Double Gamma Decay inCa40

Abstract

A positive indication of double $\gamma$ decay in ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$ may have been observed. A branching ratio $\frac{{\Gamma }_{\gamma \gamma }}{\Gamma }=\left({3.3}_{-1.2\mathrm{}}^{+0.5\mathrm{}}\right)\times {10}^{-4\mathrm{}}$ is reported for the decay of the 3.35-MeV $0_2^{+}$ state. The state was preferentially populated by the ($p,p^{\prime }$) reaction at a bombarding energy of 5.08 MeV, through a resonance in the compound nucleus ${}_{}{}^{41}{}_{}{}^{}\mathrm{Sc}$. Recoil particles were detected in an annular detector at 180°, and $\gamma$ rays were detected in two NaI(Tl) detectors at 45° and 135° with respect to the beam axis. Triple time coincidence was demanded of the three radiations, and $\gamma$-energy signals were event recorded, together with coincidence information, permitting subsequent generation of two-dimensional energy spectra ($E_{{\gamma }_1} \mathrm{vs} E_{{\gamma }_2}$) for real, correlated random, and random coincidences. The distribution of double-$\gamma$ events was analyzed by the maximum-likelihood technique and was found to be consistent with the assumption of ($E1,E1\mathrm{}$) multipolarities. A variety of background processes was considered, but none was found likely to account for the observed spectrum. A brief historical review is given, along with a summary of previous experiments which sought to observe double $\gamma$ decay in ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$ and other nuclei. Existing theories for the process are examined.