Effect of Gravity on Gamma Radiation

TLDR

The experiment used recoil‑free resonant absorption of 14.4‑keV γ‑rays in Fe‑57 over a 75‑ft vertical path, employing a 1.25‑Ci Co‑57 source, large‑window proportional counters, an enriched absorber foil, temperature‑controlled ovens, and a redesigned velocity‑monitor system to reduce systematic uncertainties and increase counting rates. The measured gravitational redshift was 0.9990 ± 0.0076 of the predicted 4.905 × 10⁻¹⁵ value, confirming the equivalence principle within experimental uncertainty.

Abstract

Recoil-free resonant absorption of the 14.4-keV $\ensuremath{\gamma}$ ray in ${\mathrm{Fe}}^{57}$ 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 ${\mathrm{Co}}^{57}$ 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 (0.9990\ifmmode\pm\else\textpm\fi{}0.0076) times the value ${4.905\ifmmode\times\else\texttimes\fi{}10}^{\ensuremath{-}15}$ of $\frac{2gh}{{c}^{2}}$ 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.