Structure in the velocity dependence of heavy-ion energy-loss straggling

(1) DEPT. OF PHYSICS, CHEMISTRY AND PHARMACY, UNIVERSITY OF SOUTHERN DENMARK, ODENSE M, DENMARK
(2) Physics Faculty, University of Duisburg-Essen, 47048 Duisburg, Germany
(3) Dept. of Experimental and Surface Physics, Johannes Kepler University, 4040 Linz-Auhof, Austria
(4) Laboratory of Ion Beam Physics, ETH Zurich, 8093 Zurich, Switzerland
(5) Dept. of Physics, Chemistry and Biology, Linköping University, 581 83 Linköping, Sweden
(6) Dept. of Physics, University of Jyväskylä, 40014 Jyväskylä, Finland
(7) Institut des Microtechnologies Appliquées Arc, Haute Ecole Arc Ingénierie, 2300 La Chaux-de-Fonds, Switzerland

Recent theoretical and experimental studies [1,2,3,4] indicate that the energy dependence of heavy-ion straggling has a multiple-peak structure. In the present work we study the systematics of this structure as a function of the atomic numbers Z1 and Z2 of ion and target. It is well known that linear straggling, bunching and packing produce separate, although not necessarily resolvable peaks in the energy dependence of straggling. More surprising is the observation [3] that charge-exchange straggling alone can produce a multiple-peak structure, although only two peaks have hitherto been unambiguously identified in calculations. Experimental studies of straggling are typically affected by insufficient knowledge of foil uniformity. To circumvent this source of error we focus on gas targets. Until now we have employed Si and Kr ions and He, N, Ne, Ar and Kr as target gases in the energy range of 0.5 - 12 MeV/u. Most measurements were performed at the K130 cyclotron at the Jyväskylä accelerator laboratory; for Si independent measurements were made at 0.5 - 2 MeV/u using the 6 MV EN tandem accelerator at ETH Zurich. While our measurements [2,4] confirm the existence of more than one peak, we have not yet identified an ion-target combination where at least two resolvable peaks have a predicted position within the energy range of any singe accelerator used. We report new calculations with the aim to confirm trends identified earlier for Kr and Si ions [3]:  
1. For charge-exchange straggling, peak height and speed increase with increasing Z1,  
2. With increasing Z2, peak velocity increases but peak height decreases with increasing Z2.  
3. The significance of bunching relative to charge exchange increases with increasing Z2.  

[1] P. Sigmund and A. Schinner, Europ. Phys. J. D 58 (2010) 105.
[2] C. Vockenhuber, J. Jensen, J. Julin, H. Kettunen, M. Laitinen, M. Rossi, T. Sajavaara, O. Osmani, A. Schinner, P. Sigmund and H. J. Whitlow, Europ. Phys. J. D 67 (2013) 145.
[3] P. Sigmund, O. Osmani and A. Schinner, Nucl. Instrum. Methods B 338 (2014) 101.
[4] C. Vockenhuber, M. Thöni, J. Jensen, K. Arstila, J. Julin, H. Kettunen, M. Laitinen, M. Rossi, T. Sajavaara, H. J. Whitlow, O. Osmani, A. Schinner and P. Sigmund (2014), presented at ICACS 26 in Debrecen (unpublished).