Proton-impact Cross Sections Research Articles

Ionization and fragmentation of CH4 by proton impact

Total ionization and fragmentation cross sections for proton impact on CH4 within the energy range of 2–10keV have been measured. Present results identify the ionization channel to be the dominant processes. Decay routs leading to the fragments CH3+, CH2+, CH+, C+and H+, points to a decreasing cross sections relative to that of ionization. Present total cross section was found to agree with pervious experimental investigations. Structure within our present total cross sections was explained with respect to interference between competing different dissociating paths of fragments as well as transition between initial and final states as well as with scattering amplitude corresponding to different reaction pathways. Comparison with suggested theoretical model points to similar trends in the total absolute fragments cross sections.

Elastic, charge transfer, and related transport cross sections for proton impact of atomic hydrogen for astrophysical and laboratory plasma modeling

Updating and extending previous work (Krstić and Schultz 1999 J. Phys. B: At. Mol. Opt. Phys. 32 3458 and other references) comprehensive calculations were performed for elastic scattering and charge transfer in proton—atomic hydrogen collisions. The results, obtained for 1301 collision energies in the center-of-mass energy range of 10−4–104 eV, are provided for integral and differential cross sections relevant to transport modeling in astrophysical and other plasma environments, and are made available through a website. Use of the data is demonstrated through a Monte Carlo transport simulation of solar wind proton propagation through atomic hydrogen gas representing a simple model of the solar wind interaction with heliospheric neutrals.

Ion induced fragmentation cross-sections of DNA constituents

Proton collision with chemical analogs for the base, the sugar and the phosphor residue of the DNA, namely pyrimidine, tetrahydrofuran and trimethyl phosphate, respectively, has been investigated. The impact energies ranged from 300 keV up to 16 MeV. For the first time, relative fragmentation cross-sections for proton impact are reported for tetrahydrofuran and trimethyl phosphate; previously reported cross sections for pyrimidine are extended for energies beyond 2500 keV. Ionization of tetrahydrofuran leads to a ring break in about 80% of all events, trimethyl phosphate predominantly fragments by bond cleavage to one of the three methyl-groups and for pyrimidine the parent ion has the highest abundance. Such comparison supports earlier findings that the sugar is the weak spot for strand breaks.

Alignment creation and deviation from statistical population distribution in hydrogen n = 3 levels observed on MSE spectra of LHD plasma

We have observed motional-Stark-effect (MSE) spectra of the Balmer-α line emitted from a heating neutral-hydrogen beam on the Large Helical Device. The polarization-resolved spectral profile indicates that the magnetic-sublevel populations in the upper level slightly deviate from the statistical distribution, i.e. alignment is created. The theoretical proton-impact cross sections with the assumption of corona equilibrium, however, lead to a profile in gross disagreement with the observed one. It is suggested that the observed profile is the result of time integration of light emission of atoms during the relaxation process, i.e. the aligned population distribution is created by the directional proton collisions and elastic proton collisions destroy the alignment, finally leading to statistical distribution.

L sub-shell cross sections for proton impact on selected 4d transition elements

L x-rays excited from thick elemental targets of Y, Zr, Nb, Mo, Rh, Pd, Ag and Sn by proton impact in the energy range 50–300 keV were measured with a high-resolution Si(Li) detector equipped with an ultra-thin window. The L sub-shell x-ray production cross sections were determined from a thick-target analysis of the x-ray yields. These were compared with the ECPSSR theory along with various atomic parameter databases and including modifications using the united-atom (UA) binding energy correction.

New Geant4 cross section models for PIXE simulation

We have implemented in the Geant4 simulation toolkit a set of new ionisation cross sections for the simulation of particle induced X-ray emission (PIXE), based upon theoretical and semi-empirical models. The set provides a model based on the so-called ECPSSR theory for the computation of K-shell ionisation cross sections for proton and alpha particle impact. A second model is based upon semi-empirical expressions proposed by Orlic for the calculation of L i -subshells ionisation cross sections for proton impact. Our developments are compared to ionisation cross sections already existing in the Geant4 toolkit and to experimental data.

Regge Oscillations in Integral Cross Sections for Proton Impact on Atomic Hydrogen

The integral cross sections for elastic scattering and spin exchange for proton impact on atomic hydrogen show several oscillations in the energy range 0.01-1.0 eV that cannot be associated with resonances or the glory effect. A complex angular momentum analysis using computed Regge trajectories shows that each peak of the oscillatory structure is predominantly associated with at most three trajectories. In this way, the peaks are related to the L=0 bound states of H+ 2. The complex angular momentum theory for integral cross sections that we introduce shows that such oscillations are a general feature of potential scattering.

