HOW ACCURATE ARE ABSOLUTE RUTHERFORD BACKSCATTERING YIELDS.

High precision backscattering assemblies for absolute yield measurements have been built and calibrated to plus or minus 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( plus or minus 0. 07) multiplied by 10**1**5 Bi atoms/cm**2. 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 Z//1/E//0 and is found to be in good agreement with theoretical estimates. Several thin gold targets, calibrated by Matteson et al. **6) to plus or minus 1% by comparison with an NBS standard, have also been measured. Thickness values are (4 plus or minus 1)% lower than the Matteson values, but no explanation has yet been found for this discrepancy.