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A search was made for changes in the two-quantum annihilation rate when positrons are annihilated in superconducting lead compared to normal lead. The change of state was brought about either by turning off or on a magnetic field or by varying the temperature of the sample. The ratio of the two-photon coincidence rate in normal lead to that in superconducting lead was found to be 1.0011±{}0.0016 in the magnetic field experiment and 1.0003±{}0.0011 in the temperature-varying experiment.
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Shafroth et al. (1956) studied this question.
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