... physics

See the work by the groups at University of California, San Diego (http://physics.ucsd.edu/research/surkogroup/positron/), Surko et al. (2000); the groups at Martin-Luther-University Halle (http://positron.physik.uni-halle.de/), at University of Michigan (http://positrons.physics.lsa.umich.edu/); and others.

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... charge

The effective charge  $Z_{\rm eff}$ is defined such that the annihilation rate per target density is $R = \sigma v = Z_{\rm eff} c \pi r_{\rm e}^2$, with $r_{\rm e}$ the classical electron radius.

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