H. Tsuchida

Mean excitation energies for the stopping power of atoms and molecules evaluated from oscillator-strength spectra

The mean excitation energy for the stopping power of matter, usually expressed by symbol I, is the only nontrivial material property in Bethe’s [Ann. Phys. 5, 325 (1930)] asymptotic stopping-power formula. It is therefore a crucial input for the evaluation of stopping power for swift charged particles. To calculate the I value of a material from its definition, it is necessary to know the oscillator-strength spectrum of the material in question over the entire range of the excitation energy. We evaluate the mean excitation energies of 32 atoms and molecules from the oscillator-strength spectra that were published by Berkowitz in 2002 [Atomic and Molecular Photoabsorption: Absolute Total Cross Sections (Academic, San Diego, 2002)]. We find that most of the present I values are consistent with those given in the literature. The I values of NO2, O3, and C60 in particular are evaluated in the present work. For buckminsterfullerene C60, an estimation of the I value is made also using the local-plasma approxima...

Ionization cross sections of C60 by fast electron impact

By means of a time-of-flight technique, we have measured cross sections for production of and ions from a gas-phase C60 target bombarded by 0.4-5.0 keV electrons. The results were in fairly good agreement with other data at overlapping energies below 1 keV. Semiclassical calculations of the single-ionization cross sections were also performed up to 10 keV using the Deutsch-Mark formula proposed for the molecule. A fairly good agreement between the experimental and theoretical cross sections was obtained both in magnitude and in energy dependence. It was found that the double and triple ionization cross sections both decrease monotonically and exhibit no hump structure, indicating that the inner-shell ionization does not play an important role in electron-impact multiple ionization of C60 .

Strong projectile dependence of C60 fragmentation by MeV-energy heavy ions

Experimental results are presented for ionization and fragmentation of C60 bombarded by various projectile ions of H+, Li1,2+, B1,2+, O2+, Si1,2+, Ag2+ and Au3+ with velocities 0.9v0-3v0 (v0: Bohr velocity). Mass distributions of light fragment ions Cn+ (n = 1-12) are found to change drastically depending on the projectile species or atomic number Zp. The result indicates that multifragmentation of C60 takes place at small impact parameters where the projectile nuclear charge is less screened. It is found that for heavy projectiles multiple ionization is apt to occur and lighter fragment ions are produced predominantly. Assuming the multifragmentation caused by penetration of the projectile through C60, energy deposition to C60 is calculated, and internal energy and ionization degree of C60 after collisions are estimated. It is concluded that the final fragment pattern is governed by the internal energy.

Cross sections for ionization and fragmentation of by fast impact

Absolute cross sections have been measured for the first time for -impact ionization and fragmentation of at proton energies ranging from 0.2 to 2.0 MeV. The cross sections were in fairly good agreement with other electron-impact data in the high-velocity region, indicating that the charge transfer is negligible in the present energy range. Theoretical cross sections for plasmon excitations were also calculated and compared with the experimental ionization cross sections. Agreement between them was within a factor of two, indicating that the plasmon excitation contributes to the total ionization by about 50%. A careful analysis of the energy dependence of the cross sections for and ions indicates that these ions are produced from the same initial excited states created in collisions with ions.

Equilibrium charge distributions of lithium ions emerging from a carbon foil

Abstract Equilibrium charge distributions of lithium ions passing through a carbon foil have been measured in the energy range 1–6 MeV. Average equilibrium charges q are found to connect smoothly with previous high energy data above 6 MeV. It was found that the present results of both charge fractions and q can be well reproduced by the empirical calculations based on the independent electron model. We also found that available semiempirical formulae for q cannot reproduce the present experimental data at all. A new semiempirical formula is presented for Li ions in the energy range 0.8–30 MeV.

