L. Toburen - Academia.edu (original) (raw)

Papers by L. Toburen

AIP Conference Proceedings, 1996

Beginning with his initial studies of the angular dependence of the spectra of electrons emitted ... more Beginning with his initial studies of the angular dependence of the spectra of electrons emitted in ion‐atom collisions, the first measurements to provide a detailed and comprehensive description of the collisional ionization process, M. Eugene Rudd contributed to an impressive list of ‘‘firsts’’ in the study of collision physics. In 1963, Gene published the first observation of a two‐center phenomena in collision physics, although it was several years before the features he observed in the spectra of ejected electrons were clearly interpreted as contributions from electron‐capture‐to‐the‐continuum, a two‐center phenomena. He contributed firsts in studies of doubly differential cross sections, inner‐shell‐ and auto‐ionization, interactions involving dressed projectiles, and interactions of ions and photons with surfaces. He also refined the experimental techniques to provide data of improved reliability in many areas where others had made pioneering studies including measurements of doubly‐differential cr...

Radiation Protection Dosimetry, 2006

The use of heavy ion beams for microbeam studies of mammalian cell response leads to a need to be... more The use of heavy ion beams for microbeam studies of mammalian cell response leads to a need to better understand interaction cross sections for collisions of heavy ions with tissue constituents. For ion energies of a few MeV u^-1 or less, ions capture electrons from the media in which they travel and undergo subsequent interactions as partially 'dressed' ions. For

Physics Letters A, 1977

Absolute cross sections for K-shell ionization of fluorine by 0.5 and 1.5 MeV protons were measur... more Absolute cross sections for K-shell ionization of fluorine by 0.5 and 1.5 MeV protons were measured for the gas targets BF3, CF4, C2F6, SF6, and TeF6. The cross sections for these molecular targets agree to within +/-5% except for SF6 where yields were approximately 10% smaller.

Zeitschrift f�r Physik, 1970

ABSTRACT

AIP Conference Proceedings, 1999

ABSTRACT Measurements of atomic inner-shell ionization cross sections for low-Z elements are comp... more ABSTRACT Measurements of atomic inner-shell ionization cross sections for low-Z elements are complicated by small fluorescence yields and because elements do not occur naturally in atomic form. These conditions favor the measurement of Auger-electron yields to determine vacancy formation and they require the use of molecular gas targets. Because molecular structure has little influence on the energy levels of inner shells it is expected to have little effect on K-shell ionization probabilities. On the other hand, Auger-electron transitions involve molecular orbitals which introduce molecular effects in the spectra and could potentially influence yields. Variations in measured atomic K-shell ionization cross sections for different molecular targets have generally been found to be small (10-15%) and explained by simple geometric effects. Some recent measurements have, however, exhibited molecular effects as large as a factor of three which are beyond simple explanations. This has revived our interest in trying to further quantify the extent of molecular effects in K-shell ionization. We have employed a new method using well tested theoretical benchmarks to derive atomic cross sections from molecular targets to study K-shell ionization of carbon and fluorine in a number of fluorocarbon molecules excited by MeV protons and lithium ions. The atomic cross sections derived from these measurements are not found to exhibit effects of molecular structure within the experimental uncertainties of approximately 10% for protons and 25% for lithium ions.

IEEE Transactions on Nuclear Science, 1979

Advances in Space Research, 1992

The biological effectiveness of radiations depends on the spatial pattern of ionizations and exci... more The biological effectiveness of radiations depends on the spatial pattern of ionizations and excitations produced by the charged particle tracks involved. Ionizations produced by both the primary ion and by energetic delta rays may contribute to the production of biologically relevant damage and to the concentration of damage which may effect the probability of repair. Although average energy concentration (dose) can be calculated using homogeneous track models, the energy is actually concentrated in small volumes containing segments of the ion and delta ray tracks. These local concentrations are studied experimentally using low pressure proportional counters, and theoretically, using Monte Carlo methods. Small volumes near an ion track may be traversed by a delta ray. If they are, the energy deposited will be similar to that produced by a single electron track in a low-energy x-ray irradiation. The probability of a delta ray interaction occurring decreases with the square of the radial distance from the track. The average energy deposited is the product of this probability and the energy deposited in an interaction. Average energy deposited calculated from measured interaction probability is in good agreement with the results of homogeneous track models.

