Discovery of

The new isotope ^{39}Na, the most neutron-rich sodium nucleus observed so far, was discovered at the RIKEN Nishina Center Radioactive Isotope Beam Factory using the projectile fragmentation of an intense ^{48}Ca beam at 345 MeV/nucleon on a beryllium target. Projectile fragments were separated and identified in flight with the large-acceptance two-stage separator BigRIPS. Nine ^{39}Na events have been unambiguously observed in this work and clearly establish the particle stability of ^{39}Na. Furthermore, the lack of observation of ^{35,36}Ne isotopes in this experiment significantly improves the overall confidence that ^{34}Ne is the neutron dripline nucleus of neon. These results provide new key information to understand nuclear binding and nuclear structure under extremely neutron-rich conditions. The newly established stability of ^{39}Na has a significant impact on nuclear models and theories predicting the neutron dripline and also provides a key to understanding the nuclear shell property of ^{39}Na at the neutron number N=28, which is normally a magic number.

Observation of a correlated free four-neutron system.

A long-standing question in nuclear physics is whether chargeless nuclear systems can exist. To our knowledge, only neutron stars represent near-pure neutron systems, where neutrons are squeezed together by the gravitational force to very high densities. The experimental search for isolated multi-neutron systems has been an ongoing quest for several decades1, with a particular focus on the four-neutron system called the tetraneutron, resulting in only a few indications of its existence so far2-4, leaving the tetraneutron an elusive nuclear system for six decades. Here we report on the observation of a resonance-like structure near threshold in the four-neutron system that is consistent with a quasi-bound tetraneutron state existing for a very short time. The measured energy and width of this state provide a key benchmark for our understanding of the nuclear force. The use of an experimental approach based on a knockout reaction at large momentum transfer with a radioactive high-energy 8He beam was key.

Probing the Symmetry Energy with the Spectral Pion Ratio.

Many neutron star properties, such as the proton fraction, reflect the symmetry energy contributions to the equation of state that dominate when neutron and proton densities differ strongly. To constrain these contributions at suprasaturation densities, we measure the spectra of charged pions produced by colliding rare isotope tin (Sn) beams with isotopically enriched Sn targets. Using ratios of the charged pion spectra measured at high transverse momenta, we deduce the slope of the symmetry energy to be 42

Mapping of a New Deformation Region around

We performed the first direct mass measurements of neutron-rich scandium, titanium, and vanadium isotopes around the neutron number 40 at the RIKEN RI Beam Factory using the time-of-flight magnetic-rigidity technique. The atomic mass excesses of ^{58-60}Sc, ^{60-62}Ti, and ^{62-64}V were measured for the first time. The experimental results show that the two-neutron separation energies in the vicinity of ^{62}Ti increase compared to neighboring nuclei. This shows that the masses of Ti isotopes near N=40 are affected by the Jahn-Teller effect. Therefore, a development of Jahn-Teller stabilization appears below the Cr isotopes, and the systematics in Sc, Ti, and V isotopes suggest that ^{62}Ti is located close to the peak of the Jahn-Teller effect.

How Different is the Core of

The structure of a neutron-rich ^{25}F nucleus is investigated by a quasifree (p,2p) knockout reaction at 270A MeV in inverse kinematics. The sum of spectroscopic factors of π0d_{5/2} orbital is found to be 1.0±0.3. However, the spectroscopic factor with residual ^{24}O nucleus being in the ground state is found to be only 0.36±0.13, while those in the excited state is 0.65±0.25. The result shows that the ^{24}O core of ^{25}F nucleus significantly differs from a free ^{24}O nucleus, and the core consists of ∼35% ^{24}O_{g.s.}. and ∼65% excited ^{24}O. The result may infer that the addition of the 0d_{5/2} proton considerably changes neutron structure in ^{25}F from that in ^{24}O, which could be a possible mechanism responsible for the oxygen dripline anomaly.

