E C Montenegro

A universal equation for the electronic stopping of ions in solids

Abstract An analytical equation for the electronic stopping of ions in solids for non-relativistic velocities, that has no adjustable parameters, is obtained in a semi-phenomenological manner. The very good agreement with experiment gives support to the physical arguments used in its derivation.

Analysis of trace elements in human hair by PIXE

Abstract The PIXE method was applied to the analysis of trace elements in scalp hair using two methods for target preparation. In the first method eight hair strands each of nearly circular cross-section and having approximately the same diameter were selected and placed on an aluminum frame. In the second method a given mass of hair was dissolved in nitric acid and a known amount of strontium was added to the solution and dripped onto a membrane filter using a micropipette. The results for the concentrations of trace elements in hair obtained by the two methods are compared and several aspects of the analysis are discussed.

CH4 ionization and dissociation by proton and electron impact

Absolute dissociative and non-dissociative ionization cross sections of CHn+ (n = 0–4) molecules by protons in the 0.5–3.5 MeV impact energy range have been measured. The present results are in good agreement with the previously published data, where overlap occurs. A decay-route model for CH4 dissociation induced by proton impact is proposed and a method to obtain the corresponding branching ratios is introduced. The consistency and extent of the proposed model and methodology is verified through its application in the case of electron-impact dissociation.

Post-collisional effects in multiple ionization of neon by protons

Absolute cross sections for single and multiple ionization of Ne by fast (0.75–3.50 MeV) protons have been measured. The comparison with available theoretical calculations shows that the inclusion of the contribution from time-delayed post-collisional mechanisms is needed for a good description of the experimental results for double and triple ionization at high velocities.

Electron loss mechanisms in collisions of He+ ions with various targets☆

Abstract The electron loss of high-velocity ions by neutral atoms is due to two different and competing mechanisms. In the screening mode, the electron loss is basically due to the nucleus-electron interaction, with the target electrons assuming the passive role of decreasing the Coulomb field of the target nucleus in the vicinity of the projectile active electron. For a fixed projectile velocity, this contribution is expected to give a non-linear dependence with the target atomic number Z2 due to the incomplete screening at the impact parameter region where the projectile ionization is more likely to occur. Within first-order theories, if the screening is completely absent, the expected dependence would be Z22; with screening, it should scale between Z2 and Z22. On the other hand, in the antiscreening mode, where the loss is due to the action of the target electrons and the target nucleus plays no active role, the expected dependence would be approximately linear with Z2. Thus, for first-order theories, the expected overall dependence with Z2 would be dominated by the screening mode as Z2 increases. We have measured total electron-loss cross sections of He+ ions impinging upon He, Ne, Ar, Kr and Xe targets in the energy range from 1.0 to 4.0 MeV to complement previous measurements and the results point towards a much smaller contribution from the screening mode than expected from first-order theories, possibly due to a saturation effect manifested only in the screening channel.

Study of a proton beam diffusing system for pixe analysis

Abstract The experimental arrangement for uniformization and profile determination of the proton beam from the Van de Graaff accelerator at PUC/RJ is described and a discussion about the effects of the diffuser thickness and relative positron to the target on the beam uniformity is presented.

A very simple way to measure coaxial cable impedance

Descrevemos um experimento simples para medir a impedancia de um cabo coaxial e a velocidade de propa- gacao de um pulso. O experimento utiliza um pulso rapido, que possui a vantagem de evitar cabos muito longos e permite a observacao da dispersao do pulso. Palavras-chave: cabo coaxial, casamento de impedancias, transmissao de sinais.

Absolute cross sections for multiple ionization of noble gases by swift proton impact

Absolute cross sections, σq+, for multiple ionization of He (q = 1), Ne (q = 1, 2, and 3), Ar (q = 1, 2, 3, and 4), Kr (q = 1, 2, 3, and 4), and Xe (q = 1, 2, 3, 4, and 5) atoms by protons in the 0.75–3.5 MeV impact energy range have been measured. The present results are in good agreement with the previously published data in the case of the single-and double-ionization cross sections, but some non-systematic discrepancies appear for higher charge states. A detailed comparison with electron impact cross sections in these gases is performed, and it shows that the cross sections for single ionization by high-velocity protons are substantially smaller than those corresponding to equi-velocity electrons for heavier targets.

The K-shell excitation function in the plane-wave Born approximation: transitions to the continuum and discrete (H atom) states

An exact analytical expression for the energy differential K-shell ionisation cross section is obtained in the framework of the plane-wave Born approximation (PWBA). The excitation function I( eta , theta ) is expressed in terms of elementary analytical functions plus a rapidly convergent power -series expansion. With a few terms of the power series the values obtained by numerical integration are reproduced within the precision inherent in the available tables. Starting from the same general formulation an exact algebraic equation for the 1s-n excitation for atomic hydrogen is also obtained.

Primary radicals production from water fragmentation by heavy ions

The importance of fragmentation of water molecules by heavy ions is discussed for several different physical environments, including tumor treatment, planetary sciences and fission reactors. The characteristics of the fragmentation yields by electrons, protons and photons are presented and compared with fragmentation yields of water molecules by C3+ and O5+ ions at energies corresponding to the Bragg peak. It is shown that for low energy electron loss and high energy electron capture, the water molecules essentially blow-out, releasing a much larger fraction of Oq+ ions as compared with electrons, protons and photons.