Yu. A. Fainberg

Passage of nitrogen and oxygen ions through carbon and celluloid films

Abstract The approach to charge-state equilibrium in v = 8 × 108 cm/s nitrogen and oxygen beams in celluloid and carbon solid films and nitrogen gas targets has been studied experimentally and theoretically. A two-film target has been used to show that the charge exchange taking place at the exit surface is of crucial importance for charge states detected outside solid targets. Electron capture cross sections inside solid targets have been calculated to be an order-of-magnitude lower than in gas targets. It is concluded that only very low lying excited states of the ions are populated within a solid, and that most ions are unexcited at least for near-equilibrium target thicknesses.

Charge distribution of nitrogen ions reflected from copper and graphite surfaces

A method for the theoretical estimation of the equilibrium charge distribution of ions passed through a media is proposed. It is based on the correction of charge-exchange cross sections for gases using a factor taking the density effect into account. The calculated charge distribution of N ions in graphite agrees well with the existing experimental data. This method makes it possible to estimate the charge distribution of fast N ions reflected from a Cu surface.

Oscillations of the charge exchange cross sections and the average equilibrium charge of helium ions

The electron loss and capture cross sections for helium ions at velocities nu =8-20*108 cm s-1 in helium, nitrogen, neon and argon have been measured and the electron capture cross sections for He+ and He2+ ions in various media have been theoretically calculated in the Oppenheimer-Brinkman-Kramers approximation. It has been found that the dependence of the electron capture cross section on the nuclear charge of the target atoms is oscillating. This phenomenon is caused by the analogous oscillations of the values of the equilibrium charge fraction and the average charge of the fast ions.

Energy and charge distributions of fast hydrogen ions and atoms reflected from Cu in the case of grazing incidence

The energy and charge distributions of protons and hydrogen atoms reflected from the Cu surface in the case of grazing incidence angles are measured at energies of incident particles (H+ and H0) of 200 and 250 keV. The charged fractions of reflected particles are analyzed. A weak dependence of the neutral fraction of reflected particles on the scattering angle is discovered for incidence angles of 1°–2° and an energy of scattered particles of 60 keV or less. It is shown that the neutral fraction of reflected particles with an energy of 60–80 keV or more is independent of the scattering angle and is determined by the ratio of the cross sections for the electron capture and loss by ions in the material.

Theoretical description of fast proton scattering from steel surface under grazing incidence

Abstract The angle-energy distributions of fast protons scattered from a steel surface under grazing incidence are studied theoretically. The presented model is based on the Monte-Carlo method and on calculations of the total collision cross-sections using the Ziegler stopping tables. The energy distributions at a fixed scattered angle are characterized by certain parameters. The 300 and 500 keV proton calculation results for a 4° grazing angle are in qualitative agreement with the authors’ experimental data.

Investigation of cross sections for the formation and destruction of negative boron ions

The electron loss and electron capture cross sections σi,i+m and σi,i−m for boron ions and atoms traveling at the velocities V=1.19 and 1.83 a.u. in H2, He, N2, Ne, Ar, and Xe are measured. The known experimental data on these cross sections at velocities near the cross-section maximum are analyzed. It is found that the electron loss cross sections can be described by a formula which was previously derived in the free-collision approximation and takes into account features of both the ions and the ambient atoms. As the nuclear charge Zt of the ambient atoms increases, the cross sections vary nonmonotonically, increasing on average as Ztt1/2. A formula based on the model of independent electrons is proposed for electron capture by ions with small values of the charge i. It describes the dependence of the electron capture cross section σi,i−1 on the mean binding energy of an electron in an ion with the charge i−1. The total electron capture cross section σi,i−1 is proportional to the number of vacancies in the unfilled electron shell nearest the nucleus. The cross sections 0σi,i−1 exhibit substantially nonmonotonic variation with Zt, increasing on average as Zt1/3.

Decrease in the observed excited-ion electron capture cross sections due to autoionization

From measurements of the cross sections for electron capture by fast helium-like ions N5+ produced by three different methods, it has been found that by increasing the number of metastable particles in an ion beam from 3% to 40% the electron capture cross section is decreased approximately twice. The use of results of previous investigations clearly shows that this decrease is due to autoionization of lithium-like ions N4+ produced from electron capture. It is suggested that the autoionization mechanism may also account for the previously observed anomalies in the electron capture cross sections for P5+, Br7+ and I7+ ions and the decreased charge-exchange cross sections for He+ and N+ metastable ions.

Energy and charge distribution of helium atoms reflected from a Cu surface under grazing incidence

The energy and charge distributions of helium ions and atoms reflected from a Cu surface at grazing incidence have been measured at the energies 250 and 300 keV. The dependence of the relative number of helium atoms in the scattered beam on the scattering angle at incidence angles less than 2° and scattered particle energies below 150 keV has been discovered. It is shown that, for a theoretical estimation of the charge fractions of particles reflected from the target surface, charge-exchange cross sections used in calculations must be corrected when solids are considered instead of gases.

Formation of lithium-like boron and nitrogen ions in the 4 P 5/2 state in gaseous media

We experimentally determined the fraction of αv of lithium-like boron B2+ and nitrogen N4+ ions in the 4P5/2 state having a velocity of 3.6 au that are formed upon capture of two (α2) electrons by hydrogen-like B4+ and N6+ ions and upon capture of one (α1) electron by helium-like (1s2s)1,3S metastable B3+ and N5+ ions in gaseous media (H2, He, N2, Ar), as well as upon passage through a celluloid film. In light-element media (H2, He), α2 increases proportional to the target thickness Tg and reaches a maximum at Tg ≈ 1016 atom/cm2 (for B ions, α2 ≈ 0.2 in H2 and α2 ≈ 0.4 in He). For boron and nitrogen ions passing through thin layers of heavier gases (N2, Ne), α2 depends considerably more weakly on Tg, and, in Ar, becomes practically constant. It is assumed that, since hydrogen and helium do not contain electrons with parallel spins, autoionizing lithium-like ions are formed as a result of successive (one by one) capture of electrons, whereas, in the heavier gases, simultaneous capture of two electrons predominates. At Tg ∼ 1015 atom/cm2, the fraction α1 of boron ions is the highest in He, ∼0.15, and the lowest in Ar, ∼0.07, being in qualitative agreement with calculations.

Study of cross sections for the loss of outer 1s, 2s, and 2p electrons by fast ions and atoms of light elements

In the case of light-element ions propagating with velocities V = 1.83 and 3.65 au in H2, He, N2, Ne, and Ar, the loss cross sections σi, i+m for m electrons (m = 1, 2, 3) are considered. The partial loss cross sections σi(nl) for one of the outer 1s, 2s, or 2p electrons are determined using the obtained data. It is shown that the experimental cross sections for the loss of the 1s and 2s electrons by positive ions qualitatively agree with the theoretical values calculated in the Born approximation. In the case of the ion velocity V = 1.83 au, the cross sections σi for 2p electrons are greater than the cross sections σi (1s) and σi (2s) by a factor of 1.2–3 for the same binding energies of electrons in the ion (Inl > 20 eV). It is found experimentally that, at V = 1.83 au, the cross sections σi (2p) for Inl ∼ 10–20 eV are less than the cross sections σi (1s) by a factor of 2–3, which is probably caused by a decrease in the screening parameter (θ2p < 1) of the outer shell of atoms.