Cross Sections For He Research Articles

Calculation of the photoionization with deexcitation cross sections of He and helium-like ions

We discuss the results of the calculation of the photoionization with de-excitation of excited He and helium-like ions Li$^{+}$ and B$^{3+}$ at high but non-relativistic photon energies $\omega $. Several lower $^{1}S$ and $^{3}S$ states are considered. We present and analyze the ratios $R_{d}^{+\ast}$ of the cross sections of photoionization with de-excitation, $\sigma_{(d)}^{+\ast}(\omega)$, and of the photo-ionization with excitation, $\sigma ^{+\ast}(\omega)$. The dependence of $R_{d}^{+\ast}$ on the excitation of the target object and the charge of its nucleus is presented. Apart to theoretical interest, results obtained can be verified using such long living excited state as $2^{3}S$ of He.

Electron-He+P-wave elastic scattering and photoabsorption in two-electron systems

In a previous paper [A. K. Bhatia, Phys. Rev. A 69, 032714 (2004)], electron-hydrogen $P$-wave scattering phase shifts were calculated using the optical potential approach based on the Feshbach projection operator formalism. This method is now extended to the singlet and triplet electron-${\mathrm{He}}^{+}$ $P$-wave scattering in the elastic region. Phase shifts are calculated using Hylleraas-type correlation functions with up to 220 terms. Results are rigorous lower bounds to the exact phase shifts, and they are compared to phase shifts obtained from the method of polarized orbitals and close-coupling calculations. The continuum functions calculated here are used to calculate photoabsorption cross sections. Photoionization cross sections of He and photodetachment cross sections of ${\mathrm{H}}^{\ensuremath{-}}$ are calculated in the elastic region---i.e., leaving ${\mathrm{He}}^{+}$ and H in their respective ground states---and compared with previous calculations. Radiative attachment rates are also calculated.

In situ cosmogenic 10Be and 21Ne in sanidine and in situ cosmogenic 3He in Fe–Ti-oxide minerals

We report concentrations of in situ cosmogenic 10Be and 21Ne from coexisting quartz and sanidine separates and of cosmogenic 3He in coexisting Fe–Ti-oxide minerals from ignimbritic successions of northern Chile (Oxaya and Lauca ignimbrites). New mineral-isotope pairs such as sanidine and Fe–Ti-oxide minerals are helpful in quantitative geomorphology for geological settings where the lithology lacks the commonly used minerals quartz, pyroxene or olivine. Production rates in sanidine and Fe–Ti-oxide minerals were determined by normalizing nuclide concentrations to established production rates in quartz. The experimentally determined production rates are compared to model production rates calculated with new cross-sections for 3He, 21Ne, and 10Be production from the individual target elements. The mean experimental 21Ne production rate for five sanidine samples is 30.4 ± 3.7 atoms g − 1 yr − 1 (30.4 ± 5.4 atoms g − 1 yr − 1 including the uncertainty of the 21Ne production rate in quartz, P 21Ne qtz). This is in excellent agreement with the modelled value of 28.3 atoms g − 1 yr − 1 , which has an estimated uncertainty of 20%. The 21Ne production rate in sanidine is thus about 50% higher than that in quartz. The cosmogenic neon in sanidine is entirely released in low temperature steps (400–600 °C) and no signs of an interfering nucleogenic neon component were observed. This is in stark contrast to quartz and makes sanidine an attractive mineral for terrestrial cosmogenic neon studies. 3He diffuses out of the sanidine structure. Preliminary results also indicate that sanidine is well suited for 10Be studies. The mean experimental 10Be production rate from two sanidine samples is 4.45 ± 0.38 atoms g − 1 yr − 1 (4.45 ± 0.42 atoms g − 1 yr − 1 including uncertainty of P 10Be qtz), very close to the modelled value of 4.55 atoms g − 1 yr − 1 , which has an estimated uncertainty of 20%. We emphasize that 10Be and 21Ne production in sanidine is composition dependent. Therefore, major element analyses should be carried out and production rates calculated on a sample by sample basis. Fe–Ti-oxide minerals retain 3He quantitatively. Experimentally derived production rates are in excellent agreement with new values derived from physical modelling. The mean experimental 3He production rate for five Fe–Ti-oxide minerals samples is 120 ± 11 atoms g − 1 yr − 1 (120 ± 12 atoms g − 1 yr − 1 including the uncertainties of P 21Ne qtz), compared to a mean modelled value of 124 atoms g − 1 yr − 1 , which has an estimated uncertainty of 20%. The variable chemical and structural composition of the solid solution lines of Fe–Ti-oxide minerals has little effect on the total 3He production rate. Cosmogenic 21Ne is not produced in significant quantities in Fe–Ti-oxide minerals due to the absence of suitable target elements.

