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Register a new userFirst observation of $^{13}$Li ground state
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The ground state of neutron-rich unbound $^{13}$Li was observed for the first time in a one-proton removal reaction from $^{14}$Be at a beam energy of 53.6 MeV/u. The $^{13}$Li ground state was reconstructed from $^{11}$Li and two neutrons giving a resonance energy of 120$^{+60}_{-80}$ keV. All events involving single and double neutron interactions in the Modular Neutron Array (MoNA) were analyzed, simulated, and fitted self-consistently. The three-body ($^{11}$Li+$n+n$) correlations within Jacobi coordinates showed strong dineutron characteristics. The decay energy spectrum of the intermediate $^{12}$Li system ($^{11}$Li+$n$) was described with an s-wave scattering length of greater than -4 fm, which is a smaller absolute value than reported in a previous measurement.
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Neutron decay spectroscopy has become a successful tool to explore nuclear properties of nuclei with the largest neutron-to-proton ratios. Resonances in nuclei located beyond the neutron dripline are accessible by kinematic reconstruction of the decay products. The development of two-neutron detection capabilities of the Modular Neutron Array (MoNA) at NSCL has opened up the possibility to search for unbound nuclei which decay by the emission of two neutrons. Specifically this exotic decay mode was observed in 16Be and 26O.
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.
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 exact nature of the lowest $K^pi =2_gamma ^+$ rotational bands in all deformed nuclei remains obscure. Traditionally they are assumed to be collective vibrations of the nuclear shape in the $gamma$ degree of freedom perpendicular to the nuclear symmetry axis. Very few such $gamma$-bands have been traced past the usual back-bending rotational alignments of high-j nucleons. We have investigated the structure of positive-parity bands in the N=90 nucleus 156Dy, using the 148Nd(12C,4n)156Dy reaction at 65 MeV, observing the resulting ${gamma}$-ray transitions with the Gammasphere array. The even- and odd-spin members of the $K^pi =2_gamma^+$ $gamma$-band are observed to 32+ and 31+ respectively. This rotational band faithfully tracks the ground-state configuration to the highest spins. The members of a possible $gamma$-vibration built on the aligned yrast S-band are observed to spins 28+ and 27+. An even-spin positive-parity band, observed to spin 24+, is a candidate for an aligned S-band built on the seniority-zero configuration of the $0_2^+$ state at 676 keV. The crossing of this band with the $0_2^+$ band is at $hbaromega$= 0.28(1) MeV and is consistent with the configuration of the $0_2^+$ band not producing any blocking of the monopole pairing.
Bound-systems of $Xi^-$--$^{14}_{}{rm N}$ are studied via $Xi^-$ capture at rest followed by emission of a twin single-$Lambda$ hypernucleus in the emulsion detectors. Two events forming extremely deep $Xi^-$ bound states were obtained by analysis of a hybrid method in the E07 experiment at J-PARC and reanalysis of the E373 experiment at KEK-PS. The decay mode of one event was assigned as $Xi^-+^{14}_{}{rm N}to^{5}_{Lambda}{rm He}$+$^{5}_{Lambda}{rm He}$+$^{4}_{}{rm He}$+n. Since there are no excited states for daughter particles, the binding energy of the $Xi^-$ hyperon, $B_{Xi^-}$, in $^{14}_{}{rm N}$ nucleus was uniquely determined to be 6.27 $pm$ 0.27 MeV. Another $Xi^-$--$^{14}_{}{rm N}$ system via the decay $^{9}_{Lambda}{rm Be}$ + $^{5}_{Lambda}{rm He}$ + n brings a $B_{Xi^-}$ value, 8.00 $pm$ 0.77 MeV or 4.96 $pm$ 0.77 MeV, where the two possible values of $B_{Xi^-}$ correspond to the ground and the excited states of the daughter $^{9}_{Lambda}{rm Be}$ nucleus, respectively. Because the $B_{Xi^-}$ values are larger than those of the previously reported events (KISO and IBUKI), which are both interpreted as the nuclear $1p$ state of the $Xi^-$--$^{14}_{}{rm N}$ system, these new events give the first indication of the nuclear $1s$ state of the $Xi$ hypernucleus, $^{15}_{Xi}{rm C}$.
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