M.J. Gaillard

Mass distribution and multiple fragmentation events in high energy cluster—cluster collisions: evidence for a predicted phase transition

Abstract Fragment size distributions including multiple fragmentation events have been measured for high energy H + 25 cluster ions (60 keV amu −1 ) colliding with a neutral C 60 target. In contrast to earlier collision experiments with a helium target the present studies do not show a U-shaped fragment mass distribution, but a single power-law falloff with increasing fragment mass. This behaviour is similar to what is known for the intermediate regime in nuclear collision physics and thus confirms a recently predicted scaling from nuclear to molecular collisions.

Fragmentation of high energy mass-selected ionic hydrogen clusters

Abstract First results on fragmentation of fast ionic hydrogen clusters ( n = 5−31, odd; 60 keV/amu) by single collision with a helium atom have been obtained with the new beams delivered by the variable energy post-accelerator of RFQ type at Institut de Physique Nucleaire de Lyon. The fragmentation apparatus and preliminary results are presented. The inclusive mass distributions of the charged fragments measured are strongly different from those resulting from lower energy collision. An important production of intermediate mass fragments is observed.

Gas-jet target for cluster fragmentation induced by high velocity collision

Abstract A gas-jet target for cluster fragmentation experiments has been studied in the inlet pressure range of 0.1–5.5 Torr corresponding to a target thickness range of 1013–1.2 × 1015 at. cm−2. The total flow has been found to be in quantitative agreement with the Poiseuille formula in the pressure range of 1.0–5.5 Torr. The profile of the jet is in good agreement with the (cos φ) 5 2 law expected in the viscous flow regime in the inlet pressure (P0) range of 0.1–5.5 Torr. From cross section measurements using the inlet pressure range of 1.5-5.5 Torr, the target thickness has been found to be proportional to P02 as expected. The target thickness characterization has been extended in the range 1013 to 1014 at. cm−2 from measurements with cluster beams.

Multifragmentation after multi-ionization of hydrogen clusters in high energy cluster–atom and cluster–cluster collisions

Abstract We report on a cluster fragmentation study involving collisions of high-energy hydrogen cluster ions with atomic helium or fullerenes. The experimental characterization of the cluster fragmentation, not only by the average fragment size distribution but also by a statistical analysis of the fragmentation events, has become possible because of a recently developed multicoincidence technique in which all the fragments of all the collisions occurring in the experiment are mass analyzed on an event by event basis. From the breakup in two fragments to the complete disintegration of the cluster, the fragmentation phenomenon exhibits a transition with an increase of the fluctuations. The fragmentation events with more than one H 3 + fragment evidence the cluster multifragmentation process. An important aspect of these results is that high-energy cluster collision can induce a reaction in the cluster.

Fragmentation of high-energy ionic hydrogen clusters by single collision with helium

Abstract Fragmentation of mass-selected 60-keV u−1-Hn+ induced by single collision with helium has been studied for various cluster sizes (n = 9, 13, 21, 25 and 31). The absolute cross sections of the charged fragments Hp+ are measured from p = 3 to n − 2. The deduced mass distributions are markedly different to those obtained at lower collision energy (where molecular evaporation is mainly involved) due to a strong production of ionic fragments with a size of p n ≤ 0.5 . Moreover, the distributions for p n are found to have scaling properties and to follow a power law A−T, where A is the normalized fragment mass ( p n ) and τ an exponent close to 2.6.

Neutral fragment production induced by collision of 60-keV u−1-H25+ clusters with helium

We report on measurements of the neutral fragment yields resulting from the collision of 60-keV u−1-H25+ clusters with helium. The sum of the mass numbers of all the neutral fragments (Sneutral) coming from each cluster is measured. The histogram obtained can be roughly divided into three Sneutral value ranges corresponding to evaporation (2u ⩽ Sneutral ⩽ 10 u, even), electron capture (Sneutral = 25 u) and ionization (11 u ⩽ Sneutral ⩽ 23 u, mainly). The ionization process which corresponds to both odd and even values of Sneutral appears to be involved in about 83% of the dissociation events observed. These results, and their comparison with the inclusive ionic fragment distributions previously measured, show the importance of multi-ionization of the incident cluster during the collision. Moreover, the fragmentation of the unstable multicharged cluster leads to the production of several light Hp+ fragments (p ⩽ 5).

Energy loss and charge exchange processes of high energy heavy ions channeled in crystals

The interaction of moving ions with single crystals is very sensitive to the orientation of the incident beam with respect to the crystalline directions of the target. Our experiments show that high energy heavy ion channeling deeply modifies their slowing down and charge exchange processes. This is due to the fact that channeled ions interact only with outershell target electrons, which means that the electron density they experience is very low and that the binding energy, and then the momentum distribution of these electrons, are quite different from the corresponding average values associated to random incidence. The two experimental studies presented here show the reduction of the energy loss rate for fast channeled heavy ions and illustrate the two aspects of channeling effects on charge exchange, the reduction of electron loss on one hand, and of electron capture on the other hand.In the first study we have obtained the charge distributions and energy loss spectra of 27 MeV/u Xe35+ ions transmitted...

Ionic hydrogen clusters : structure and fragmentation

The structure of Hn + hydrogen clusters which are a nucleation of H2 molecules around a H3 + core has been theoretically studied for n=5−13 (odd) using ab initio methods. For n larger than 9, the predicted structures can be thought as an addition of H2 molecules around a weakly deformed H9 + core. Besides, mass-selected ionic hydrogen cluster beams of high energy are now available at the cluster acceleration facility at the Institut de Physique Nucleaire de Lyon. Fragmentation of 60-keV/amu — Hn + clusters induced by single collision on helium atoms is investigated for n = 9, 25 and 31. The deduced inclusive mass distributions of the charged fragments are measured from mass 3 to n−2. These distributions appear strongly different from those obtained at lower energy. An important production of intermediate-mass fragments is observed.

Physics with high-energy cluster beams

Abstract The interest of physicists in working with high energy cluster beams is increasing, in particular in France where the hydrogen cluster accelerator at Lyon has been recently upgraded to deliver Hn+ beams in the energy range 60−100 keV u , n As a matter of fact, the use of mass-selected high-energy clusters as projectiles appears to be an important source of information on the interaction of particles and clusters with matter (solid, gas, electron, plasma) and related phenomena, and on the clusters themselves. Results (experiments and Monte Carlo simulation) on the interaction of Hn+ clusters with thin foils and some perspectives of investigation with fast clusters are presented.