Halo Structure of the Neutron-Dripline Nucleus

The heaviest bound isotope of boron ^{19}B has been investigated using exclusive measurements of its Coulomb dissociation, into ^{17}B and two neutrons, in collisions with Pb at 220 MeV/nucleon. Enhanced electric dipole (E1) strength is observed just above the two-neutron decay threshold with an integrated E1 strength of B(E1)=1.64±0.06(stat)±0.12(sys) e^{2} fm^{2} for relative energies below 6 MeV. This feature, known as a soft E1 excitation, provides the first firm evidence that ^{19}B has a prominent two-neutron halo. Three-body calculations that reproduce the energy spectrum indicate that the valence neutrons have a significant s-wave configuration and exhibit a dineutronlike correlation.

g Factor of the

The gyromagnetic factor of the low-lying E=251.96(9) keV isomeric state of the nucleus ^{99}Zr was measured using the time-dependent perturbed angular distribution technique. This level is assigned a spin and parity of J^{π}=7/2^{+}, with a half-life of T_{1/2}=336(5) ns. The isomer was produced and spin aligned via the abrasion-fission of a ^{238}U primary beam at RIKEN RIBF. A magnetic moment |µ|=2.31(14)µ_{N} was deduced showing that this isomer is not single particle in nature. A comparison of the experimental values with interacting boson-fermion model IBFM-1 results shows that this state is strongly mixed with a main νd_{5/2} composition. Furthermore, it was found that monopole single-particle evolution changes significantly with the appearance of collective modes, likely due to type-II shell evolution.

Swelling of Doubly Magic

Interaction cross sections for ^{42-51}Ca on a carbon target at 280 MeV/nucleon have been measured for the first time. The neutron number dependence of derived root-mean-square matter radii shows a significant increase beyond the neutron magic number N=28. Furthermore, this enhancement of matter radii is much larger than that of the previously measured charge radii, indicating a novel growth in neutron skin thickness. A simple examination based on the Fermi-type distribution, and mean field calculations point out that this anomalous enhancement of the nuclear size beyond N=28 results from an enlargement of the core by a sudden increase in the surface diffuseness of the neutron density distribution, which implies the swelling of the bare ^{48}Ca core in Ca isotopes beyond N=28.

Fragmentation of Single-Particle Strength around the Doubly Magic Nucleus

Spectroscopic factors of neutron-hole and proton-hole states in ^{131}Sn and ^{131}In, respectively, were measured using one-nucleon removal reactions from doubly magic ^{132}Sn at relativistic energies. For ^{131}In, a 2910(50)-keV γ ray was observed for the first time and tentatively assigned to a decay from a 5/2^{-} state at 3275(50) keV to the known 1/2^{-} level at 365 keV. The spectroscopic factors determined for this new excited state and three other single-hole states provide first evidence for a strong fragmentation of single-hole strength in ^{131}Sn and ^{131}In. The experimental results are compared to theoretical calculations based on the relativistic particle-vibration coupling model and to experimental information for single-hole states in the stable doubly magic nucleus ^{208}Pb.

Location of the Neutron Dripline at Fluorine and Neon.

A search for the heaviest isotopes of fluorine, neon, and sodium was conducted by fragmentation of an intense ^{48}Ca beam at 345 MeV/nucleon with a 20-mm-thick beryllium target and identification of isotopes in the large-acceptance separator BigRIPS at the RIKEN Radioactive Isotope Beam Factory. No events were observed for ^{32,33}F, ^{35,36}Ne, and ^{38}Na and only one event for ^{39}Na after extensive running. Comparison with predicted yields excludes the existence of bound states of these unobserved isotopes with high confidence levels. The present work indicates that ^{31}F and ^{34}Ne are the heaviest bound isotopes of fluorine and neon, respectively. The neutron dripline has thus been experimentally confirmed up to neon for the first time since ^{24}O was confirmed to be the dripline nucleus nearly 20 years ago. These data provide new keys to understanding the nuclear stability at extremely neutron-rich conditions.

