Measurement of the stopping cross section for protons in copper by backscattering using various methods for foil-thickness determination

Abstract

Copper films evaporated on pyrolitic carbon are used to determine the stopping cross section of copper for protons in the energy range from 60 to 500 keV by evaluating the energy width of Rutherford backscattering (RBS)-spectra. For this purpose we need both the number of atoms per unit area of the copper-foils and the spectrum widths. The number of atoms per unit area is determined for all copper films by a quartz-thickness monitor during evaporation, by weighing on a microbalance and by RBS, where a reference target of known thickness has been used to obtain absolute values without the input of stopping cross section data. The results of the quartz monitor and of RBS always agree within 2%, they agree with the results obtained by weighing within a mean relative error of less than 6% for each target. From the width of the RBS-spectra the stopping cross section is extracted by an iteration procedure which starts with Warters' approximation [1] and afterwards corrects for the small systematic errors arising there. The resulting stopping cross section data lead to a maximum at 125 keV. They are in excellent agreement with the data of Luomajärvi [2] and in fair agreement with the data of Mertens and Krist [3], In comparison with the Andersen and Ziegler curve, our data are lower in the whole energy range.