Two-Photon Decay of the 1.76-MeV0+State ofZr90

Abstract

Measurements of the probability and the energy distribution for the two-photon decay from the 1.76-MeV ${0}^{+}$ first excited state to the ${0}^{+}$ ground state of $^{90}\mathrm{Zr}$ have been performed using a sum-coincidence technique with an antidetector and large lead shields. This improved experimental technique made it possible to eliminate unfavorable coincident events which would disturb true events from the two-photon process. The ratio of two-photon decay to the sum of internal-pair decay and internal-conversion decay, $\frac{{T}_{\ensuremath{\gamma}\ensuremath{\gamma}}}{{T}_{\ensuremath{\pi}+e}}$, has been found to be (5.1 \ifmmode\pm\else\textpm\fi{} 2.5) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}4}$. This value is larger than the upper limits reported recently by other workers, and not consistent with any values calculated based on existing theories. The energy spectrum of one of two photons has also been observed. This spectrum suggests reasonably that two-photon decay takes place as an ($E1,E1$) transition via giant-dipole-resonance states. A discussion of the theoretical significance of this study is given.