We perform the first direct mass measurements of neutron-rich calcium isotopes beyond neutron number 34 at the RIKEN Radioactive Isotope Beam Factory by using the time-of-flight magnetic-rigidity technique. The atomic mass excesses of ^{55-57}Ca are determined for the first time to be -18650(160), -13510(250), and -7370(990)  keV, respectively. We examine the emergence of neutron magicity at N=34 based on the new atomic masses. The new masses provide experimental evidence for the appearance of a sizable energy gap between the neutron 2p_{1/2} and 1f_{5/2} orbitals in ^{54}Ca, comparable to the gap between the neutron 2p_{3/2} and 2p_{1/2} orbitals in ^{52}Ca. For the ^{56}Ca nucleus, an open-shell property in neutrons is suggested.

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We analyze recently measured total reaction cross sections for [24][25][26][27][28][29][30][31][32][33][34][35][36][37][38] Mg isotopes incident on 12 C targets at 240 MeV/nucleon by using the folding model and antisymmetrized molecular dynamics (AMD). The folding model well reproduces the measured reaction cross sections, when the projectile densities are evaluated by the deformed Woods-Saxon (def-WS) model with AMD deformation. Matter radii of [24][25][26][27][28][29][30][31][32][33][34][35][36][37][38] Mg are then deduced from the measured reaction cross sections by fine tuning the parameters of the def-WS model. The deduced matter radii are largely enhanced by nuclear deformation. Fully microscopic AMD calculations with no free parameter well reproduce the deduced matter radii for [24][25][26][27][28][29][30][31][32][33][34][35][36] Mg, but still considerably underestimate them for 37,38 Mg. The large matter radii suggest that 37,38 Mg are candidates for deformed halo nucleus. AMD also reproduces other existing measured ground-state properties (spin parity, total binding energy, and one-neutron separation energy) of Mg isotopes. Neutron-number (N ) dependence of deformation parameter is predicted by AMD. Large deformation is seen from 31 Mg with N = 19 to a drip-line nucleus 40 Mg with N = 28, indicating that both the N = 20 and 28 magicities disappear. N dependence of neutron skin thickness is also predicted by AMD.

The nuclear shell structure, which originates in the nearly independent motion of nucleons in an average potential, provides an important guide for our understanding of nuclear structure and the underlying nuclear forces. Its most remarkable fingerprint is the existence of the so-called magic numbers of protons and neutrons associated with extra stability. Although the introduction of a phenomenological spin–orbit (SO) coupling force in 1949 helped in explaining the magic numbers, its origins are still open questions. Here, we present experimental evidence for the smallest SO-originated magic number (subshell closure) at the proton number six in 13–20C obtained from systematic analysis of point-proton distribution radii, electromagnetic transition rates and atomic masses of light nuclei. Performing ab initio calculations on 14,15C, we show that the observed proton distribution radii and subshell closure can be explained by the state-of-the-art nuclear theory with chiral nucleon–nucleon and three-nucleon forces, which are rooted in the quantum chromodynamics.

