Exotic nuclei explored at in-flight separators

One of the most exotic light neutron-rich nuclei currently accessible for experimental study is ^{40}Mg, which lies at the intersection of the nucleon magic number N=28 and the neutron drip line. Low-lying excited states of ^{40}Mg have been studied for the first time following a one-proton removal reaction from ^{41}Al, performed at the Radioactive Isotope Beam Factory of RIKEN Nishina Center with the DALI2 γ-ray array and the ZeroDegree spectrometer. Two γ-ray transitions were observed, suggesting an excitation spectrum that shows unexpected properties as compared to both the systematics along the Z=12, N≥20  Mg isotopes and available state-of-the-art theoretical model predictions. A possible explanation for the observed structure involves weak-binding effects in the low-lying excitation spectrum.

https://link.aps.org/accepted/10.1103/PhysRevLett.122.052501

First Spectroscopy of the Near Drip-line Nucleus ^{40}Mg.

We present a review of recent works on clustering phenomena in unstable nuclei studied by antisymmetrized molecular dynamics (AMD). The AMD studies in these decades have uncovered novel types of clustering phenomena brought about by the excess neutrons. Among them, this review focuses on the molecule-like structure of unstable nuclei. One of the earliest discussions on the clustering in unstable nuclei was made for neutron-rich Be and B isotopes. AMD calculations predicted that the ground state clustering is enhanced or reduced depending on the number of excess neutrons. Today, the experiments are confirming this prediction as the change of the proton radii. Behind this enhancement and reduction of the clustering, there are underlying shell effects called molecular- and atomic-orbits. These orbits form covalent and ionic bonding of the clusters analogous to the atomic molecules. It was found that this "molecular-orbit picture" reasonably explains the low-lying spectra of Be isotopes. The molecular-orbit picture is extended to other systems having parity asymmetric cluster cores and to the three cluster systems. O and Ne isotopes are the candidates of the former, while the $3\alpha$ linear chains in C isotopes are the latter. For both subjects, many intensive studies are now in progress. We also pay a special attention to the observables which are the fingerprint of the clustering. In particular, we focus on the monopole and dipole transitions which are recently regarded as good probe for the clustering. We discuss how they have and will reveal the exotic clustering.

Antisymmetrized molecular dynamics studies for exotic clustering phenomena in neutron-rich nuclei

Matter radii of the $^{34--40}\mathrm{Mg}$ nuclei are investigated by self-consistent Hartree-Fock-Bogolyubov calculations assuming the axial symmetry. With the semirealistic M3Y-P6 interaction, the $N$ dependence of the matter radii observed in the experiments is reproduced excellently. Both the pairing and the deformation play significant roles in an intertwined manner. The $^{35}\mathrm{Mg}$ nucleus has a smaller radius than the neighboring even-$N$ nuclei, which is attributed to its smaller deformation. On the other hand, a neutron halo is obtained in $^{37}\mathrm{Mg}$. We point out that, in contrast to the pairing antihalo effect that may operate on the even-$N$ nuclei, the pair correlation enhances halos in odd-$N$ nuclei, owing to the new mechanism which we call unpaired-particle haloing. The halo in $^{37}\mathrm{Mg}$ is predicted to have a peanut shape in its intrinsic state, reflecting $p$-wave contribution as in $^{40}\mathrm{Mg}$. The $N$ dependence of the deformation is significant again, by which the single-particle level dominated by the $p$-wave component comes down.

Intertwined effects of pairing and deformation on neutron halos in magnesium isotopes

Background: Very neutron-rich isotopes, including Ne30-32, in the vicinity of N=20 are known to exhibit ground states dominated by fp-shell intruder configurations: the “island of inversion.” Systematics for the Ne-isotopic chain suggest that such configurations may be in strong competition with normal shell-model configurations in the ground state of Ne29. Purpose: A determination of the structure of Ne29 is thus important to delineate the extent of the island of inversion and better understand structural evolution in neutron-rich Ne isotopes. This is accomplished here through a combined investigation of nuclear and Coulomb-induced one-neutron removal reactions. Method: Cross sections for one-neutron removal on carbon and lead targets and the parallel momentum distribution of the Ne28 residues from the carbon target are measured at around 240 MeV/nucleon. The measurements are compared with reaction calculations combined with spectroscopic information from SDPF-M shell-model wave functions. Results: The deduced width of the inclusive parallel momentum distribution, 98(12) MeV/c (FWHM), suggests that the ground state of Ne29 has a spin parity of 3/2-. Detailed comparisons of the measured inclusive and partial cross sections of the two targets and the parallel momentum distribution of the carbon target with reaction calculations, combined with spectroscopic information from large-scale shell-model calculations, are all consistent with a 3/2- spin-parity assignment. Conclusions: The results indicate that Ne29 lies within the island of inversion and that the ground state of Ne29 is dominated by a Ne28(0+1)xp3/2 neutron configuration. Combined with recently measured interaction cross sections, it is concluded that Ne29 may exhibit a moderately developed halo-like distribution

/pdf/one-neutron-removal-from-ne29-defining-the-lower-limits-of-353kwwjlt3.pdf

One-neutron removal from Ne29: Defining the lower limits of the island of inversion

Precise reaction cross sections (${\ensuremath{\sigma}}_{R}$) for $^{24\ensuremath{-}38}\mathrm{Mg}$ on C targets at energies around 240 MeV/nucleon have been measured at the Radioactive Isotope Beam Factory at RIKEN. The ${\ensuremath{\sigma}}_{R}$ for $^{36\ensuremath{-}38}\mathrm{Mg}$ have been measured for the first time. An enhancement of ${\ensuremath{\sigma}}_{R}$ compared to the systematics for spherical stable nuclei has been observed, especially in the neutron-rich region, which reflects the deformation of those isotopes. In the vicinity of the drip line the ${\ensuremath{\sigma}}_{R}$ for $^{37}\mathrm{Mg}$ is especially large. It is shown by analysis using a recently developed theoretical method that this prominent enhancement of ${\ensuremath{\sigma}}_{R}$ for $^{37}\mathrm{Mg}$ should come from the $p$-orbital halo formation breaking the $N=28$ shell gap.

Evidence of halo structure in Mg 37 observed via reaction cross sections and intruder orbitals beyond the island of inversion

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Evidence of halo structure in Mg 37 observed via reaction cross sections and intruder orbitals beyond the island of inversion