Upper limit on the two-photon emission branch for the 0 2 + → 0 1 + transition in 98 Mo

Background: Two-photon emission, while well known in atomic physics, is a rare second-order process in nuclear physics with only three cases where a two-photon branch is measured. The limited knowledge stems from the experimental difficulty in resolving two-photon emission from dominant single-photon emission, restricting practical cases for study to ${0}^{+}\\ensuremath{\\rightarrow}{0}^{+}$ ($E0$) transitions, since single-photon emission is forbidden. In practical terms, this limits the range of easily accessible cases to even-even nuclei with the unusual property of a first excited state with spin/parity of 0${}^{+}$.Purpose: Two-photon branches are measured for the closed-shell nuclei, ${}^{16}\\mathrm{O}$, ${}^{40}\\mathrm{Ca}$, and ${}^{90}\\mathrm{Zr}$. The intention of the present work was to obtain data for a case which was not a closed-shell nucleus. Of the possible nuclei relevant to such a study, ${}^{98}Mo$ was chosen as its first-excited state is 0${}^{+}$ and lies below 1 MeV, meaning that internal pair transitions are not allowed.Method: The first excited state (${J}^{\\ensuremath{\\pi}}={0}^{+}$) in ${}^{98}Mo$ was excited in resonant inelastic proton scattering using a 6.7-MeV proton beam. The population of the state was selected using an annular double-sided silicon strip detector (DSSD). The decay of the state by conversion electrons was observed using the same DSSD, while Gammasphere was used to detect possible two-photon events.Results: An upper limit on the two-photon branch obtained was 1 $\\ifmmode\\times\\else\\texttimes\\fi{}$ 10${}^{\\ensuremath{-}4}$ at the 95% confidence level (CL). Conclusions The upper limit obtained is smaller than any other previously obtained two-photon branch. Phase space considerations suggest that the actual value of the branching ratio in this case may be significantly smaller than the upper limit obtained.