J. L. Shinpaugh

Electron emission from amorphous solid water induced by passage of energetic protons and fluorine ions.

Abstract Absolute doubly differential electron emission yields were measured from thin films of amorphous solid water (ASW) after the transmission of 6 MeV protons and 19 MeV (1 MeV/nucleon) fluorine ions. The ASW films were frozen on thin (1-µm) copper foils cooled to approximately 50 K. Electrons emitted from the films were detected as a function of angle in both the forward and backward direction and as a function of the film thickness. Electron energies were determined by measuring the ejected electron time of flight, a technique that optimizes the accuracy of measuring low-energy electron yields, where the effects of molecular environment on electron transport are expected to be most evident. Relative electron emission yields were normalized to an absolute scale by comparison of the integrated total yields for proton-induced electron emission from the copper substrate to values published previously. The absolute doubly differential yields from ASW are presented along with integrated values, providing single differential and total electron emission yields. These data may provide benchmark tests of Monte Carlo track structure codes commonly used for assessing the effects of radiation quality on biological effectiveness.

Anomalous q dependence of 0' binary encounter electron production in energetic collisions of Fq+ (q=3-9) with He and H2 targets

The projectile charge state dependence of binary encounter electron (BEe) production was studied at zero degrees with respect to the beam direction for collisions of 19 and 28.5 MeV F(3(sup)-or+9)+= with H2 and He targets. The measured double differential cross sections for BEe production were found to be increasingly enhanced with decreasing projectile charge state contrary to the usual screening behaviour. The relative enhancement of the BEe F8+ yields over the F9+ yields was found to be about 15% increasing to about 50% for F3+ projectiles. Plausible mechanisms are discussed; however, no theoretical explanation of this anomalous enhancement is established.

A zero-degree tandem electron spectrometer and rydberg analyzer for projectile electron studies in ion-atom collisions☆

Abstract An electron spectrometer system consisting of a tandem 45° parallel-plate spectrometer preceded by a Rydberg analyzer has been developed to study projectile electron capture, ionization and excitation by 0° spectroscopy. This system has the ability to resolve a large number of excited and continuum atomic states of the projectile and has a low kinematic line broadening. High-n Rydberg electrons are field-ionized and decelerated in an inhomogeneous-field Rydberg analyzer. The field-ionized electrons are then energy-dispersed according to n-value. The design and performance of the system is presented with some results of continuum (cusp), Rydberg, Coster-Kronig and highly resolved (ΔE/E = 0.1–0.2%) K-Auger electrons for 0.5–1.75 MeV/amu, Fq++He, Ne, H2 collisions.

Collisions of highly charged ions with electrons, atoms and surfaces

At the Oak Ridge Multicharged Ion Source Facility, an experimental atomic collisions physics program is centered around a recently upgraded Electron Cyclotron Resonance (ECR) multicharged ion source. The 10 GHz CAPRICE source has been in operation since October 22, 1992, and has provided more intense, higher charge ion beams than our previous ECR ion source. Intense metallic beams have recently become available with the installation of a metallic oven on the source. In addition to measurements of electron-impact excitation, carried out in collaboration with the Joint Institute for Laboratory Astrophysics (JILA), experiments are presently on-line to study electron-impact ionization, low-energy ion-atom collisions, and ion-surface interactions. A brief summary of our various activities with an emphasis on the new capabilities is presented.

High resolution studies of electron capture and excitation by 0° projectile electron spectroscopy

Abstract A tandem 45° parallel-plate electron spectrometer has been developed for high resolution electron studies of atomic collision processes using the method of 0° electron spectroscopy. High resolution projectile K-Auger electron spectra have been obtained in a state-selective study of transfer-excitation (TE) and single excitation in collisions of 4.5–33 MeV F6+ with He and H2 targets. A high resolution measurement of 9.5 MeV F7+ with He yielded preliminary information on state-selective capture to the metastable F7+ (1s2s) state. Future plans include studies of Fq+ + T (q: 6−9; T: H2, He, Ne) to determine absolute state-selective cross sections for transfer-excitation, single and double electron capture.

Doubly Differential Electron Yields from Thin Copper Foils Induced by Fast Ion Impact

The double‐differential yields of electrons ejected from thin metal foils can provide detailed information on the transport properties of secondary electrons produced within condensed phase material by fast charged particles. Models of charged‐particle track structure in condensed phase material depend on theoretical elastic and inelastic scattering cross sections based on material parameters. Because of the uncertainties in theoretical methods used to describe low‐energy electron scattering in condensed matter and the infeasibility of direct measurements of cross sections in the condensed phase, electron transport models are tested by comparison of calculated and measured spectra of electrons that have undergone transport through the bulk and exited through the surface. Since energy and angular resolved electron yields provide the most stringent tests of theory, and electron scattering cross sections are most uncertain for low energies, we have applied time‐of‐flight methods to focus on the low‐energy el...

