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

Background: Two-photon emission, while well known in atomic physics, is arare 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+ ⊿ 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, 160,40Ca, and 90Zr. 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, 98Mo 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 = 0+) in 98Mo 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 x 10-4 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. [ABSTRACT FROM AUTHOR]