Cross section calculations in condensed media: charged particles in liquid water.

Ionisation cross sections for charged particles are deduced within the framework of the relativistic plane wave first Born approximation and the classical electromagnetic theory. The macroscopic dielectric response function is related to the atomic generalised oscillator strength. Interaction cross sections for proton impact on liquid water are presented and discussed. A semi-empirical model for the angular distribution of secondary electrons produced by proton impact on liquid water, which is based on the Bethe approach, is presented.

Theoretical calculation of single ionization in collisions between protons and low-Zmolecules at intermediate and high energies

Single ionization total cross sections for proton impact on ${\mathrm{N}}_{2},$ CO, ${\mathrm{CH}}_{4},$ and ${\mathrm{CO}}_{2}$ are calculated by applying the continuum-distorted-wave-eikonal-initial-state model. Two different approximations have been considered for molecular targets: Bragg's additivity rule and a molecular representation of the bound state target wave function. In the latter, the cross section is approximated by a linear combination of atomic cross sections where the coefficients are determined from a population analysis and the binding energies have been extracted from experimental spectra. The results from both models are compared with experimental data.

On the accuracy of the L-subshell ionization cross sections for proton impact I. Spectrum fitting

The L X-ray spectrum of thorium following 1 MeV proton impact has been measured using a Si(Li) detector. The spectrum was analysed by various commonly used procedures. It is shown that the separation of the L γ group into X-ray transitions of L 1 and L 2 subshells is affected by the natural lineshape model. The common practice of neglecting the natural lineshape (Lorentzian or other shapes where there are many-body effects) introduces systematic errors which are larger than the frequently-cited error bars. In the example used here, the effects of neglecting natural lineshape are similar in size to the difference in theoretical L 1/L 3 and L 2/L 3 subshell ionization cross section ratios predicted for protons by the ECPSSR and coupled channel calculations. Similar systematic errors will arise in other contexts. The optimum accuracy inherent in Si(Li) spectroscopy will only be realised if full account is taken of natural lineshapes in the treatment of the X-ray spectra.

Calculation of the cross sections for positron- and proton-impact ionization of helium.

Cross sections for positron- and proton-impact ionization of atomic helium have been calculated using a coupled-state method. The single-electron Hamiltonian of the helium atom was diagonalized in a finite Hilbert space. We included nine radial pseudostates for each s, p, d, and f angular momentum. The time-dependent Schr\"odinger equation was solved by using a unitary matrix, U matrix, and a hyperbolic trajectory for both projectiles. Our positron results, covering the energy range 60--1000 eV, are compared with available measurements and theoretical calculations. Agreement with the most recent measurement is reasonably good over the entire positron energy range. The proton-impact cross sections are obtained for the proton energy range 10--1000 keV and also show good agreement with the measurements over the entire energy range. We conclude that the lower ionization cross sections for positron impact in comparison with those for proton impact are due to the bigger deflection suffered by the positron in the collision.

SEMIEMPIRICAL FORMULAS FOR CALCULATION OF L SUBSHELL IONIZATION CROSS SECTIONS

Presented are new parameters for the calculation of L subshell ionization cross sections for proton impact using a semiempirical expression. A similar paper was published by our group in early 1993 but the fitting parameters were obtained by using only 2295 experimental L shell cross section data. Since then a large number of experimental data have become available and therefore a new fitting has been performed employing more than 5000 data points. All available data were fitted separately for L1, L2 and L3 subshells. For targets with low atomic numbers (14≤Z≤42), only coefficients for L tot were obtained. Because of the slight Z dependence of the universal function, data were also devised into five sub-groups according to their atomic numbers and fitted separately within each group for L1, L2 and L3 subshells. To extend the energy range of validity of the new fitting function theoretical values were used in the high energy region where experimental data were lacking. Results are compared with ECPSSR predictions and discussed.

Electron-electron interactions in fast neutral-neutral collisions

Differential electron emission is studied for 50–500 keV H + and H atom impact on helium. Using the first Born formulation, it is shown that projectile electron-target electron interactions are expected to dominate the differential cross sections for low energy target electron emission induced by fast neutral projectile impact on any target. Measurements of the 15° electron emission were made in order to investigate this prediction. For low impact energies, a constant ratio between the hydrogen atom and proton impact cross sections was found for emitted electron velocities less than half the projectile velocity, V P. But as the collision energy increased, for electron velocities less than 0.25 V P, the cross section ratio increased as the emitted electron velocity decreased. This is interpreted as a signature of projectile electron-target electron interactions becoming dominant for distant collisions between neutral particles.