Radiation Protection Dosimetry, 1990

ABSTRACT Distributions of the specific energy deposited in nanometre-sized volumes have been meas... more ABSTRACT Distributions of the specific energy deposited in nanometre-sized volumes have been measured as a function of the radial distance from the path of 5.9 MeV.amu-1 uranium and 13.0 to 17.2 MeV.amu-1 germanium beams. Tissue volumes of 125, 250, 500 and 1000 nm diameter were simulated using a grid-walled proportional counter capable of two-dimensional motion relative to the ion beam. Radial distances from 0 to 8000 nm were investigated. Values of the mean specific energy were obtained from these distributions and compared with radial dose profiles calculated from track structure models. Excellent agreement was observed between the measured mean specific energy and the calculated radial dose for small distances relative to the track path. At longer distances, the mean specific energy deposited in small volumes became larger than the calculated average dose, reflecting the stochastic nature of the energy deposition process, i.e. at greater distances from the ion path a small site was either hit, by the traversal of secondary electron or absorption of a photon, or received no energy. If one accounts for the fraction of the particles that deposit no energy in the site, then the average specific energy deposition per particle is in excellent agreement with model calculations.

Journal of the International Commission on Radiation Units and Measurements

Electron spectra produced within a solid target irradiated by fast charged particles differ in ma... more Electron spectra produced within a solid target irradiated by fast charged particles differ in many respects from spectra obtained from a gas target. Unfortunately, it is very difficult to measure directly the spectrum of internal electrons produced in the solid so this has been done only for a few limited cases. Generally, the spectra of electrons are recorded outside the solid, and they are modified by interactions during migration to the surface as well as by the transmission through the surface. Additional electrons produced in these interactions are included in the observed spectra (Figure 5.1). To analyze the process of secondary electron emission from solids, the sequence of events is divided into three stages: 1) production of internal secondaries as a result of the interaction between the primary (incident) particle and the target electrons and atoms, 2) the migration of the liberated electrons to the surface, and 3) their escape through the surface into vacuum. These three stages are shown schematically in Figure 5.1. This is a simplified picture since the secondary electron is shown as continuing its motion through the solid along a straight line without any further collisions, and the incident particle makes only a single collision. In the case of incident electrons, it is not possible to distinguish between secondaries or primaries in the spectra obtained outside the solid. A large fraction of the electrons in the spectra are primary electrons which have been deflected or slowed down inside the target. Generally, a reflected or transmitted electron undergoes. so many collisions that it is impossible to distinguish between stages 1 and 2. A typical external spectrum resulting from electron bombardment of solids is displayed in Figure 5.2. Since it is not possible to distinguish between secondary electrons and primary electrons that have been slowed down, it is customary to divide the emitted electrons into two groups: those with energies less than 50 eV which are taken to be the true secondaries and those between 50 e V and the primary energy E which are the reflected (backscattered) electrons. The total yield of electrons with energies up to 50 eV is called 0 and that of the reflected electrons 1/. Of course, this arbitrary division does not reflect the origin of a particular electron, but it is convenient since the behavior of slow electrons in solids differs from that of electrons with energies above 50 eV. In contrast, for ion-induced electron emission all electrons are secondary particles. This fact makes the interpretation of ion-induced electron spectra simpler than electron-induced spectra. The rapid development of ultrahigh vacuum (UHV) technique during the last twenty years has stimulated a renewed activity in studies of secondary electron emission from solids since the number and the corresponding energy distribution of electrons with energies below 10 eV depend critically on the surface conditions. Generally, only results obtained from clean surfaces prepared by ion sputtering or in-situ film deposition can be considered as reliable for these low energies. With increasing exit energies, the requirements for the experimental procedure become less critical, and for the highest exit energies considered (1 to 3 MeV), the surface preparation hardly plays any role. In many cases, one may utilize the similarity between electron spectra induced by electrons and those induced by fast protons. Stages 2) and 3) do not depend on the type of primary particle, and for electrons as well as protons of high speed, several theoretical treatments (see Sections 2.6,2.7, and 2.8) predict that the cross section for ionization of the i'th subshell depends essentially only on the speed of the primary particle. Unfortunately, experimental data comparing spectra from electon and proton impact on solids are scarce. Examples are the energy spectra from polycrystalline niobium obtained for 3-keV electron and 400-keV proton impact by Musket (1975) as shown in Figure 5.3. Even though the impact speeds are not the same, the spectra are indeed very similar. This similarity simplifies the discussion of external electron spectra at low ejection energies induced by electron bombardment, since there exists a large amount of high quality data on spectra obtained by fast protons.

