Thomas M. Rosseel

A design for a high intensity slow positron facility using forward scattered radiation from an electron linear accelerator

A tungsten moderator will be placed behind the target of the Oak Ridge Electron Linear Accelerator (ORELA) to convert gamma radiation to slow positrons. These will be extracted and led through evacuated solenoids to an experiment room. A Penning trap will be used to extend the slow positron pulses to achieve duty factors of 10% or greater. The facility will be used for atomic and molecular physics studies, positron microscopy, and materials research. Operations will be inexpensive and will not interfere with the normal function of ORELA, the measurement of neutron cross sections by flight-time spectrometry.

Electron transfer processes in collisions of highly charged energetic (0.1–1.0 MeV/nucleon) ions with helium atoms

Abstract We have investigated charge transfer in collisions of energetic (0.1−1 MeV/nucleon) highly charged ions with helium atoms with the principal aim of clarifying the nature of two-electron processes. The sensitivity of partial charge-changing cross sections (i.e., single- and double-charge transfer, transfer ionization (TI), and single and double ionization) to core configuration and scaling rules for one- and two-electron process were investigated with iodine ions ( q =5+ → 26+) and uranium ions ( q =17+ → 44+) using an ion-charge state, recoil-ion coincidence method. Using zero-degree electron spectroscopy in coincidence with charge transfer, we found that at the higher energies, as in the case of 0.1 MeV/nucleon ions previously reported, TI involves the transfer of two electrons to a higher correlated state followed by loss of one electron to the continuum. In addition, we observe very high Rydberg electrons in coincidence with TI, implying a possible up-down correlation in the pair transfer. In addition, we made measurements of VUV photons emitted at the collision in coincidence with He + and He 2+ recoils. The results show that TI leads to capture into lower n states than single-charge transfer.

L-shell X-ray production cross sections in Nd, Gd, Ho, Yb, Au and Pb for 25-MeV carbon and 32-MeV oxygen ions☆

Abstract L-shell X-ray production cross sections in 60 Nd, 64 Gd, 67 Ho, 70 Yb, 79 Au and 82 Pb have been measured for incident 25 MeV 6 12 C + q ( q = 4, 5, 6) and 32 MeV 8 16 O + q ( q = 5, 7, 8) ions. Measurements were made on targets ranging in thickness from 1 to 100 μg/cm 2 . Enhancement in the L-shell X-ray production cross section for projectiles with one or two K-shell vacancies over those for projectiles with no K-shell vacancies is observed. The sum of direct ionization to the continuum (DI) plus electron capture (EC) to the L, M, N… shells and EC to the K-shell of the projectile have been extracted from the data. Calculations in the first Born approximation are ~10 times larger than the data. Predictions of the ECPSSR theory that accounts for the energy-loss, Coulomb deflection, perturbed-stationary state and relativistic effects are in good agreement with the data for both ions.

Application of heavy-ion-induced X-ray satellite emission to alloys

High-resolution measurements of heavy-ion-induced X-ray satellite emission (HIXSE) spectra have been obtained for a series of Mo-Ti binary alloys and V(FexCoyNiz)3 long-range-ordered (LRO) alloys. Variations in the relative intensity distribution of the satellite lines of these alloys were observed and are attributed to the influence of the valence-electron distribution or chemical environment of the emitting target atom. Results are also presented which demonstrate the ability of this method to differentiate between different crystal structures of the same alloy. The sensitivity of HIXSE to changes in the valence-electron environment of alloys may provide a means of probing metal valence states in metals and alloys.

Dielectronic Excitation and Recombination in Crystal Channels

In previous papers, in this volume, electron-ion recombination processes have been studied using either free-electron targets or targets of “almost-free” electrons which are loosely bound to atoms or molecules. In the former case, the techniques generally involve either merged electron-ion beams in “single-pass” configuration or in electron “coolers” in storage rings or observation of photons and associated charge state fractions in electron beam ion sources, in either the EBIS or EBIT configuration. These techniques are direct and, in the case of merged or crossed beam experiments, offer very high energy resolution.

Heavy-Section Steel Technology and Irradiation Programs—Retrospective and Prospective Views

In 1965, the Atomic Energy Commission (AEC), at the advice of the Advisory Committee on Reactor Safeguards (ACRS), initiated the process that resulted in the establishment of the Heavy Section Steel Technology (HSST) Program at Oak Ridge National Laboratory (ORNL). In 1989, the Heavy-Section Steel Irradiation (HSSI) Program, formerly the HSST task on irradiation effects, was formed as a separate program, and, in 2007, the HSST/HSSI Programs, sponsored by the U.S. Nuclear Regulatory Commission (USNRC), celebrated 40 years of continuous research oriented toward the safety of light-water nuclear reactor pressure vessels (RPV). This paper presents a summary of results from those programs with a view to future activities.

High-resolution HIXSE studies of the chemical environment of sulfur implanted in quartz glass

High-resolution sulfur Kα X-ray satellite spectra produced by 30 MeV Cl ion irradiation of quartz glass containing ion-implanted sulfur have been obtained. The normalized emission spectra exhibited variations in the intensity distribution of the satellite structure for different implant doses. Correlation of parameters describing the satellite intensity profile to the dose and the local chemical environment suggest that a method for nondestructively probing implant profiles, free from restrictions on the Z of the implant and matrix, may be possible.

High-resolution heavy-ion-induced X-ray satellite emission study of implanted sulfur as a probe of co-implanted oxygen in an oxide substrate

Abstract Using high-resolution measurements, the relative fluorescence yields of the sulfur K α L n X-ray satellite lines produced by 24 MeV Si q + ion irradiation of a series of quartz glass targets co-implanted to equal doses and depths of sulfur and oxygen have been obtained and compared to the yields from targets implanted to the same depth with sulfur only. The data exhibit lower yields in the higher-order KL n satellite lines for the co-implanted oxygen targets but similar dose dependent curves. These results are consistent with oxygen increasing the valence electron density of the local sulfur environment. They also suggest that, with an appropriate choice of a probe ion, HIXSE may be capable of detecting impurity ions in matrices that normally mask the impurity, e.g., oxygen in oxide substrates.

Electron ion interactions in crystal channels: collisions in ultra-dense electron media

Dielectronic excitation of H-like S, Ca and Ti is shown to occur in the dense electron gas of a crystal channel. Cross sections for collisional ionization of the short lived excited states can then be determined. Ionic excitation can also be achieved by resonant coherent excitation in which case specific m states can be excited for further study.

Chemical effects on the L heavy-ion-induced X-ray satellite emission (HIXSE) spectra of niobium alloys and compounds

Abstract The Nb L X-ray satellites produced by 36 MeV Cl ion bombardment of a series of Nb alloys and compounds have been examined using a high-resolution (8 eV at 2.2 keV) Bragg-crystal spectrometer. Variations in the normalized emission spectra of the L α n M m and L β n M m satellite lines are observed and attributed to the influence of the valence-electron distribution or chemical environment of the emitting Nb ion. Difference spectra, generated by using the Nb metal spectrum as the subtrahend, amplified those variations. Results will be presented which demonstrate the chemical sensitivity of the L X-ray satellites. Potential applications to materials problems are also discussed.