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Register a new userEvidence of a new state in $^{11}$Be observed in the $^{11}$Li $beta$-decay
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Coincidences between charged particles emitted in the $beta$-decay of $^{11}$Li were observed using highly segmented detectors. The breakup channels involving three particles were studied in full kinematics allowing for the reconstruction of the excitation energy of the $^{11}$Be states participating in the decay. In particular, the contribution of a previously unobserved state at 16.3 MeV in $^{11}$Be has been identified selecting the $alpha$ + $^7$He$toalpha$ + $^6$He+n channel. The angular correlations between the $alpha$ particle and the center of mass of the $^6$He+n system favors spin and parity assignment of 3/2$^-$ for this state as well as for the previously known state at 18 MeV.
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The ($^{11}$B,$^{11}$Li) double charge-exchange reaction (DCER) at $E(^{11}$B)/$A$=80 MeV was measured for the first time to demonstrate the feasibility of the reaction for studying neutrino nuclear responses for double beta decays (DBD). The $^{13}$C($^{11}$B,$^{11}$Li)$^{13}$O reaction shows strengths at the ground state and low and high excitation giant resonance regions. The $^{56}$Fe ($^{11}$B,$^{11}$Li) $^{56}$Ni reaction shows the large strengths in the possible double giant resonance region and beyond, but shows no strengths in the low excitation region below 5 MeV, suggesting strong concentration of the DBD strength at the high excitation region. The DCER is used to evaluate the spin isospin strengths relevant to DBD responses.
The kinematics of two-neutron emission following the $beta$-decay of $^{11}$Li was investigated for the first time by detecting the two neutrons in coincidence and by measuring their angle and energy. An array of liquid-scintillator neutron detectors was used to reject cosmic-ray and $gamma$-ray backgrounds by pulse-shape discrimination. Cross-talk events in which two detectors are fired by a single neutron were rejected using a filter tested on the $beta$-1n emitter $^9$Li. A large cross-talk rejection rate is obtained ($> 95 %$) over most of the energy range of interest. Application to $^{11}$Li data leads to a significant number of events interpreted as $beta$-2n decay. A discrete neutron line at $approx$ 2 MeV indicates sequential two-neutron emission, possibly from the unbound state at 10.6 MeV excitation energy in $^{11}$Be.
The formation of a dineutron in the nucleus $^{11}$Li is found to be localized to the surface region. The experiment measured the intrinsic momentum of the struck neutron in $^{11}$Li via the $(p,pn)$ knockout reaction at 246 MeV/nucleon. The correlation angle between the two neutrons is, for the first time, measured as a function of the intrinsic neutron momentum. A comparison with reaction calculations reveals the localization of the dineutron at $rsim3.6$ fm. The results also support the density dependence of dineutron formation as deduced from Hartree-Fock-Bogoliubov calculations for nuclear matter.
The structure of the extremely proton-rich nucleus $^{11}_{~8}$O$_3$, the mirror of the two-neutron halo nucleus $^{11}_{~3}$Li$_8$, has been studied experimentally for the first time. Following two-neutron knockout reactions with a $^{13}$O beam, the $^{11}$O decay products were detected after two-proton emission and used to construct an invariant-mass spectrum. A broad peak of width $sim$3,MeV was observed. Within the Gamow coupled-channel approach, it was concluded that this peak is a multiplet with contributions from the four-lowest $^{11}$O resonant states: $J^{pi}$=3/2$^-_1$, 3/2$^-_2$, 5/2$^+_1$, and 5/2$^+_2$. The widths and configurations of these states show strong, non-monotonic dependencies on the depth of the $p$-$^9$C potential. This unusual behavior is due to the presence of a broad threshold resonant state in $^{10}$N, which is an analog of the virtual state in $^{10}$Li in the presence of the Coulomb potential. After optimizing the model to the data, only a moderate isospin asymmetry between ground states of $^{11}$O and $^{11}$Li was found.
The neutron-rich $^{11}$Li halo nucleus is unique among nuclei with known separation energies by its ability to emit a proton and a neutron in a $beta$ decay process. The branching ratio towards this rare decay mode is evaluated within a three-body model for the initial bound state and with Coulomb three-body final scattering states. The branching ratio should be comprised between two extreme cases, i.e. a lower bound $6 times 10^{-12}$ obtained with a pure Coulomb wave and an upper bound $5 times 10^{-10}$ obtained with a plane wave. A simple model with modified Coulomb waves provides plausible values between between $0.8 times 10^{-10}$ and $2.2 times 10^{-10}$ with most probable total energies of the proton and neutron between 0.15 and 0.3 MeV.
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