Improved Value for the Gamow-Teller Strength of the

A record number of ^{100}Sn nuclei was detected and new isotopic species toward the proton dripline were discovered at the RIKEN Nishina Center. Decay spectroscopy was performed with the high-efficiency detector arrays WAS3ABi and EURICA. Both the half-life and the ß-decay end point energy of ^{100}Sn were measured more precisely than the literature values. The value and the uncertainty of the resulting strength for the pure 0^{+}→1^{+} Gamow-Teller decay was improved to B_{GT}=4.4_{-0.7}^{+0.9}. A discrimination between different model calculations was possible for the first time, and the level scheme of ^{100}In is investigated further.

Proton Shell Evolution below

The ß-delayed γ-ray spectroscopy of neutron-rich ^{123,125}Ag isotopes is investigated at the Radioactive Isotope Beam Factory of RIKEN, and the long-predicted 1/2^{-} ß-emitting isomers in ^{123,125}Ag are identified for the first time. With the new experimental results, the systematic trend of energy spacing between the lowest 9/2^{+} and 1/2^{-} levels is extended in Ag isotopes up to N=78, providing a clear signal for the reduction of the Z=40 subshell gap in Ag towards N=82. Shell-model calculations with the state-of-the-art V_{MU} plus M3Y spin-orbit interaction give a satisfactory description of the low-lying states in ^{123,125}Ag. The tensor force is found to play a crucial role in the evolution of the size of the Z=40 subshell gap. The observed inversion of the single-particle levels around ^{123}Ag can be well interpreted in terms of the monopole shift of the π1g_{9/2} orbitals mainly caused by the increasing occupation of ν1h_{11/2} orbitals.

Extraction of the Landau-Migdal Parameter from the Gamow-Teller Giant Resonance in

The key parameter to discuss the possibility of the pion condensation in nuclear matter, i.e., the so-called Landau-Migdal parameter g^{'}, was extracted by measuring the double-differential cross sections for the (p,n) reaction at 216 MeV/u on a neutron-rich doubly magic unstable nucleus, ^{132}Sn with the quality comparable to data taken with stable nuclei. The extracted strengths for Gamow-Teller (GT) transitions from ^{132}Sn leading to ^{132}Sb exhibit the GT giant resonance (GTR) at the excitation energy of 16.3±0.4(stat)±0.4(syst) MeV with the width of Γ=4.7±0.8 MeV. The integrated GT strength up to E_{x}=25 MeV is S_{GT}^{-}=53±5(stat)_{-10}^{+11}(syst), corresponding to 56% of Ikeda's sum rule of 3(N-Z)=96. The present result accurately constrains the Landau-Migdal parameter as g^{'}=0.68±0.07, thanks to the high sensitivity of the GTR energy to g^{'}. In combination with previous studies on the GTR for ^{90}Zr and ^{208}Pb, the result of this work shows the constancy of this parameter in the nuclear chart region with (N-Z)/A=0.11 to 0.24 and A=90 to 208.

Magic Nature of Neutrons in

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.

Discovery of

The discovery of the important neutron-rich nucleus _{20}^{60}Ca_{40} and seven others near the limits of nuclear stability is reported from the fragmentation of a 345 MeV/u ^{70}Zn projectile beam on ^{9}Be targets at the radioactive ion-beam factory of the RIKEN Nishina Center. The produced fragments were analyzed and unambiguously identified using the BigRIPS two-stage in-flight separator. The eight new neutron-rich nuclei discovered, ^{47}P, ^{49}S, ^{52}Cl, ^{54}Ar, ^{57}K, ^{59,60}Ca, and ^{62}Sc, are the most neutron-rich isotopes of the respective elements. In addition, one event consistent with ^{59}K was registered. The results are compared with the drip lines predicted by a variety of mass models and it is found that the models in best agreement with the observed limits of existence in the explored region tend to predict the even-mass Ca isotopes to be bound out to at least ^{70}Ca.

