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Recoil-free resonant absorption of the 14. 4-keV ray in Fe^57 has been employed to measure the effect of gravity over a 75-ft vertical path in the Jefferson Laboratory, in an improved version of the experiment of Pound and Rebka. A Co^57 source, initially 1. 25 Ci, large-windowed proportional counters, and an enriched absorber foil 15 in. in diameter permitted a much increased counting rate. The employment of temperature-regulated ovens for source and absorbers and a redesigned monitor system to detect variations in waveform of the source velocity effected a reduction in systematic uncertainties. The result found was (0. 99900. 0076) times the value 4. 90510^-15 of 2gh{c^2} predicted from the principle of equivalence. The range given here is the statistical standard deviation set by the number of counts involved. An estimated limit of systematic error is 0. 010.
Pound et al. (Mon,) studied this question.
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