Structure of unbound neutron-rich 9^{9}He studied using single-neutron transfer

The 8He(d,p) reaction was studied in inverse kinematics at 15.4A MeV using the MUST2 Si-CsI array in order to shed light on the level structure of 9He. The well known 16O(d,p)17O reaction, performed here in reverse kinematics, was used as a test to validate the experimental methods. The 9He missing mass spectrum was deduced from the kinetic energies and emission angles of the recoiling protons. Several structures were observed above the neutron-emission threshold and the angular distributions were used to deduce the multipolarity of the transitions. This work confirms that the ground state of 9He is located very close to the neutron threshold of 8He and supports the occurrence of parity inversion in 9He.Comment: Exp\'erience GANIL/SPIRAL1/MUST

The N = 16 spherical shell closure in 24O

The unbound excited states of the neutron drip-line isotope 24O have been investigated via the 24O(p,p')23O+n reaction in inverse kinematics at a beam energy of 62 MeV/nucleon. The decay energy spectrum of 24O* was reconstructed from the momenta of 23O and the neutron. The spin-parity of the first excited state, observed at Ex = 4.65 +/- 0.14 MeV, was determined to be Jpi = 2+ from the angular distribution of the cross section. Higher lying states were also observed. The quadrupole transition parameter beta2 of the 2+ state was deduced, for the first time, to be 0.15 +/- 0.04. The relatively high excitation energy and small beta2 value are indicative of the N = 16 shell closure in 24O.Comment: to be submitted to Physical Review Letter

Spectroscopy of 32Ne and the Island of Inversion

We report on the first spectroscopic study of the N=22 nucleus 32Ne at the newly completed RIKEN Radioactive Ion Beam Factory. A single gamma-ray line with an energy of 722(9) keV was observed in both inelastic scattering of a 226 MeV/u 32Ne beam on a Carbon target and proton removal from 33Na at 245 MeV/u. This transition is assigned to the de-excitation of the first J^pi = 2+ state in 32Ne to the 0+ ground state. Interpreted through comparison with state-of-the-art shell model calculations, the low excitation energy demonstrates that the Island of Inversion extends to at least N=22 for the Ne isotopes.Comment: Accepted for publication in Phys. Rev. Lett. 11 pages, 3 figure

Search for low lying dipole strength in the neutron rich nucleus 26^{26}Ne

Coulomb excitation of the exotic neutron-rich nucleus $^{26}$Ne on a $^{nat}$Pb target was measured at 58 A.MeV in order to search for low-lying E1 strength above the neutron emission threshold. Data were also taken on an $^{nat}$Al target to estimate the nuclear contribution. The radioactive beam was produced by fragmentation of a 95 A.MeV $^{40}$Ar beam delivered by the RIKEN Research Facility. The set-up included a NaI gamma-ray array, a charged fragment hodoscope and a neutron wall. Using the invariant mass method in the $^{25}$Ne+n channel, we observe a sizable amount of E1 strength between 6 and 10 MeV. The reconstructed $^{26}$Ne angular distribution confirms its E1 nature. A reduced dipole transition probability of B(E1)=0.49$\pm$0.16 $e^2fm^2$ is deduced. For the first time, the decay pattern of low-lying strength in a neutron-rich nucleus is obtained. The results are discussed in terms of a pygmy resonance centered around 9 MeV

Linear Responses in Time-dependent Hartree-Fock-Bogoliubov Method with Gogny Interaction

A numerical method to integrate the time-dependent Hartree-Fock Bogoliubov (TDHFB) equations with Gogny interaction is proposed. The feasibility of the TDHFB code is illustrated by the conservation of the energy, particle numbers, and center-of-mass in the small amplitude vibrations of oxygen 20. The TDHFB code is applied to the isoscalar quadrupole and/or isovector dipole vibrations in the linear (small amplitude) region in oxygen isotopes (masses A = 18,20,22 and 24), titanium isotopes (A = 44,50,52 and 54), neon isotope (A = 26), and magnesium isotopes (A = 24 and 34). The isoscalar quadrupole and isovector dipole strength functions are calculated from the expectation values of the isoscalar quadrupole and isovector dipole moments.Comment: 10 pages, 13 figure

