N. V. de Castro Faria

Charge equilibrium in swift hydrogen beams emerging from carbon foils

Abstract Neutral and negative equilibrium fractions for protons and deuterons emerging from carbon foils were measured in the incident energy range 250–4000 keV/u. Simultaneous detection of the positive and the neutral (or negative) component of the system was employed and special care was taken with problems due to the contamination of the exit surface.

Isotopic effects in the production of neutrals and negative ions from the foil break-up of H2+, HD+ and D2+

The yields of neutral (H or D) or negatively charged (H- or D-) fragments per atom of the same species have been measured in incident diatomic molecular beams. These fragments result from the break-up in thin carbon foil and the yields have been measured over a wide range of thicknesses and for incident energies of 0.5, 0.7 and 0.9 MeV u-1. Compared with those for incident H+ or D+ beams at the corresponding conditions, these yields can differ considerably from unity. Both the enhancement in the production of neutrals and the inhibition in the production of negative ions exhibit at dependence on the reduced mass of the chemically equivalent systems. The observed isotopic dependence corroborates some simple models commonly put forward to explain these anomalous yields.

K-shell ionisation cross sections for W, Au and U by low-velocity protons

Proton-induced K-shell ionisation cross sections for W, Au and U by low-velocity protons were obtained from thick-target measurements. For the first time the lowest incident energy reached a value less than ten times the binding energy of the K-shell electron (less than nine times in the case of Au). Possible sources of error are thoroughly examined and a comparison with other available experimental results and theoretical values is presented and discussed.

Atomic negative ions from molecular beams traversing carbon foils

Abstract Diatomic (H2+, D2+, HD+, HHe+) and triatomic (H3+) molecular beams were used to produce H− and D− ions afte thin carbon foils. The dwell time covered a region from 0.75 to 15 fs, depending on the nature of the incident beam. The ratio Φ M Φ A has been measured. Here, ΦM is the number of negative ions H− (or D−) per atom of the same species molecule and ΦA is the atomic equilibrium fraction at the same velocity. As a general rule, this ratio is larger than 1 for very short dwell times, reaches a minimum less than 1 and tends slowly to 1 for very large dwell times. The inhibition in the production of negative ions exhibits a clear isotopic dependence.

Collisional production of metastable hydrogen atoms from cold H2

We have investigated the production of metastable H(22S) coming from the dissociation of cold H2 molecules by electron impact. Two detectors have analysed the neutral fragments as a function of their time-of-flight through localized quenching of H(22S) in H(22P) and Lyman-a detection.

He− formation by two- and three-electron capture

Cross sections for double- and triple-electron capture by He+ and He2+ ions, respectively, have been measured in target gases He (in the first case), Ne, Ar, and Kr. Long-lived metastable states of the negative ion He− were formed and detected. The bombarding helium ions energy range was 0.75–4 MeV and the capture cross sections were measured by the initial-growth method. The data show evidence of an independent electron capture process.

Charge-exchanging cross sections in collisions of fast hydrogen species with light noble gases

Abstract Cross sections for single- and double-electron capture and loss for hydrogen species in collisions with He, Ne and Ar atoms have been determined. Pure beams of H + (D + ), H 0 and H − obtained from a 4 MV Van de Graaff accelerator were passed through a differentially pumped gas cell. The three resulting charge states were magnetically separated and simultaneously recorded. The cross sections were obtained from the normalized growth curves as a function of the target thickness.