Physical Review, 1968

... The results presented here were included in a thesis submitted by LH Toburen to the faculty o... more ... The results presented here were included in a thesis submitted by LH Toburen to the faculty of Vanderbilt University in partial fulfillment of the requirements for the degree of Doctor of Philosophy. t Present address: Osaka Laboratory, Atomic Energy Research Institute, 508 Mii ...

Physical Review, 1969

The double-electron-capture cross sections were measured for protons in the energy range from 75 ... more The double-electron-capture cross sections were measured for protons in the energy range from 75 to 250 keV incident upon the target gases H2, He, Ar, Kr, N2, and H2O. A power law extrapolation of the present results for H2 and He joins smoothly with previously measured cross sections at higher- and lower-impact energies; however, wide discrepancies exist between measured and calculated cross section values. For the heavier target gases, a power law extrapolation of the present results is in disagreement with previous measurements. This disagreement is attributed to the power law being inappropriate at energies where electron capture from inner electron shells is possible.

Inner-Shell and X-Ray Physics of Atoms and Solids, 1981

The electron spectra of autoionizing levels in helium excited by photons, electrons and light ion... more The electron spectra of autoionizing levels in helium excited by photons, electrons and light ions has been the subject of extensive studies1. Due to Wigner’s spin conservation rule no appreciable formation of triplet levels in doubly excited HeI** has been found by proton impact on He target gas. However, triplet states can be formed via electron exchange2, using bombardment of structured ions such as He+. For sufficiently low-velocity ions autoionization may be influenced by the proximity of the projectile during the decay of the helium excited states. Decay of excited states of the quasi-molecule formed during slow collisions has been observed to produce shifts in the energy of excited states3 as well as Stark mixing of magnetic substates which decay in the electric field of the projectile. The latter effect influences the angular distributions of ejected electrons.4 Little is known regarding the excitation of helium autoionization states by heavy multiply charged ions. Excitation by fast 30 MeV 05+ resulted in strong excitation of He(2s2p)1D series with essentially no triplet state excitation5. The present measurements extend the range of velocity and charge states of the incident ions to provide greater detail on the relative importance of collision parameters on the excitation and de-excitation process.

Cross sections for K-shell ionization of fluorine and carbon have been measured from Auger-electr... more Cross sections for K-shell ionization of fluorine and carbon have been measured from Auger-electron yields for 2-MeV proton impact on CH_3F, CHF_3, CF_4, C_2F_6, and C_4F_8. Ejected electrons were electrostatically analyzed for electron-emission angles from 20^circ to 125^circ with respect to the incident beam. Absolute cross sections were determined by integrating the doubly differential yields with respect to emission angle

Physical Review A, 1972

ABSTRACT

Radiation Protection Dosimetry, 2002

When charged particles slow in tissue they undergo electron capture and loss processes that can h... more When charged particles slow in tissue they undergo electron capture and loss processes that can have profound effects on subsequent interaction cross sections. Although a large amount of data exists for the interaction of bare charged particles with atoms and molecules, few experiments have been reported for these 'dressed' particles. Projectile electrons contribute to an impact-parameter-dependent screening of the projectile charge that precludes straightforward scaling of energy loss cross sections from those of bare charged particles. The objective of this work is to develop an analytical model for the energy-loss-dependent effects of screening on differential ionisation cross sections that can be used in track structure calculations for high LET ions. As a first step a model of differential ionisation cross sections for bare ions has been combined with a simple screening model to explore cross sections for intermediate and low energy dressed ions in collisions with atomic and molecular gas targets. The model is described briefly and preliminary results compared to measured ejected electron energy spectra.