Determination of plasma trace elements in tumor-bearing animals by proton-induced X-ray emission spectroscopy.

Although altered levels of circulating essential trace elements are known to accompany malignant disease, the lack of sensitivity of conventional detection methods has generally limited their study to clinical conditions involving extensive disease (i.e., significant tumor burden). As such, the application of altered trace element levels as potential prognostic guides or as response indicators subsequent to treatment has been of limited use. During this study, proton-induced X-ray emission spectroscopy was evaluated as a tool to determine trace element imbalances in a murine tumor model. Using plasma from C57B1/6 mice bearing the syngeneic Lewis lung carcinoma (LLCa), levels of Fe, Cu, and Zn, as well as changes in the Cu/Zn ratio, were measured in animals carrying an increasing primary tumor burden. The plasma levels of Fe, Cu, and Zn were found to decrease significantly 7 d following implants of LLCa cells with no significant change observed in the Cu/Zn ratio. By d 21, however, an increase in the Cu/Zn ratio was found to accompany increased growth of the LLCa tumor; the plasma levels of Cu had returned to normal levels, whereas both the Fe and Zn plasma levels remained lowered. Collectively, the results suggest that although a net change in individual plasma trace element concentrations might not be accurately associated with tumor growth, a clear relationship was established between the Cu/Zn ratio and tumor size.

Electron emission from condensed phase material induced by fast protons

Monte Carlo track simulation has become an important tool in radiobiology. Monte Carlo transport codes commonly rely on elastic and inelastic electron scattering cross sections determined using theoretical methods supplemented with gas-phase data; experimental condensed phase data are often unavailable or infeasible. The largest uncertainties in the theoretical methods exist for low-energy electrons, which are important for simulating electron track ends. To test the reliability of these codes to deal with low-energy electron transport, yields of low-energy secondary electrons ejected from thin foils have been measured following passage of fast protons. Fast ions, where interaction cross sections are well known, provide the initial spectrum of low-energy electrons that subsequently undergo elastic and inelastic scattering in the material before exiting the foil surface and being detected. These data, measured as a function of the energy and angle of the emerging electrons, can provide tests of the physics of electron transport. Initial measurements from amorphous solid water frozen to a copper substrate indicated substantial disagreement with MC simulation, although questions remained because of target charging. More recent studies, using different freezing techniques, do not exhibit charging, but confirm the disagreement seen earlier between theory and experiment. One now has additional data on the absolute differential electron yields from copper, aluminum and gold, as well as for thin films of frozen hydrocarbons. Representative data are presented.

Laboratory Data Needs and Applications for Assessing Radiation Effects in Biological Materials

All types of ionizing radiation interact with material by producing atomic or molecular ions, excited states, and secondary electrons. Still, different types of radiation lead to quite different yields of biological damage. It is generally believed that the spatial distributions of ionization and excitation produced by the slowing down of charged particles, particularly electrons, govern the yields of bioactive molecular species. The assessment of these spatial patterns of ionization and excitation depend largely on the production cross sections for secondary electrons, the energies and angular correlations of their production, and the subsequent differential cross sections for their energy loss in the media of interest. The most thorough assessment of spatial patterns of energy deposition by charged particles is obtained using Monte Carlo simulation of charged particle track structure based on the available database of interaction cross sections. This step‐by‐step analysis of the interactions of charged ...

SU-F-T-127: Charged Particle Transport in Condensed Media

PURPOSE Provide quality interaction cross sections for charged particle Monte Carlo (MC) track structure codes and evaluate low energy electron transport in condensed media. METHODS MC methods and codes are often used to model or simulate charged particle radiation transport in matter. Detailed (or event-by-event) track structure simulations are of special interest for the modeling of the physical and chemical stages of radiation action with matter and the initial radiation damage to biological systems. They require reliable interaction cross sections of all radiation qualities considered (e.g., electrons, protons, alpha particles, light and heavy ions) with the target material under consideration, mainly in the condensed phase, including liquids and solids. Interaction cross sections are calculated using the dielectric formalism, a mixture of first principles, theoretical modeling and experimental information for bulk and surface transport and implemented into MC track structure codes. Secondary electron emission yields from amorphous solid water (ASW) and thin metal foils (copper and gold) after fast proton impact are simulated and measured experimentally. RESULTS After considering different transport models for bulk and surface transport and a careful modeling of the experimental geometry, simulated secondary electron emission yields follow the trends of experimental data well. Furthermore, yields for electron emissions below 50 eV are sensitive to differential elastic scattering cross sections. CONCLUSION Low-energy electron transport in condensed media is still a challenge for detailed track structure simulation codes. Interaction cross-sections, transport models, and target geometry need to be considered adequately. The research was funded in part by NSF Major Research Instrumentation Program 2009, Award Number: 0923270 and by NIH/NCI R01 CA93351.