Excitation of the 2p level in atomic hydrogen by He + and H + impact between 70 and 700 keV

Total cross sections for H + and He + impact excitation of atomic hydrogen to the 2p level have been measured for projectile energies between 70 and 700 keV. The cross sections for proton impact above 200 keV are in very good agreement with results of the first Born approximation. He + and H + impact—when compared on an equal velocity scale—result within the error bars of 12% in equal cross section values.

Analytical formulas for ionization cross sections and coster-kronig corrected fluorescence yields of the L1, L2, and L3 subshells

We present analytical formulas for three L-subshell ionization cross sections for proton impact based on the recent experimental data and for the Coster-Kronig corrected fluorescence yields based on the theoretical values due to Chen et al. and due to McGuire. Total L X-ray production cross sections were calculated using the analytical formulas and compared with experimental values. As a result, our semi-empirical values using the formulas fitted to the values of Chen et al. coincided with the experimental values very well. Furthermore our values for L α , L β , L γ , L η , and L l X-ray production cross sections calculated using the analytical L1, L2, and L3 Subshell ionization cross sections, and the theoretical values of the fluorescent yields and of the Coster-Kronig transition probabilities due to Chen et al. also agreed with the experimental values.

Pulsed crossed-beam study of the ionisation of atomic hydrogen by electron impact

A pulsed crossed-beam technique incorporating time of flight spectroscopy has been successfully developed and applied to measurements of the electron impact ionisation cross sections of atomic hydrogen in the range 14.6-4000 eV. The method has been derived from a crossed-beam coincidence technique. By substituting a pulsed proton beam for the pulsed electron beam in the present work, measured electron impact ionisation cross sections have been normalised by reference to known equivelocity proton impact cross sections for both ionisation and charge transfer. The results resolve the discrepancy between earlier less accurate cross sections measured using the modulated cross-beam technique. They provide a reliable check on the range of validity of theoretical predictions over a wide energy range.

Application of a first-Born-type approximation for 2l-electron capture in ion-atom collisions

Calculations for electron capture to the nl=2l (l=0,1) states in bare ion-atom collisions are presented using a recently developed first Born-type approximation. The evaluations of total cross sections for proton impact on hydrogen and helium targets are compared with other theoretical and experimental results.

Projectile-charge dependence of inner-shell-ionization cross sections in a high-velocity region

Ratios of ionization cross sections for proton impact to those for $^{3}\mathrm{H}\mathrm{e}\ensuremath{-}\mathrm{i}\mathrm{o}\mathrm{n}$ impact have been measured on $\mathrm{Al}K$ and $\mathrm{Y}L$ shells at projectile energies of 7, 9, 12, 16, and 21 MeV /amu. The ratio tends to unity with an increase in the projectile energy for both the $\mathrm{Al}K$ and $\mathrm{Y}L$ ionizations. This behavior can well be explained in terms of the Binstock-Reading theory combined with the distant-collision theory of Hill and Merzbacher: The polarization effect in distant collisions and the decreasing electron-density effect in close collisions cancel out each other in this high-energy region.

Efficiencies for production of atomic nitrogen and oxygen by relativistic proton impact in air

Relativistic electron and proton impact cross sections are obtained and represented by analytic forms which span the energy range from threshold to 109 eV. For ionization processes, the Massey–Mohr continuum generalized oscillator strength surface is parametrized. Parameters are determined by simultaneous fitting to (1) empirical data, (2) the Bethe sum rule, and (3) doubly differential cross sections for ionization. Branching ratios for dissociation and predissociation from important states of N2 and O2 are determined. The efficiency for the production of atomic nitrogen and oxygen by protons with kinetic energy less than 1 GeV is determined using these branching ratio and cross section assignments.

Analytic curve fits to helium transition integrals

It is pointed out that a helium transition integral, and hence its associated generalized oscillator strength, has eight poles in the q plane, where q is the momentum transfer. They constitute four conjugate pairs lying on the imaginary axis. This analytic behaviour is exhibited in the formulae derived by Crothers and McEachran in 1970, whereas the expression assumed by Lassettre in 1965 contains only one pair of poles. The further six poles are directly attributable to the indistinguishability of the bound electrons. The accurate numerical 11S to 21S generalized oscillator strengths calculated by Bell et al. in 1969 are curve-fitted by interpolating six of the values with an analytic expression exhibiting the required eight poles, whose orders are partially determined by the requirements of orthogonality and the correct asymptotic fall-off with respect to q. The accurate numerical 11S to 21S first Born proton impact cross sections obtained by Bell et al. in 1968 are reproduced with less than 0.1% error over the entire range (5 to 5000 keV) of proton impact energies.