Radiation Protection Dosimetry, 2011

Monte Carlo track simulation has become an important tool in radiobiology. Monte Carlo transport ... more 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.

Radiation and Environmental Biophysics, 1998

It is widely accepted that an understanding of the detailed structure of charged particle tracks ... more It is widely accepted that an understanding of the detailed structure of charged particle tracks is essential for interpreting the mechanistic consequences of energy deposition by high linear energy transfer (LET) radiation. The spatial relationship of events along the path of a charged particle, including excitation, ionization, and charge-transfer, govern subsequent chemical, biochemical, and biological reactions that can lead to adverse biologic effects. The determination of spatial patterns of ionization and excitation relies on a broad range of cross-section data relating the interactions of charged particles to the molecular constituents of the absorbing medium. It is important that these data be absolute in magnitude, comprehensive in scope, and reliable if accurate assessment of track structure parameters is to be achieved. Great strides have been made in the development of this database, understanding the underlying theory, and developing analytic models, particularly for interactions involving electrons and protons with atoms and molecules. The database is less comprehensive for interactions involving heavier charged particles, especially those that carry bound electrons, and for interactions in condensed phase media. Although there has been considerable progress in understanding the physical mechanisms for interactions involving fast heavy ions and atomic targets during the past few years, we still lack sufficient understanding to confidently predict cross-sections for these ions with biologically relevant material. In addition, little is known of the interaction cross-sections for heavy charged particles as they near the end of their track, i.e., for low velocity ions where collision theory is less well developed and where the particle's net charge fluctuates owing to electron capture and loss processes. This presentation focuses on the current status of ionization and charge-transfer data. Compilations, reviews, Internet sources, theoretical models, and recent data applicable to track structure calculations are discussed.

Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 1989

The physical structure of charged-particle tracks plays an important role in determinin g the res... more The physical structure of charged-particle tracks plays an important role in determinin g the response of a stopping material to the absorption of energy following irradiation by charged particles and neutrons. For biological systems, and for microelectronic circuits, the response is sensitive to the energy deposited in submicron-size volume elements. Because of the stochastic nature of energy deposition in and near charged particle tracks, there may be wide variations in the actual quantities of energy deposited in critical volume elements. Since direct measurements of energy deposition in condensed material (solids or liquids) are not technologically feasible, the effects of charged-particle track structure are usually estimated from one of several model calculations; the most commonly used are homogeneous track structure models. Recent interest in the radiation biology of high Linear-Energy-Transfer (LET) radiation has spurred interest in testing these model descriptions of the structure of high-energy heavy-ion tracks. As one means of providing such tests, experiments were recently conducted at the GSI-Darmstadt, UNILAC accelerator, to measure energy deposition in small volumes as a function of the radial distance from the path of fast, heavy ions. These measurements, conducted in collaboration with research groups of Dr. G. Kraft at GSI and Prof. Schmidt-Bijcking of the University of Frankfurt, were made for simulated tissue volumes 0.5 and 1.0 pm in diameter, located from 0 to 10 pm from the path of Ge ions having energies from 13.0-to 17.2-MeV/amu. Excellent agreement was observed between model calculations and measured dose distributions for radial distances up to a few micrometers in simulated tissue. At greater distances the actual measured dose in irradiated volumes was much more than the calculated average value. These differences reflect the stochastic nature of energy deposition in which a large fraction of the volume elements receive no dose from a given particle traversal, but elements which do receive energy receive relatively large amounts. This may have important consequences for effects which occur with a nonlinear or threshold energy response. * Work Supported by the Office of Health and Environmental Research (OHER), US Department of Energy under Contract DE-ACO6-76RL0 1830.

