LARGE ENERGY-LOSS STRAGGLING OF PROTONS AND HE IONS IN MYLAR FOILS

Nuclear reaction analysis often requires polymer or thin metal foils to be inserted in front of the detection system to stop scattered incident ions from entering the detector. It is important to completely understand the straggling processes in the foil if true target depth profiles are to be extracted from the energy loss profiles of the reaction product particles. Generally Bohr straggling is used to account for energy spreads in these foils. This, however, has no ion energy dependence, is good mainly for energy losses much less than 20% and can underestimate straggling by a factor of 5 or more for large energy losses. This paper reports on proton and helium ion straggling in foils where the fractional energy loss (DELTA-E/E) exceeds the Bohr limit of 20% and gives simple methods of calculating the straggling from the stopping power of the ions in the foils.