IN-SITU THICKNESS CALIBRATION OF THIN TARGETS USED IN THE DETERMINATION OF STOPPING POWER

We present a simple device, which allows us to determine the areal mas s density of thin targets in UHV with an accuracy better than 3%. It i s a modification of the principle used by Andersen et al. [Nucl. Instr . and Meth. 149 (1978) 137]. We compare the Rutherford backscattering rate from a reference target to that of the target under investigation . Both targets are alternately exposed to the beam. As a reference tar get we use a polished cone made of commercial aluminum alloy, which ro tates at constant speed upstream of the measured target. In this way, we can obtain absolute stopping cross sections of highly reactive targ ets, produced in-situ by evaporation.