AIP Conference Proceedings, 1996

Beginning with his initial studies of the angular dependence of the spectra of electrons emitted ... more Beginning with his initial studies of the angular dependence of the spectra of electrons emitted in ion‐atom collisions, the first measurements to provide a detailed and comprehensive description of the collisional ionization process, M. Eugene Rudd contributed to an impressive list of ‘‘firsts’’ in the study of collision physics. In 1963, Gene published the first observation of a two‐center phenomena in collision physics, although it was several years before the features he observed in the spectra of ejected electrons were clearly interpreted as contributions from electron‐capture‐to‐the‐continuum, a two‐center phenomena. He contributed firsts in studies of doubly differential cross sections, inner‐shell‐ and auto‐ionization, interactions involving dressed projectiles, and interactions of ions and photons with surfaces. He also refined the experimental techniques to provide data of improved reliability in many areas where others had made pioneering studies including measurements of doubly‐differential cr...

Radiation Protection Dosimetry, 2006

The use of heavy ion beams for microbeam studies of mammalian cell response leads to a need to be... more The use of heavy ion beams for microbeam studies of mammalian cell response leads to a need to better understand interaction cross sections for collisions of heavy ions with tissue constituents. For ion energies of a few MeV u^-1 or less, ions capture electrons from the media in which they travel and undergo subsequent interactions as partially 'dressed' ions. For

Physics Letters A, 1977

Absolute cross sections for K-shell ionization of fluorine by 0.5 and 1.5 MeV protons were measur... more Absolute cross sections for K-shell ionization of fluorine by 0.5 and 1.5 MeV protons were measured for the gas targets BF3, CF4, C2F6, SF6, and TeF6. The cross sections for these molecular targets agree to within +/-5% except for SF6 where yields were approximately 10% smaller.

Zeitschrift f�r Physik, 1970

ABSTRACT

AIP Conference Proceedings, 1999

ABSTRACT Measurements of atomic inner-shell ionization cross sections for low-Z elements are comp... more ABSTRACT Measurements of atomic inner-shell ionization cross sections for low-Z elements are complicated by small fluorescence yields and because elements do not occur naturally in atomic form. These conditions favor the measurement of Auger-electron yields to determine vacancy formation and they require the use of molecular gas targets. Because molecular structure has little influence on the energy levels of inner shells it is expected to have little effect on K-shell ionization probabilities. On the other hand, Auger-electron transitions involve molecular orbitals which introduce molecular effects in the spectra and could potentially influence yields. Variations in measured atomic K-shell ionization cross sections for different molecular targets have generally been found to be small (10-15%) and explained by simple geometric effects. Some recent measurements have, however, exhibited molecular effects as large as a factor of three which are beyond simple explanations. This has revived our interest in trying to further quantify the extent of molecular effects in K-shell ionization. We have employed a new method using well tested theoretical benchmarks to derive atomic cross sections from molecular targets to study K-shell ionization of carbon and fluorine in a number of fluorocarbon molecules excited by MeV protons and lithium ions. The atomic cross sections derived from these measurements are not found to exhibit effects of molecular structure within the experimental uncertainties of approximately 10% for protons and 25% for lithium ions.

IEEE Transactions on Nuclear Science, 1979

Advances in Space Research, 1992

The biological effectiveness of radiations depends on the spatial pattern of ionizations and exci... more The biological effectiveness of radiations depends on the spatial pattern of ionizations and excitations produced by the charged particle tracks involved. Ionizations produced by both the primary ion and by energetic delta rays may contribute to the production of biologically relevant damage and to the concentration of damage which may effect the probability of repair. Although average energy concentration (dose) can be calculated using homogeneous track models, the energy is actually concentrated in small volumes containing segments of the ion and delta ray tracks. These local concentrations are studied experimentally using low pressure proportional counters, and theoretically, using Monte Carlo methods. Small volumes near an ion track may be traversed by a delta ray. If they are, the energy deposited will be similar to that produced by a single electron track in a low-energy x-ray irradiation. The probability of a delta ray interaction occurring decreases with the square of the radial distance from the track. The average energy deposited is the product of this probability and the energy deposited in an interaction. Average energy deposited calculated from measured interaction probability is in good agreement with the results of homogeneous track models.

