Target Nuclear Charge Research Articles

Strong influence of the solid target material on the convoy electron production

We have investigated the dependence of the yield of convoy electrons induced by energetic (MeV) light ions (H +, H + 2) on the target material. A large variety of target elements (C, Al, Ni, Cu, Pd, Ag, Sm, Gd Au, Bi) has been studied for the first time under controlled surface conditions (ultrahigh vacuum p UHV <10 −7 Pa). The results exhibit a systematic dependence of the convoy electron yield Y C on the Fermi energy E F of the target material in contrast to common theoretical models, predicting Y C to be a monotonie function of the target nuclear charge Z 5 T .

Electron transfer and ionization in collisions between protons and the ions He+, Li2+, Be3+, B4+, and C5+ studied with the use of a Sturmian basis.

Total cross sections are reported for electron transfer and ionization in collisions between protons and the hydrogenic ions ${\mathrm{Be}}^{3+}$, ${\mathrm{B}}^{4+}$, and ${\mathrm{C}}^{5+}$, and results are presented for ${\mathrm{He}}^{+}$ and ${\mathrm{Li}}^{2+}$ in addition to those previously reported. Proton energies relative to the target ion are in the range 17.5--150 keV for ${\mathrm{He}}^{+}$ targets, increasing with the target's nuclear charge to 150--600 keV for ${\mathrm{C}}^{5+}$ targets. Within these energy ranges the electron-transfer cross sections reach their peak values, and the ionization cross sections approach peak values from below. A coupled-Sturmian-pseudostate approach has been taken, and the largest-basis results are estimated to be converged to within 10% at energies where the electron-transfer cross sections peak. With sufficiently large bases, the cross-section curves vary smoothly and regularly with energy and the target's nuclear charge. Scaling rules for these curves are examined and compared with the simple scaling laws in the Born approximations.

Target nuclear charge dependence of 1s2to 1s2p and 1s2to 1s3p excitation cross sections of Fe24+projectiles in the intermediate velocity range

Excitation cross sections into the 1s2p and 1s3p states of 400 MeVFe24+ projectiles colliding with He, N2, Ar and Kr targets have been extracted from the intensities of helium-like 2P to 1S and 3P to 1S transitions de-exciting the ion after one-electron 1s to 2p and 1s to 3p excitation. The exciting partner being neutral, the capture channel is closed, and these experiments thus give an opportunity to study the pure distortion effect even in the case of quasisymmetric systems at intermediate velocity (Vp/Ve approximately 0.6). A striking feature of the results is the experimental evidence of saturation in the excitation cross section as the nuclear charge of the exciting partner increases. This trend is supported by theoretical predictions of a new method based on the Schwinger variational principle that was developed for that purpose.

Impact parameter dependent k-shell ionization probabilities in ion-atom collisions with different asymmetry

We present experimental probabilities P κ ( θ) for target K-shell ionization in 1.26 MeV/u He → Te, Dy; N → In, Nd collisions up to very large scattering angles ( θ ≲ 170°). The P κ ( θ) are nearly constant for θ > 10°. Groups of collision systems with about the same sum of projectile and target nuclear charge (Z P + Z T = united atom system) and collision velocity were s probabilities are within these groups of collision systems nearly proportional to the square of the projectile nuclear charge even up to Z p Z T = 0.7 . The calculations in the semiclassical approximation (SCA) by Trautmann et al. are compatible with the experimental results when the recoil effect is included and wave functions of the united atom system are used.

Polarisation of the proton-induced L X-radiation in 3d transition elements

The polarisation of the Ll X-ray line of 27Co, 28Ni, 29Cu and 30Zn targets induced by proton impact has been measured at ion energies ranging from 60 keV to 2.4 MeV. The polarisation is attributed to the collisionally induced alignment of the 2p3/2 vacancy. Measured polarisation values show strong dependence on the target nuclear charge. The small values found particularly in Co and Ni are attributed to a dealignment after the collision caused by angular momentum recoupling between the 2p3/2 vacancy and the partially filled 3d shell. For Cu and Zn the dealignment is expected to be small, and the collisionally induced alignment has been derived from the measured polarisation values. The experimental alignment data agree with the theory only qualitatively.

Proton energies at the maximum of the electronic stopping cross section in materials with 57⩽ Z2⩽83

Energy losses for 30−350 keV protons in thin foils of La, Nd, Tb, Dy, Lu, Ta, Re, Ir, Pt, Au, and Bi are measured using a magnetic spectrometer. One goal of our experiments is to check the accuracy of Ziegler's interpolation of stopping cross sections [2] for protons in a mass region where this interpolation is based only on few or even no experimental data. Although not being of obvious physical importance, the proton energy at maximum stopping, E( S max), turns out to give some indication for the precision of Ziegler's stopping function. Ziegler composed these functions of a low energy part and a high energy part with no special condition for the location of the maximum. Nevertheless, our experiments strongly support most of the values for E( S max) as deduced from Ziegler's functions: E( S max) is oscillating as a function of the target's nuclear charge as is the stopping cross section itself, thus reflecting the electronic structure of the materials. On the other hand, our experimental data give some indication that the interpolated function establishes too low values for the stopping cross sections of the heavy metals.