Electron loss and capture cross-sections of 1.0–3.5 MeV H0 and H+ in carbon foils

Abstract The attenuation of H 0 beam transmitted thin carbon foils of 2–20 μg/cm 2 in thickness was measured in the incident energy range from 1.0 to 3.5 MeV. Considering two components of H 0 in the ground state and H + , the electron loss cross-sections of H 0 and the electron capture cross-sections of H + were derived from the measured foil-thickness dependence of the transmitted H 0 yields. Both the present electron loss and electron capture cross-sections are larger than those obtained below 2.4 MeV with quite similar experimental methods by Gaillard et al. The present results are also slightly larger than the values below 2.5 MeV derived by applying an additivity rule to the measured charge changing cross-sections with various carbon-containing gas targets by Toburen et al. The theoretical prediction based on the Born approximation by Gillespie is found to give an excellent agreement with our electron loss cross-sections. As for the electron capture cross-section, although the OBK approximation gives slightly larger values, it can well reproduce the obtained incident energy dependence.

Kinetic energies of fragment ions produced in collisions of 2 MeV Si4+ with C60

The fragmentation of C60 induced by the impacts of 2 MeV Si4+ ions has been studied using time-of-flight techniques. From the peak profile analysis, initial kinetic energies of the fragment ions were obtained. For all the fragment ions investigated the energy distributions peaked at zero kinetic energy, and the total width of the distribution exhibited a strong dependence on the size of fragment ions. For the small fragment ions the kinetic energies decreased monotonously from 6 eV (C+) to 0.85 eV (C12+), while for the multiply ionized parent ions and their daughter ions these values were nearly constant of about 0.04 eV. The results indicate that a large energy transfer from projectiles to C60 takes place during collisions, resulting in high excitation followed by multifragmentation.

Positive and negative cluster ions from liquid ethanol by fast ion bombardment

Secondary ion mass spectra have been measured for the first time for a liquid ethanol target bombarded by 2.0 MeV He(+) ions. Positive and negative ion spectra exhibit evidently a series of cluster ions of the forms [(EtOH)(n)H](+) and [(EtOH)(n)-H](-), respectively, in addition to light fragment ions from intact parent molecules. It was found that these cluster ions are produced only from liquid phase ethanol. Both positive and negative secondary ion spectra show similar cluster size distributions with almost the same decay slope. We also present for the first time the cluster ion distribution emitted from the liquid at different liquid temperatures.

Observation of transient lattice vacancies produced during high-energy ion irradiation of Ni foils

Real-time positron annihilation spectroscopy has been applied for the first time for the investigation of lattice vacancies produced during ion irradiation. Measurements were performed for thin nickel foils irradiated with 2.5 MeV C ions. Doppler broadenings of positron annihilation γ-rays were measured alternately during beam-on and beam-off conditions. It was found that the Doppler broadening line-shape parameter measured during irradiation is larger than those obtained before and after irradiation. This evidently implies that transient or non-survivable vacancy defects are produced during ion irradiation. On the other hand, no such significant change in the line-shape parameter was observed for other face-centred-cubic metal forms of aluminium.

Dissociation of biomolecules in liquid environments during fast heavy-ion irradiation

The effect of aqueous environment on fast heavy-ion radiation damage of biomolecules was studied by comparative experiments using liquid- and gas-phase amino acid targets. Three types of amino acids with different chemical structures were used: glycine, proline, and hydroxyproline. Ion-induced reaction products were analyzed by time-of-flight secondary-ion mass spectrometry. The results showed that fragments from the amino acids resulting from the C-Cα bond cleavage were the major products for both types of targets. For liquid-phase targets, specific products originating from chemical reactions in solutions were observed. Interestingly, multiple dissociated atomic fragments were negligible for the liquid-phase targets. We found that the ratio of multifragment to total fragment ion yields was approximately half of that for gas-phase targets. This finding agreed with the results of other studies on biomolecular cluster targets. It is concluded that the suppression of molecular multifragmentation is caused by the energy dispersion to numerous water molecules surrounding the biomolecular solutes.