Radiation Protection Dosimetry, 1990

ABSTRACT Distributions of the specific energy deposited in nanometre-sized volumes have been meas... more ABSTRACT Distributions of the specific energy deposited in nanometre-sized volumes have been measured as a function of the radial distance from the path of 5.9 MeV.amu-1 uranium and 13.0 to 17.2 MeV.amu-1 germanium beams. Tissue volumes of 125, 250, 500 and 1000 nm diameter were simulated using a grid-walled proportional counter capable of two-dimensional motion relative to the ion beam. Radial distances from 0 to 8000 nm were investigated. Values of the mean specific energy were obtained from these distributions and compared with radial dose profiles calculated from track structure models. Excellent agreement was observed between the measured mean specific energy and the calculated radial dose for small distances relative to the track path. At longer distances, the mean specific energy deposited in small volumes became larger than the calculated average dose, reflecting the stochastic nature of the energy deposition process, i.e. at greater distances from the ion path a small site was either hit, by the traversal of secondary electron or absorption of a photon, or received no energy. If one accounts for the fraction of the particles that deposit no energy in the site, then the average specific energy deposition per particle is in excellent agreement with model calculations.

Journal of the International Commission on Radiation Units and Measurements

Electron spectra produced within a solid target irradiated by fast charged particles differ in ma... more Electron spectra produced within a solid target irradiated by fast charged particles differ in many respects from spectra obtained from a gas target. Unfortunately, it is very difficult to measure directly the spectrum of internal electrons produced in the solid so this has been done only for a few limited cases. Generally, the spectra of electrons are recorded outside the solid, and they are modified by interactions during migration to the surface as well as by the transmission through the surface. Additional electrons produced in these interactions are included in the observed spectra (Figure 5.1). To analyze the process of secondary electron emission from solids, the sequence of events is divided into three stages: 1) production of internal secondaries as a result of the interaction between the primary (incident) particle and the target electrons and atoms, 2) the migration of the liberated electrons to the surface, and 3) their escape through the surface into vacuum. These three stages are shown schematically in Figure 5.1. This is a simplified picture since the secondary electron is shown as continuing its motion through the solid along a straight line without any further collisions, and the incident particle makes only a single collision. In the case of incident electrons, it is not possible to distinguish between secondaries or primaries in the spectra obtained outside the solid. A large fraction of the electrons in the spectra are primary electrons which have been deflected or slowed down inside the target. Generally, a reflected or transmitted electron undergoes. so many collisions that it is impossible to distinguish between stages 1 and 2. A typical external spectrum resulting from electron bombardment of solids is displayed in Figure 5.2. Since it is not possible to distinguish between secondary electrons and primary electrons that have been slowed down, it is customary to divide the emitted electrons into two groups: those with energies less than 50 eV which are taken to be the true secondaries and those between 50 e V and the primary energy E which are the reflected (backscattered) electrons. The total yield of electrons with energies up to 50 eV is called 0 and that of the reflected electrons 1/. Of course, this arbitrary division does not reflect the origin of a particular electron, but it is convenient since the behavior of slow electrons in solids differs from that of electrons with energies above 50 eV. In contrast, for ion-induced electron emission all electrons are secondary particles. This fact makes the interpretation of ion-induced electron spectra simpler than electron-induced spectra. The rapid development of ultrahigh vacuum (UHV) technique during the last twenty years has stimulated a renewed activity in studies of secondary electron emission from solids since the number and the corresponding energy distribution of electrons with energies below 10 eV depend critically on the surface conditions. Generally, only results obtained from clean surfaces prepared by ion sputtering or in-situ film deposition can be considered as reliable for these low energies. With increasing exit energies, the requirements for the experimental procedure become less critical, and for the highest exit energies considered (1 to 3 MeV), the surface preparation hardly plays any role. In many cases, one may utilize the similarity between electron spectra induced by electrons and those induced by fast protons. Stages 2) and 3) do not depend on the type of primary particle, and for electrons as well as protons of high speed, several theoretical treatments (see Sections 2.6,2.7, and 2.8) predict that the cross section for ionization of the i'th subshell depends essentially only on the speed of the primary particle. Unfortunately, experimental data comparing spectra from electon and proton impact on solids are scarce. Examples are the energy spectra from polycrystalline niobium obtained for 3-keV electron and 400-keV proton impact by Musket (1975) as shown in Figure 5.3. Even though the impact speeds are not the same, the spectra are indeed very similar. This similarity simplifies the discussion of external electron spectra at low ejection energies induced by electron bombardment, since there exists a large amount of high quality data on spectra obtained by fast protons.

