Energy deposition by nonequilibrium charge states of MeV I127 in Au

Petter Ström,Daniel Primetzhofer

doi:10.1103/physreva.103.022803

Abstract

The energy loss of iodine ions at initial charge states up to 25+ and energies up to 36 MeV in self-supporting gold foils between 37 and 107 nm of thickness was measured with an electron mirror type time-of-flight detector. An excess energy loss of 130 keV was observed at 36 MeV for charge state 25+ compared to 16+, and an energy loss deficit of 100 keV was observed for charge state 8+. The charge state equilibration length for 36-MeV iodine was estimated to lie between 3 and 7 nm, corresponding to an equilibration time between 0.4 and 0.9 fs. This result is relevant both for nanostructure fabrication with MeV ion beams and for depth profiling based on ion beam analysis data in cases where the charge state of the primary ion is far from the equilibrium value in the sample under study. A comparison to published data on charge state equilibration for various projectile-target combinations and energies from 10 keV to 6 GeV indicated that the energy scaling of the equilibration length observed at high energy is invalid for projectile velocities on the order of the Bohr velocity and below. The measurement further provided equilibrium values of the electronic stopping power for iodine in gold at ten energies between 10 and 36 MeV.

Highlights

Studies of the energy loss experienced by energetic ions as they traverse matter are relevant both from a fundamental perspective as they permit us to obtain knowledge about ionsolid interactions, and for applications as they provide input data for quantitative ion beam analysis or allow us to predict the response of a material to irradiation [1,2,3,4]
An ion impinging on a target in an initial charge state different from the average value eventually established as a result of electron exchange with the material exhibits a modified energy deposition during the charge state equilibration
The uncertainty in foil thickness is obtained from the difference between the individual measurement points on each sample, while possible errors in energy loss contain three components: (i) the estimated fit error for the peak position, (ii) the drift of the ToF calibration measured as the difference in peak position for measurements without Au foil at the start and end of the run, and (iii) the error in ion energy estimated from the error in the analyzing magnet field

Summary

Introduction

Studies of the energy loss experienced by energetic ions as they traverse matter are relevant both from a fundamental perspective as they permit us to obtain knowledge about ionsolid interactions, and for applications as they provide input data for quantitative ion beam analysis or allow us to predict the response of a material to irradiation [1,2,3,4]. An ion impinging on a target in an initial charge state different from the average value eventually established as a result of electron exchange with the material exhibits a modified energy deposition during the charge state equilibration. I.e., approximately up to the Bohr velocity v0 (≈2.19 × 106 m/s), this effect has to be accounted for in modeling of dE /dx for ions heavier than protons [8] despite the equilibrium charge state being close to 0, as experiments show characteristic effects of charge exchange on dE /dx even for simple systems such as He [9]. For heavy ions in the MeV range, apparently, the magnitude of the deviation

Results

Discussion

Conclusion