(e, 2e) studies of H2 in the intermediate energy regime

(e, 2e) studies have been conducted ionizing H2 from the X1Σ+g state to the X2Σ+g H+2 ground state. The electron energy ranged from 10 eV through to 40 eV above the ionization threshold for experiments performed in a coplanar symmetric geometry, with both scattered and ejected electrons sharing equal energy. Results are also presented for asymmetric scattering where one of the electrons carried 80 eV energy away from the interaction in a forward direction at an angle of 35° to the incident electron beam. These experiments were performed to test the hypothesis that the two nuclei in the molecule act as a ‘double scattering’ centre, with the possibility of producing Young's double-slit-type interference in the resulting differential cross section. New features are found in the ionization cross section for H2 in a coplanar symmetric geometry; however it is unclear at present if these are due to Young's double-slit interference.

Calculation of the Cross Section of Helium Ionization by an Electron Impact with the Formation of a Helium Ion in an Excited State

The total cross sections of He and He+ ionization by an electron impact are calculated in the first Born approximation. Calculations of the matrix elements are carried out by the Fock-Dirac multiconfiguration relativistic method using an intermediate type of coupling with orthogonal functions of the initial and final states. A single-electron wave function of the continuous spectrum for an Auger electron is obtained using the Fock-Dirac single-configuration method. The results of the calculations performed with orthogonal and nonorthogonal wave functions of the initial and final states are compared. The ionization cross sections are calculated for cases in which a knock-on electron of the continuous spectrum is described by both the orthogonal and nonorthogonal wave functions with respect to the wave functions of the core electrons.

Ionization of Molecular Hydrogen

A general model, based on a theoretically calculated ionization oscillator strength and an experimentally determined excitation shape function, has been obtained for calculating the molecular hydrogen electron-impact ionization cross section of a transition between any discrete vibrational levels of the neutral X 1Σ state and ionic X 2Σ state. Specifically, the excitation shape function and ionization oscillator strength for transitions from the vi = 0 level of the X 1Σ neutral state to the discrete levels of the X 2Σ ionic state are derived from analyzing several experimental measurements. The derived oscillator strength is found to be consistent with the 1994 photoabsorption measurements of Samson & Haddad and the 1977 theoretical cross sections of Flannery and coworkers. The derived excitation function, along with the oscillator strengths for transitions involving the vi > 0 level calculated from the data of Flannery and coworkers, permits an accurate calculation of the nondissociative cross sections of H2 between any discrete vibrational levels over a wide energy range.

Absolute differential cross sections for direct scattering of keV O+ ions by He, H2, N2, and O2

Accurate atomic scattering cross sections are essential for modeling energetic particle precipitation into the upper atmosphere. Here, measurements of absolute differential cross sections for direct scattering of O+ ions by He, H2, N2, and O2 are reported. The measured cross sections include contributions from both elastic and inelastic scattering processes. Data are presented for 0.5‐, 1.5‐, and 5‐keV projectile energies over the laboratory angular range 0.22°–4.68°. Additionally, a simple numerical technique is used to estimate the scattering cross sections for He, N2, and O2 at 0.1 keV. At a given energy, the cross sections for different targets exhibit a high degree of similarity in both amplitude and angular dependence. This observation suggests that the cross sections for targets such as O and H, which are much more difficult to study in the laboratory and for which measurements are not available, may be approximated by an average of the data presented here.

Total electron-impact ionization cross sections of helium

By accurate measurement of total cross section ratios for electron-impact ionization and photoionization, total electron-impact ionization cross sections of He were determined. Relative standard uncertainties as low as 1.6 to 2% in the electron energy range from 100 to 4000 eV were achieved using published cross section data for photoionization. Results are discussed in terms of theoretical predictions and by comparison with former experimental results. The data are in excellent agreement with the results by Rejoub et al (2002 Phys. Rev. A 65 042713) and represent an accurate and reliable database for testing theoretical models.