Excitation of the Isovector Spin Monopole Resonance via the Exothermic

The (^{12}N, ^{12}C) charge-exchange reaction at 175 MeV/u was developed as a novel probe for studying the isovector spin giant monopole resonance (IVSMR), whose properties are important for better understanding the bulk properties of nuclei and asymmetric nuclear matter. This probe, now available through the production of ^{12}N as a secondary rare-isotope beam, is exothermic, is strongly absorbed at the surface of the target nucleus, and provides selectivity for spin-transfer excitations. All three properties enhance the excitation of the IVSMR compared to other, primarily light-ion, probes, which have been used to study the IVSMR thus far. The ^{90}Zr(^{12}N,^{12}C) reaction was measured and the excitation energy spectra up to about 70 MeV for both the spin-transfer and non-spin-transfer channels were deduced separately by tagging the decay by γ emission from the ^{12}C ejectile. Besides the well-known Gamow-Teller and isobaric analog transitions, a clear signature of the IVSMR was identified. By comparing with the results from light-ion reactions on the same target nucleus and theoretical predictions, the suitability of this new probe for studying the IVSMR was confirmed.

Spectroscopy of Pionic Atoms in

We observed the atomic 1s and 2p states of π^{-} bound to ^{121}Sn nuclei as distinct peak structures in the missing mass spectra of the ^{122}Sn(d,^{3}He) nuclear reaction. A very intense deuteron beam and a spectrometer with a large angular acceptance let us achieve a potential of discovery, which includes the capability of determining the angle-dependent cross sections with high statistics. The 2p state in a Sn nucleus was observed for the first time. The binding energies and widths of the pionic states are determined and found to be consistent with previous experimental results of other Sn isotopes. The spectrum is measured at finite reaction angles for the first time. The formation cross sections at the reaction angles between 0° and 2° are determined. The observed reaction-angle dependence of each state is reproduced by theoretical calculations. However, the quantitative comparison with our high-precision data reveals a significant discrepancy between the measured and calculated formation cross sections of the pionic 1s state.

First Observation of

The most neutron-rich boron isotopes ^{20}B and ^{21}B have been observed for the first time following proton removal from ^{22}N and ^{22}C at energies around 230 MeV/nucleon. Both nuclei were found to exist as resonances which were detected through their decay into ^{19}B and one or two neutrons. Two-proton removal from ^{22}N populated a prominent resonancelike structure in ^{20}B at around 2.5 MeV above the one-neutron decay threshold, which is interpreted as arising from the closely spaced 1^{-},2^{-} ground-state doublet predicted by the shell model. In the case of proton removal from ^{22}C, the ^{19}B plus one- and two-neutron channels were consistent with the population of a resonance in ^{21}B 2.47±0.19 MeV above the two-neutron decay threshold, which is found to exhibit direct two-neutron decay. The ground-state mass excesses determined for ^{20,21}B are found to be in agreement with mass surface extrapolations derived within the latest atomic-mass evaluations.

Discovery of

In this Letter, the observation of two previously unknown isotopes is presented for the first time: ^{72}Rb with 14 observed events and ^{77}Zr with one observed event. From the nonobservation of the less proton-rich nucleus ^{73}Rb, we derive an upper limit for the ground-state half-life of 81 ns, consistent with the previous upper limit of 30 ns. For ^{72}Rb, we have measured a half-life of 103(22) ns. This observation of a relatively long-lived odd-odd nucleus, ^{72}Rb, with a less exotic odd-even neighbor, ^{73}Rb, being unbound shows the diffuseness of the proton drip line and the possibility of sandbanks to exist beyond it. The ^{72}Rb half-life is consistent with a 5^{+}→5/2^{-} proton decay with an energy of 800-900 keV, in agreement with the atomic mass evaluation proton-separation energy as well as results from the finite-range droplet model and shell model calculations using the GXPF1A interaction. However, we cannot explicitly exclude the possibility of a proton transition between 9^{+}(^{72}Rb)→9/2^{+}(^{71}Kr) isomeric states with a broken mirror symmetry. These results imply that ^{72}Kr is a strong waiting point in x-ray burst rp-process scenarios.