Excitation and ionisation in collisions of negatively charged projectiles with atoms

Semiclassical coupled-states calculations have been carried out for collisions involving structureless positively and negatively charged projectiles to explore the dependence of the excitation and ionisation cross sections on the sign of the projectile charge. Both asymmetric collisions, where the magnitude of the projectile charge, Zp, is much less than the target nuclear charge, ZT, so that the perturbation of the target by the projectile is weak, and symmetric collisions, where mod ZP mod approximately=ZT, have been considered. The Coulomb deflection of muons is found to have an appreciable effect on the K-shell ionisation cross section. For the symmetric collision case, the authors' considered excitation and ionisation of atomic hydrogen targets both by protons and antiprotons and by electrons. For protons and antiprotons the ratios of the coupled-state cross sections to the corresponding first Born cross sections do not show the same relation to the first Born that is exhibited by the copper ionisation results, and are in fact remarkably the same for ZP=-1 as for ZP=+1. These results demonstrate the projectile charge asymmetry of the collision, that is due to the presence of the charge transfer channel for the positive projectiles, but its absence for the negative projectiles. For e-+H collisions coupled-state corrections to the first Born approximation are found to be appreciable, and overall good agreement with experiment is found.

Learning from numerical calculations of ion-atom collisions

Violent collisions of two independent many-particle systems, victims, are discussed in the atomic sphere. The asymmetric region where the charge of the projectile Z p is less than the target nuclear charge Z N is now well understood, though interesting details still need to be worked out. Negatively charged projectiles offer a new illustration of Fadeev re-arrangement collisions. Multi-electron coherence effects illustrate the richness of the field but a symmetric ( Z p ∼ Z N) collision treatment is needed. A new one-and-a-half center expansion method promises a solution to this problem. Future areas of interest are discussed.

Inner-shell-electron capture byH+,He2+, andLi3+projectiles from CH4, Ne, and Ar

The authors have measured cross sections for the capture of $K$-shell electrons from Ne and C (in C${\mathrm{H}}_{4}$) by ${\mathrm{H}}^{+}$, ${\mathrm{He}}^{2+}$, and ${\mathrm{Li}}^{3+}$ projectiles, and of $L$-shell electrons from Ar by ${\mathrm{H}}^{+}$ in an energy range from 0.4 to 3 MeV/amu. Detection of $K$- or $L$-Auger electrons from the target in coincidence with charge-changing projectiles was used to obtain the ratio of capture to total vacancy-production cross sections from the inner shell of interest. This ratio was found to become as high as 43% for the case of ${\mathrm{He}}^{2+}$ on C${\mathrm{H}}_{4}$, giving direct evidence that inner-shell capture becomes an important inner-shell vacancy-production mechanism as one approaches symmetric collisions. Independent measurements of total inner-shell vacancy production were performed, allowing the capture cross sections to be deduced. The capture cross sections increase slightly less rapidly with projectile charge than would be expected on the basis of first-order perturbation-theory Oppenheimer-Brinkman-Kramers (OBK) scaling rules, while scaling from one target shell or nuclear charge to another shell seems better described by these rules at high velocity. Breakdown of the OBK scaling rules occurs at low velocities. A comparison of experimental cross sections with various theoretical results is presented and discussed.

Total Born-approximation cross sections for single-electron loss by atoms and ions colliding with atoms

The first Born approximation (FBA) is applied to the calculation of single-electron-loss cross sections for various ions and atoms containing from one to seven electrons. Screened hydrogenic wave functions are used for the states of the electron ejected from the projectile, and Hartree-Fock elastic and incoherent scattering factors are used to describe the target. The effect of the target atom on the scaling of projectile ionization cross sections with repect to the projectile nuclear charge is explored in the case of hydrogenlike ions. Also examined is the scaling of the cross section with respect to the target nuclear charge for electron loss by ${\mathrm{Fe}}^{+25}$ in collision with neutral atoms ranging from H to Fe. These results are compared to those of the binary-encounter approximation (BEA) and to the FBA for the case of ionization by completely stripped target ions. Electron-loss cross sections are also calculated for the ions ${\mathrm{O}}^{+i}(i=3\ensuremath{-}7)$ and ${\mathrm{N}}^{+i}(i=0\ensuremath{-}6)$ in collision with He targets in the energy range of \ensuremath{\sim}0.1 to 100 MeV/nucleon. These results are found to be in excellent agreement with the available data near the peak of the ionization cross section.