Abstract
A gamma-gamma coincidence counting method is used to detect pair emission from an excited state of ${\mathrm{Mg}}^{24}$. The positrons are stopped in absorbing material surrounding the source and detected by their annihilation radiation. The coincidence rate due to annihilation radiation is distinguished from the rate due to cascaded nuclear gamma-rays by the sharp 180-degree angular correlation of the annihilation radiation. Pair emission from ${\mathrm{Mg}}^{24}$ is distinguished from gamma-ray pair production in the material surrounding the source by using materials of different atomic number $Z$. The efficiency of the apparatus for detecting positrons is determined by observing the coincidence rate from a ${\mathrm{Na}}^{22}$ source of known disintegration rate. This efficiency, combined with the fraction of the observed coincidence rate that is due to pair emission from ${\mathrm{Mg}}^{24}$ and with the known disintegration rate of the parent ${\mathrm{Na}}^{24}$ source yields a value of (6.7\ifmmode\pm\else\textpm\fi{}1.0)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ for the internal pair-conversion coefficient of the 2.76-Mev transition in ${\mathrm{Mg}}^{24}$. This value agrees with that calculated by Rose for electric quadrupole transitions of this energy.
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