Excited states in neutron rich boron isotopes

Publisher's version/PDFNew experimental results on in-beam gamma spectroscopy of neutron rich boron isotopes are presented for [superscript 17]B and its neutron removal fragments [superscript 14,15]B, after scattering with a H[subscript 2] target. A gamma transition for [superscript 17]B is observed at 1089 [plus or minus] 15 keV. The fragment [superscript 15]B is observed abundantly associated with a gamma transition of 1336 [plus or minus] 10 keV. This suggests for the first time a core-excited structure for [superscript 17]B thereby providing a new insight into its structure. Observations for [superscript 12,14]B are also presented. The data set provides a useful systematic study of first excited states of neutron rich boron isotopes showing the dramatic drop in excitation energy beyond N = 8

One- and two-neutron removal reactions from the most neutron-rich carbon isotopes

The structure of $^{19,20,22}$C has been investigated using high-energy (about 240 MeV/nucleon) one- and two-neutron removal reactions on a carbon target. Measurements were made of the inclusive cross sections and momentum distributions for the charged residues. Narrow momentum distributions were observed for one-neutron removal from $^{19}$C and $^{20}$C and two-neutron removal from $^{22}$C. Two-neutron removal from $^{20}$C resulted in a relatively broad momentum distribution. The results are compared with eikonal-model calculations combined with shell-model structure information. The neutron-removal cross sections and associated momentum distributions are calculated for transitions to both the particle-bound and particle-unbound final states. The calculations take into account the population of the mass $A-1$ reaction residues, $^{A-1}$C, and, following one-neutron emission after one-neutron removal, the mass $A-2$ two-neutron removal residues, $^{A-2}$C. The smaller contributions of direct two-neutron removal, that populate the $^{A-2}$C residues in a single step, are also computed. The data and calculations are shown to be in good overall agreement and consistent with the predicted shell-model ground state configurations and the one-neutron overlaps with low-lying states in $^{18-21}$C. These suggest significant $\nu{s}_{1/2}^2$ valence neutron configurations in both $^{20}$C and $^{22}$C. The results for $^{22}$C strongly support the picture of $^{22}$C as a two-neutron halo nucleus with a dominant $\nu{s}_{1/2}^2$ ground state configuration.Comment: 13 pages, 8 figure

Experimental investigation of ground-state properties of ⁷H with transfer reactions

The properties of nuclei with extreme neutron–to–proton ratios, far from those naturally occurring on Earth, are key to understand nuclear forces and how nucleons hold together to form nuclei. 7H, with six neutrons and a single proton, is the nuclear system with the most unbalanced neutron–to–proton ratio known so far. However, its sheer existence and properties are still a challenge for experimental efforts and theoretical models. Here we report experimental evidences on the formation of 7H as a resonance, detected with independent observables, and the first measurement of the structure of its ground state. The resonance is found at ∼0.7 MeV above the 3H+4n mass, with a narrow width of ∼0.2 MeV and a 1/2+ spin and parity. These data are consistent with a 7H as a 3H core surrounded by an extended four-neutron halo, with a unique four-neutron decay and a relatively long half-life thanks to neutron pairing; a prime example of new phenomena occurring in what would be the most pure-neutron nuclear matter we can access in the laboratory.</p

Isoscalar response of Ni-68 to alpha-particle and deuteron probes

Isoscalar giant resonances have been measured in the unstable Ni-68 nucleus using inelastic alpha and deuteron scattering at 50A MeV in inverse kinematics with the active target MAYA at GANIL. Using alpha scattering, the extracted isoscalar giant monopole resonance (ISGMR) centroid was determined to be 21.1 +/- 1.9 MeV and the isoscalar giant quadrupole resonance (ISGQR) to be 15.9 +/- 1.3MeV. Indications for soft isoscalar monopole and dipole modes are provided. Results obtained with both (alpha, alpha') and (d, d') probes are compatible. The evolution of isoscalar giant resonances along the Ni isotopic chain from Ni-56 to Ni-68 is discussed.</p