because of the innovative method we use to generate and monochromatize the positron beam, the entire system is inexpensive, compact and portable.

Evaluation of 166m Ho: In search of photonuclear cross sections and trigger-level paths

Part of the Army Research Laboratory’s battery program is research in the viability of nuclear batteries. Particularly attractive is a nuclear battery that can be stored without loss of energy, and then turned on by some external signal. This paper describes the progress toward that goal.