Physical Review, 1968

... The results presented here were included in a thesis submitted by LH Toburen to the faculty o... more ... The results presented here were included in a thesis submitted by LH Toburen to the faculty of Vanderbilt University in partial fulfillment of the requirements for the degree of Doctor of Philosophy. t Present address: Osaka Laboratory, Atomic Energy Research Institute, 508 Mii ...

Physical Review, 1969

The double-electron-capture cross sections were measured for protons in the energy range from 75 ... more The double-electron-capture cross sections were measured for protons in the energy range from 75 to 250 keV incident upon the target gases H2, He, Ar, Kr, N2, and H2O. A power law extrapolation of the present results for H2 and He joins smoothly with previously measured cross sections at higher- and lower-impact energies; however, wide discrepancies exist between measured and calculated cross section values. For the heavier target gases, a power law extrapolation of the present results is in disagreement with previous measurements. This disagreement is attributed to the power law being inappropriate at energies where electron capture from inner electron shells is possible.

Inner-Shell and X-Ray Physics of Atoms and Solids, 1981

The electron spectra of autoionizing levels in helium excited by photons, electrons and light ion... more The electron spectra of autoionizing levels in helium excited by photons, electrons and light ions has been the subject of extensive studies1. Due to Wigner’s spin conservation rule no appreciable formation of triplet levels in doubly excited HeI** has been found by proton impact on He target gas. However, triplet states can be formed via electron exchange2, using bombardment of structured ions such as He+. For sufficiently low-velocity ions autoionization may be influenced by the proximity of the projectile during the decay of the helium excited states. Decay of excited states of the quasi-molecule formed during slow collisions has been observed to produce shifts in the energy of excited states3 as well as Stark mixing of magnetic substates which decay in the electric field of the projectile. The latter effect influences the angular distributions of ejected electrons.4 Little is known regarding the excitation of helium autoionization states by heavy multiply charged ions. Excitation by fast 30 MeV 05+ resulted in strong excitation of He(2s2p)1D series with essentially no triplet state excitation5. The present measurements extend the range of velocity and charge states of the incident ions to provide greater detail on the relative importance of collision parameters on the excitation and de-excitation process.

Cross sections for K-shell ionization of fluorine and carbon have been measured from Auger-electr... more Cross sections for K-shell ionization of fluorine and carbon have been measured from Auger-electron yields for 2-MeV proton impact on CH_3F, CHF_3, CF_4, C_2F_6, and C_4F_8. Ejected electrons were electrostatically analyzed for electron-emission angles from 20^circ to 125^circ with respect to the incident beam. Absolute cross sections were determined by integrating the doubly differential yields with respect to emission angle

Physical Review A, 1972

ABSTRACT

Radiation Protection Dosimetry, 2002

When charged particles slow in tissue they undergo electron capture and loss processes that can h... more When charged particles slow in tissue they undergo electron capture and loss processes that can have profound effects on subsequent interaction cross sections. Although a large amount of data exists for the interaction of bare charged particles with atoms and molecules, few experiments have been reported for these 'dressed' particles. Projectile electrons contribute to an impact-parameter-dependent screening of the projectile charge that precludes straightforward scaling of energy loss cross sections from those of bare charged particles. The objective of this work is to develop an analytical model for the energy-loss-dependent effects of screening on differential ionisation cross sections that can be used in track structure calculations for high LET ions. As a first step a model of differential ionisation cross sections for bare ions has been combined with a simple screening model to explore cross sections for intermediate and low energy dressed ions in collisions with atomic and molecular gas targets. The model is described briefly and preliminary results compared to measured ejected electron energy spectra.

