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Register a new userSelective population and neutron decay of the first excited state of semi-magic O-23
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We have observed an excited state in the neutron-rich semi-magic nucleus O-23 for the first time. No such states have been found in previous searches using gamma-ray spectroscopy. The observation of a resonance in n-fragment coincidence measurements confirms the speculation in the literature that the lowest excited state is neutron unbound and establishes positive evidence for a 2.8(1) MeV excitation energy of the first excited state in O-23. The non-observation of a predicted second excited state is explained assuming selectivity of inner-shell knockout reactions.
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Two-proton decay of the $^{17}$Ne low-lying states populated in the $^1$H($^{18}$Ne,$d$)$^{17}$Ne transfer reaction was studied. The two-proton width $Gamma_{2p}$ of the $^{17}$Ne first excited $3/2^-$ state at $E^*=1.288$ MeV is of importance for the two-proton radioactivity theory and nuclear-astrophysics applications. A dedicated search for the two-proton emission of this state was performed leading to the new upper limit obtained for the width ratio $Gamma_{2p}/Gamma_{gamma} < 1.6(3) times 10^{-4}$. A novel, combined mass method is suggested and tested capable to improve the resolution of the experiment which is a prime significance for the study of nuclear states with extremely small particle-to-gamma width ratios $Gamma_{mathrm{part}}/Gamma_{gamma}$. The condition $Gamma_{mathrm{part}} ll Gamma_{gamma}$ is quite common for the states of astrophysical interest which makes the proposed approach promising in this field.
The first excited state in neutron-rich 23O was observed in a (2p1n) knock-out reaction from 26Ne on a beryllium target at a beam energy of 86 MeV/A. The state is unbound with respect to neutron emission and was reconstructed from the invariant mass from the 22O fragment and the neutron. It is unbound by 45(2) keV corresponding to an excitation energy of 2.8(1) MeV. The non-observation of further resonances implies a predominantly direct reaction mechanism of the employed three-nucleon-removal reaction which suggests the assignment of the observed resonance to be the 5/2+ hole state.
A two-neutron unbound excited state of $^{24}$O was populated through a (d,d) reaction at 83.4 MeV/nucleon. A state at $E = 715 pm 110$ (stat) $pm 45 $ (sys) keV with a width of $Gamma < 2$ MeV was observed above the two-neutron separation energy placing it at 7.65 $pm$ 0.2 MeV with respect to the ground state. Three-body correlations for the decay of $^{24}$O $rightarrow$ $^{22}$O + $2n$ show clear evidence for a sequential decay through an intermediate state in $^{23}$O. Neither a di-neutron nor phase-space model for the three-body breakup were able to describe these correlations.
The CUORICINO experiment was an array of 62 TeO$_{2}$ single-crystal bolometers with a total $^{130}$Te mass of $11.3,$kg. The experiment finished in 2008 after more than 3 years of active operating time. Searches for both $0 u$ and $2 u$ double-beta decay to the first excited $0^{+}$ state in $^{130}$Xe were performed by studying different coincidence scenarios. The analysis was based on data representing a total exposure of N($^{130}$Te)$cdot$t=$9.5times10^{25},$y. No evidence for a signal was found. The resulting lower limits on the half lives are $T^{2 u}_{1/2}(^{130} Terightarrow^{130} Xe^{*})>1.3times10^{23},$y (90% C.L.), and $T^{0 u}_{1/2}(^{130} Terightarrow^{130} Xe^{*})>9.4times10^{23},$y (90% C.L.).
Effect of the tensor force on $beta$?-decay is studied in the framework of the proton-neutron random-phase-approximation (RPA) with the Skyrme force. The investigation is performed for even-even semi-magic and magic nuclei, $^{34}$Si, $^{68}$, $^{78}$Ni and $^{132}$Sn. The tensor correlation induces strong impact on low-lying Gamow-Teller state. In particular, it improves the ?$beta$-decay half-lives. $Q$ and $ft$ values are also investigated and compared with experimental data.
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