Efficient Method for Lorentz Integral Transforms of Reaction Cross Sections

The Lorentz Integral Transform (LIT) method is reformulated via the Lanczos algorithm both for inclusive and exclusive reactions. The new technique is tested for the total photoabsorption cross section of 3H and 4He. Due to the rapid convergence of the algorithm one has a decrease in cpu time by two orders of magnitude, but at the same time an excellent agreement with the results of a conventional LIT calculation. The present work opens up the possibility of ab initio calculations for inclusive and exclusive processes for A greater equal 6 with inclusion of complete final state interactions.

High-resolution state-selected ion-molecule reaction studies using pulsed field ionization photoelectron-secondary ion coincidence method

We have developed an octopole-quadrupole photoionization apparatus at the Advanced Light Source for absolute integral cross-section measurements of rovibrational-state-selected ion-molecule reactions. This apparatus consists of a high-resolution photoionization ion source, a wired ion gate lens, a dual radio-frequency (rf) octopole ion guide reaction gas cell, and a quadrupole mass spectrometer for reactant and product ion detection. The unique feature of this apparatus is the implementation of the high-resolution pulsed field ionization-photoelectron (PFI-PE)-photoion coincidence (PFI-PEPICO) technique, which has allowed the rotational-state selection of diatomic ions for ion-molecule reaction studies. The novel application of the wired ion gate lens for the rejection of false coincidence background ions is described. This application, along with the differential-ion-gate scheme, has made possible the measurements of rovibrational-state-selected absolute integral reaction cross sections for ion-molecule collisions using the PFI-PE-secondary ion coincidence (PFI-PESICO) method. The successful measurement of absolute state-selected cross sections for H2+(X,v+,N+)+Ar(Ne) with v+ up to 17 [the third to the last vibrational state of H2+(X)] demonstrates the high sensitivity of this differential-ion-gate PFI-PESICO method. In order to gain a detailed understanding and to obtain optimal performance of the wired ion gate lens for PFI-PESICO measurements, we have carried out ion trajectory calculations of reactant ions between the photoionization region and the rf-octopole ion guide. On the basis of these calculations, possible future improvements for the application of this differential-ion-gate PFI-PESICO scheme are discussed.

Euclidian response of light nuclei

We discuss the longitudinal and transverse quasi-elastic cross-sections for 3He and 4He in terms of the Euclidian response, which can be directly calculated from the ground-state wave function. We find that the main open problem in quasi-elastic scattering, the excess of transverse strength, can be understood as a consequence of meson exchange currents, once the n-p short-range and tensor correlations in both initial and final state are included.

Cross section of 3He( 3He,2p) 4He measured near the Gamow peak

We measured the cross section of 3He( 3He,2p) 4He in the energy region of 30–50 keV center of mass energy by using the OCEAN facility. The resulting astrophysical S-factors are in agreement with the existing data and their total errors are as good as those of the previous experiment. For further study we tried the measurements at lower energies than E cm = 30 keV. It will require running times of one month with more improvements such as background reduction and steady operation of the detection system.

Hyperspherical close-coupling calculations for charge-transfer cross sections inHe2++H(1s)collisions at low energies

A theory for ion-atom collisions at low energies based on the hyperspherical close-coupling (HSCC) method is presented. In hyperspherical coordinates the wave function is expanded in analogy to the Born-Oppenheimer approximation where the adiabatic channel functions are calculated with B-spline basis functions while the coupled hyperradial equations are solved by a combination of R-matrix propagation and the slow/smooth variable discretization method. The HSCC method is applied to calculate charge-transfer cross sections for He{sup 2+}+H(1s){yields}He{sup +}(n=2)+H{sup +} reactions at center-of-mass energies from 10 eV to 4 keV. The results are shown to be in general good agreement with calculations based on the molecular orbital (MO) expansion method where electron translation factors (ETF's) or switching functions have been incorporated in each MO. However, discrepancies were found at very low energies. It is shown that the HSCC method can be used to study low-energy ion-atom collisions without the need to introduce the ad hoc ETF's, and the results are free from ambiguities associated with the traditional MO expansion approach.