Radiation Protection Dosimetry, 2011

Monte Carlo track simulation has become an important tool in radiobiology. Monte Carlo transport ... more 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.

Radiation and Environmental Biophysics, 1998

It is widely accepted that an understanding of the detailed structure of charged particle tracks ... more It is widely accepted that an understanding of the detailed structure of charged particle tracks is essential for interpreting the mechanistic consequences of energy deposition by high linear energy transfer (LET) radiation. The spatial relationship of events along the path of a charged particle, including excitation, ionization, and charge-transfer, govern subsequent chemical, biochemical, and biological reactions that can lead to adverse biologic effects. The determination of spatial patterns of ionization and excitation relies on a broad range of cross-section data relating the interactions of charged particles to the molecular constituents of the absorbing medium. It is important that these data be absolute in magnitude, comprehensive in scope, and reliable if accurate assessment of track structure parameters is to be achieved. Great strides have been made in the development of this database, understanding the underlying theory, and developing analytic models, particularly for interactions involving electrons and protons with atoms and molecules. The database is less comprehensive for interactions involving heavier charged particles, especially those that carry bound electrons, and for interactions in condensed phase media. Although there has been considerable progress in understanding the physical mechanisms for interactions involving fast heavy ions and atomic targets during the past few years, we still lack sufficient understanding to confidently predict cross-sections for these ions with biologically relevant material. In addition, little is known of the interaction cross-sections for heavy charged particles as they near the end of their track, i.e., for low velocity ions where collision theory is less well developed and where the particle's net charge fluctuates owing to electron capture and loss processes. This presentation focuses on the current status of ionization and charge-transfer data. Compilations, reviews, Internet sources, theoretical models, and recent data applicable to track structure calculations are discussed.

Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 1989

The physical structure of charged-particle tracks plays an important role in determinin g the res... more The physical structure of charged-particle tracks plays an important role in determinin g the response of a stopping material to the absorption of energy following irradiation by charged particles and neutrons. For biological systems, and for microelectronic circuits, the response is sensitive to the energy deposited in submicron-size volume elements. Because of the stochastic nature of energy deposition in and near charged particle tracks, there may be wide variations in the actual quantities of energy deposited in critical volume elements. Since direct measurements of energy deposition in condensed material (solids or liquids) are not technologically feasible, the effects of charged-particle track structure are usually estimated from one of several model calculations; the most commonly used are homogeneous track structure models. Recent interest in the radiation biology of high Linear-Energy-Transfer (LET) radiation has spurred interest in testing these model descriptions of the structure of high-energy heavy-ion tracks. As one means of providing such tests, experiments were recently conducted at the GSI-Darmstadt, UNILAC accelerator, to measure energy deposition in small volumes as a function of the radial distance from the path of fast, heavy ions. These measurements, conducted in collaboration with research groups of Dr. G. Kraft at GSI and Prof. Schmidt-Bijcking of the University of Frankfurt, were made for simulated tissue volumes 0.5 and 1.0 pm in diameter, located from 0 to 10 pm from the path of Ge ions having energies from 13.0-to 17.2-MeV/amu. Excellent agreement was observed between model calculations and measured dose distributions for radial distances up to a few micrometers in simulated tissue. At greater distances the actual measured dose in irradiated volumes was much more than the calculated average value. These differences reflect the stochastic nature of energy deposition in which a large fraction of the volume elements receive no dose from a given particle traversal, but elements which do receive energy receive relatively large amounts. This may have important consequences for effects which occur with a nonlinear or threshold energy response. * Work Supported by the Office of Health and Environmental Research (OHER), US Department of Energy under Contract DE-ACO6-76RL0 1830.