Precise reaction cross sections (oR) for 24_38M g on C targets at energies around 240 M eV /nucleon have been measured at the Radioactive Isotope Beam Factory at RIKEN. The oR for 36-38 Mg have been measured for the first time. An enhancement o f oR 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 aR for 37Mg is especially large. It is shown by analysis using a recently developed theoretical method that this prominent enhancement of oR for 37Mg should come from the p-orbital halo formation breaking the N = 28 shell gap.Since the early years of the study of atomic nuclei, the nuclear shell model has been the basic framework for understanding nuclear structure. The high stability of nuclei with certain numbers of neutrons (or protons) observed in stable nuclei indicates the existence of the shells filled at certain so-called "magic numbers." Studies in the last few decades have revealed that those magic numbers are sometimes broken or changed in unstable nuclei [1], The breakdown of the N = 20 shell gap between the sd and f p shells has been extensively studied since the irregularities in binding energies and 2+ excitation energies were observed in neutron-rich nuclei around N = 20 [2-6]. The term "island of inversion" was applied to this region [6] and deformed nuclear structures related to the changing of shell structures have been reported in this region [7]. The vanishing of the N = 28 shell closure has been also extensively studied, starting from neutronrich S-Ar isotopes [8][9][10][11][12][13][14]. The development of deformation observed in those nuclei could be interpreted as degeneracy of the f p shell, which induces strong quadrupole deformation [9][10][11][13][14][15][16][17][18]. Such deformation has been reported also for Si isotopes [19,20], and studies have recently indicated that this * takechi @ np.gs .niigata-u. ac .jp PACS number(s): 21.10.Gv, 25.60.Dz phenomenon could be seen even in a very neutron-rich Mg region [21].The purpose of our present study is to elucidate the changes of nuclear structures, such as a development of deformation, a breakdown of the magic numbers and possible halo formation in Mg isotopes, from the stability line to the vicinity of the neutron drip line. For this purpose, precise measurements of reaction cross sections for 24_38Mg have been performed at the Radioactive Isotope Beam Factory (RIBF) at RIKEN. The reaction cross section aR or interaction cross section ay reflects the nuclear size, and has been a powerful probe in searching for halo formation since the first study by Tanihata et al. [22], Recently, measurements of o, for Ne isotopes performed at RIBF [23] have successfully revealed the halo structure of 3lNe in which the sd-pf shell inversion associated with nuclear deformation causes the formation of a halo [23][24][25]. Moreover, theoretical studies on those data have shown that a precise data set on crR is v...

Nuclear magnetic resonance (NMR) spectrum of the short-lived nucleus 17 N (I = 1/2, T 1/2 = 4.17s) in liquid water was measured by means of the β-NMR technique to clarify the chemical species formed by nitrogen ions injected into water. We have improved the spectral resolution to 5ppm in the full width at half maximum which is about 1/40 times compared to the previous study. The shape of the obtained spectrum indicated that it may consist of multiple resonance lines rather than a single line. Some possibilities regarding the chemical states of nitrogen in water are discussed based on the present result. Keywords β-NMR • Nitrogen ion • H 2 O • Spin-spin coupling This article is part of the Topical Collection on

The low-lying structure of the neutron-rich nucleus (50)Ar has been investigated at the Radioactive Isotope Beam Factory using in-beam γ-ray spectroscopy with (9)Be((54)Ca,(50)Ar+γ)X, (9)Be((55)Sc,(50)Ar+γ)X, and (9)Be((56)Ti,(50)Ar+γ)X multinucleon removal reactions at ∼220  MeV/u. A γ-ray peak at 1178(18) keV is reported and assigned as the transition from the first 2(+) state to the 0(+) ground state. A weaker, tentative line at 1582(38) keV is suggested as the 4(1)(+)→2(1)(+) transition. The experimental results are compared to large-scale shell-model calculations performed in the sdpf model space using the SDPF-MU effective interaction with modifications based on recent experimental data for exotic calcium and potassium isotopes. The modified Hamiltonian provides a satisfactory description of the new experimental results for (50)Ar and, more generally, reproduces the energy systematics of low-lying states in neutron-rich Ar isotopes rather well. The shell-model calculations indicate that the N=32 subshell gap in (50)Ar is similar in magnitude to those in (52)Ca and (54)Ti and, notably, predict an N=34 subshell closure in (52)Ar that is larger than the one recently reported in (54)Ca.

Very neutron-rich Z ∼ 60 isotopes produced by in-flight fission of 345 MeV/nucleon 238 U beam at the RI Beam Factory, RIKEN Nishina Center have been studied by delayed γ-ray spectroscopy. New isomers are discovered in the neutron-rich N = 100 isotones, 162 Sm, 163 Eu, and 164 Gd. Half-lives, γ-ray energies and relative intensities of these isomers were obtained. Level schemes were proposed for these nuclei and the first 2 + and 4 + states were assigned for the even-even nuclei. configurations with similar excitation energy. The results suggest that neutron-rich N = 100 nuclei are well deformed and the deformation gets larger as Z decreases to 62. The onset of K isomers with the same configuration at almost the same energy in N = 100 isotones indicates that the neutron single-particle structures of neutron-rich isotones down to Z = 62 do not change significantly from those of the Z = 70 stable nuclei. Systematics of the excitation energies of new isomers can be explained without the predicted N = 100 shell gap.