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The single ionization cross sections for He and H 2 by positron impact have been measured in the first few eV above threshold and found to exhibit a different energy dependence from the corresponding electron results. If the data, between 1 and 3 eV above threshold, are fitted by a power law, exponents of 1.99 6 0.19 and 1.70 6 0.11 are obtained for He and H 2 , respectively. This agrees qualitatively with extensions of the Wannier theory in that the exponent is larger than for electron impact.doi:10.1103/physrevlett.77.1250 pmid:10063029 fatcat:mtcdeqo2tze7rmopo34hr3ky7u