HOW ACCURATE ARE ABSOLUTE RUTHERFORD BACKSCATTERING YIELDS

High precision backscattering assemblies for absolute yield measurements have been built and calibrated to ±1% on three different MeV ion accelerators. The Bi-content in a set of Bi-implanted silicon standards (from the recent Karlsruhe 'round-robin' project) has been accurately measured in each of these assemblies: the resulting value is 4.77(±0.07) × 1015 Bi atoms/cm2. Unexpectedly large deviations from the Rutherford scattering law were observed, viz. approximately 4% for 1.0 MeV He on Bi. The magnitude of this screening correction has been measured over a wide range of Z1/E0 and is found to be in good agreement with theoretical estimates. Several thin gold targets, calibrated by Matteson et al.6) to ±1% by comparison with an NBS standard, have also been measured. Our thickness values are (4±1)% lower than the Matteson values, but no explanation has yet been found for this discrepancy. © 1979.