ENERGY-LOSS STRAGGLING OF MEV IONS IN THIN SOLID FILMS

High resolution measurements of the energy loss distribution for 0.5 MeV/u N-15 ions in solids are obtained using the H-1(N-15, alpha-gamma)C-12 resonant nuclear reaction at 6.4 MeV and thin film sandwich structures composed of a target material on a silicon substrate with an intermediate hydrocarbon layer. Energy loss and energy loss straggling are determined for targets with 6 less-than-or-equal-to Z2 less-than-or-equal-to 79. It is shown that measurements of the variance of the energy loss distribution as a function of film thickness can be used to determine the contribution from thickness variations such as surface roughness. Excellent agreement is found with the results of atomic force profilometry. In this way, the most significant experimental error in the determination of straggling cross sections in solids can be eliminated. Calculations based on the plane-wave Born approximation are performed for representative cases to determine straggling cross sections and charge exchange cross sections near equilibrium. These are used to estimate the contribution of fluctuations in the charge state distribution (CSDF) of the traversing ions to the total straggling. Experimental results are also compared with various other theoretical estimates such as the formalisms of Bohr and Bethe-Livingston. It is found that the data is very well described by the sum of the CSDF estimates and the Bethe-Livingston approximation for fully stripped ions.