The preferred XPS methodology for measurement of SiOz film thickness on polished silicon surfaces is discussed. A precise measurement of the photoelectron attenuation length was made using nuclear reaction analysis (NRA) to calibrate the film thicknesses. Anodic oxide films on Si, which are very reproducible in thickness, are used as test samples. Appropriate corrections fur the problem of adventitious carbon are shown. Under some conditions, the phenomenon of photoelectron difliaction has a significant effect on the measurements, and improvements to the precision by rotating the sample about the normal during data acquisition are demonstrated.

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Direct measurements of doubly differential (angle and energy) distributions of backscattered positrons are reported using an electrostatically guided slow-positron beam. Backscattering yields for 35-keV positrons incident normally on the target were measured as a function of both scattering angle 8 and target atomic number Z (4~ZS 82). Absolute backscattering coefficients for positrons are com-

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