Photodisintegration cross sections of 3He and 4He at low energies

Photonuclear reactions of light nuclei give us important informations on nuclear reactions relevant to astrophysics. In this paper we will present the new experimental results on the photonuclear reaction cross sections of 3He and 4He in the energy region up to ∼30MeV using a real photon beam and an active target.

Cross section of 3He( 3He,2p) 4He measured near the Gamow energy

Cross section of 3He( 3He,2p) 4He measured near the Gamow energy

Improvements on the quasifree absorption model for electron scattering

The quasifree nonempirical model proposed by Staszewka et al. in 1983 for the imaginary part of the electron scattering optical potential is revised, in order to improve its foundations, accuracy, and range of applicability. The importance of relativistic and many-body effects for heavy atoms is shown and some corrections proposed. A criterion for selecting the {delta} parameter is discussed. The model is also shown to admit considerable simplifications without loss of accuracy. Calculated elastic (differential and integral) and inelastic cross sections for He to Xe noble gases and also for N{sub 2}, CO, and CO{sub 2} molecules are compared to experimental values.

Total electron scattering cross sections of He, Ne and Ar, in the energy range 4eV–2 keV

Total scattering cross sections of the rare gases He, Ne and Ar have been measured,for electrons in the energy range from 4 eV to 2 keV. The measurements wereperformed using a linear transmission device and an electron energy analyser,with an angular resolution better than 1.1 × 10−3 srand with an energy resolution far below the first excitation threshold in the threeatoms. The uncertainties of the experimental results are generally smaller than3%. With a few exceptions, the present results are in good agreement with thedata of other authors. Systematic deviations were found between the presentresults and the data available for the total scattering cross sections of He athigh electron energies. The deviations increase with increasing energyand reach about 13% at 2 keV. It appears that different levels of theseparation of forward-scattered electrons are responsible for these systematicdeviations.

Elastic and rotationally inelastic differential cross sections for He+H2O collisions

Elastic and rotationally inelastic cross sections have been measured for He+H2O scattering at two collision energies, 66.3 and 99.0 meV, using the crossed molecular beam technique. The inelastic events are detected by time-of-flight analysis of the scattered He atoms. The data are converted to elastic differential cross sections and inelastic angular-dependent energy loss spectra in the center-of-mass system. They are compared with averaged, full close-coupling calculations of state-to-state cross sections for rotational excitation based on a newly calculated ab initio potential using symmetry-adapted perturbation theory. The agreement with the elastic differential cross sections is excellent. The energy loss spectra are reproduced satisfactorily and among the largest differential cross sections that contributed to the measurements are excitations around all three possible axes for ΔJ=1 but a preference of the excitation around the in-plane C axis for ΔJ=2 transitions.

A precise measurement of the spin-dependent neutron scattering length of 3He

The poor knowledge of the spin-dependent neutron scattering length of 3He has until now handicapped nuclear four body theory and the interpretation of excitations in the quantum liquid. We have measured, for the first time directly, the real part of the bound incoherent neutron scattering length, bi′ of 3He. A neutron spin echo spectrometer was used to detect pseudomagnetic precession of polarised neutrons passing through polarised 3He gas. Any absolute calibrations of sample and beam parameters were avoided using simple transmission measurements with non-polarised neutrons. The only a priory information required was the spin-dependent neutron absorption cross section of 3He. The result is bi′ = -2.365(20) fm, which reduces the prior uncertainty by a factor 30. The corresponding new value of the bound incoherent scattering cross section is σi = 1.532(12) barn. Including the known value of the coherent neutron scattering length, we obtain new values for the real parts of the free triplet and singlet neutron scattering lengths, a-′ = 7.370(58) fm and a+′ = 3.278(53) fm.

Regularities and irregularities in partial photoionization cross sections of He

We have combined B-spline basis sets with R-matrix theory for the description of photoionization in two-electron systems and applied this combination to the study of irregularities in He photoionization when doubly excited resonances up to the $N=9$ threshold of ${\mathrm{He}}^{+}$ are reached. Both total and partial photoionization cross sections have been obtained. Since the magnitude of the resonance effects is very similar for all partial cross sections, we believe that experimental investigations of photoionization cross sections leaving ${\mathrm{He}}^{+}$ in a highly excited state could provide the most information about possible long-range fluctuations in the photoionization cross sections.