Unveiling the intruder deformed 0${}_2^{+}$ state in ${}^{34}$Si

Abstract

The 0${}_2^{+}$ state in ${}^{34}$Si has been populated at the {\sc Ganil/Lise3} facility through the ${\hat{I}}^2$-decay of a newly discovered 1${}^{+}$ isomer in ${}^{34}$Al of 26(1) ms half-life. The simultaneous detection of $e^{+}{e}^{-}$ pairs allowed the determination of the excitation energy E(0${}_2^{+}$)=2719(3) keV and the half-life T${}_{1/2}$=19.4(7) ns, from which an electric monopole strength of $\ddot{I}{}^2$(E0)=13.0(0.9)$\times 10^{-3}$ was deduced. The 2${}_1^{+}$ state is observed to decay both to the 0${}_1^{+}$ ground state and to the newly observed 0${}_2^{+}$ state (via a 607(2) keV transition) with a ratio R(2${}_1^{+}$$\to 0_1^{+}/2_1^{+}$$\to 0_2^{+}$)=1380(717). Gathering all information, a weak mixing with the 0${}_1^{+}$ and a large deformation parameter of ${\hat{I}}^2$=0.29(4) are found for the 0${}_2^{+}$ state, in good agreement with shell model calculations using a new {\sc sdpf-u-mix} interaction allowing \textit{np-nh} excitations across the N=20 shell gap.

5 pages, 3 figures, accepted for publication in Physical Review Letters