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تسجيل مستخدم جديدUnveiling the intruder deformed 0$^+_2$ state in $^{34}$Si
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The 0$^+_2$ state in $^{34}$Si has been populated at the {sc Ganil/Lise3} facility through the $beta$-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 $rho^2$(E0)=13.0(0.9)$times10^{-3}$ 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$$rightarrow0^+_1/2^+_1$$rightarrow0^+_2$)=1380(717). Gathering all information, a weak mixing with the 0$^+_1$ and a large deformation parameter of $beta$=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.
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The structure of $^{34}$Si was studied through $gamma$ spectroscopy separately in the $beta^-$ decays of $^{34}$Mg and $^{34}$Al at the ISOLDE facility of CERN. Different configurations in $^{34}$Si were populated independently from the two recently
identified $beta$-decaying states in $^{34}$Al having spin-parity assignments $J^pi = 4^-$ dominated by the normal configuration $pi (d_{5/2})^{-1} otimes u (f_{7/2})$ and $J^pi = 1^+$ by the intruder configuration $pi (d_{5/2})^{-1} otimes u (d_{3/2})^{-1}(f_{7/2})^{2}$. The paper reports on spectroscopic properties of $^{34}$Si such as an extended level scheme, spin and parity assignments based on log($ft$) values and $gamma$-ray branching ratios, absolute $beta$ feeding intensities and neutron emission probabilities. A total of 11 newly identified levels and 26 transitions were added to the previously known level scheme of $^{34}$Si. Large scale shell-model calculations using the {sc sdpf-u-mix} interaction, able to treat higher order intruder configurations, are compared with the new results and conclusions are drawn concerning the predictive power of {sc sdpf-u-mix}, the $N=20$ shell gap, the level of mixing between normal and intruder configurations for the 0$_1^+$, 0$_2^+$ and 2$_1^+$ states and the absence of triaxial deformation in $^{34}$Si.
We report on the observation of a new isomeric state in $^{68}$Ni. We suggest that the newly observed state at 168(1) keV above the first 2$^+$ state is a $pi(2p-2h)$ 0$^{+}$ state across the major Z=28 shell gap. Comparison with theoretical calculat
ions indicates a pure proton intruder configuration and the deduced low-lying structure of this key nucleus suggests a possible shape coexistence scenario involving a highly deformed state.
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