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.

Precise Spectroscopy of the 3n and 3p Systems via the

To search for low-energy resonant structures in isospin T=3/2 three-body systems, we have performed the experiments ^{3}H(t,^{3}He)3n and ^{3}He(^{3}He,t)3p at intermediate energies. For the 3n experiment, we have newly developed a thick Ti-^{3}H target that has the largest tritium thickness among targets of this type ever made. The 3n experiment for the first time covered the momentum-transfer region as low as 15 MeV/c, which provides ideal conditions for producing fragile systems. However, in the excitation-energy spectra we obtained, we did not observe any distinct peak structures. This is in sharp contrast to tetraneutron spectra. The distributions of the 3n and 3p spectra are found to be similar, except for the displacement in energy due to Coulomb repulsion. Comparisons with theoretical calculations suggest that three-body correlations exist in the 3n and 3p systems, although not enough to produce a resonant peak.

Excited-State Half-Lives in

The known I^{π}=8_{1}^{+}, E_{x}=2129-keV isomer in the semimagic nucleus ^{130}Cd_{82} was populated in the projectile fission of a ^{238}U beam at the Radioactive Isotope Beam Factory at RIKEN. The high counting statistics of the accumulated data allowed us to determine the excitation energy, E_{x}=2001.2(7) keV, and half-life, T_{1/2}=57(3) ns, of the I^{π}=6_{1}^{+} state based on γγ coincidence information. Furthermore, the half-life of the 8_{1}^{+} state, T_{1/2}=224(4) ns, was remeasured with high precision. The new experimental information, combined with available data for ^{134}Sn and large-scale shell model calculations, allowed us to extract proton and neutron effective charges for ^{132}Sn, a doubly magic nucleus far-off stability. A comparison to analogous information for ^{100}Sn provides first reliable information regarding the isospin dependence of the isoscalar and isovector effective charges in heavy nuclei.

First Exploration of Monopole-Driven Shell Evolution above the N=126 Shell Closure: New Millisecond Isomers in

Isomer spectroscopy of heavy neutron-rich nuclei beyond the N=126 closed shell has been performed for the first time at the Radioactive Isotope Beam Factory of the RIKEN Nishina Center. New millisecond isomers have been identified at low excitation energies, 985.3(19) keV in ^{213}Tl and 874(5) keV in ^{215}Tl. The measured half-lives of 1.34(5) ms in ^{213}Tl and 3.0(3) ms in ^{215}Tl suggest spins and parities 11/2^{-} with the single proton-hole configuration πh_{11/2} as leading component. They are populated via E1 transitions by the decay of higher-lying isomeric states with proposed spin and parity 17/2^{+}, interpreted as arising from a single πs_{1/2} proton hole coupled to the 8^{+} seniority isomer in the ^{A+1}Pb cores. The lowering of the 11/2^{-} states is ascribed to an increase of the πh_{11/2} proton effective single-particle energy as the second νg_{9/2} orbital is filled by neutrons, owing to a significant reduction of the proton-neutron monopole interaction between the πh_{11/2} and νg_{9/2} orbitals. The new ms isomers provide the first experimental observation of shell evolution in the almost unexplored N>126 nuclear region below doubly magic ^{208}Pb.

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.

First Application of Mass Measurements with the Rare-RI Ring Reveals the Solar r-Process Abundance Trend at A=122 and A=123.

The Rare-RI Ring (R3) is a recently commissioned cyclotronlike storage ring mass spectrometer dedicated to mass measurements of exotic nuclei far from stability at Radioactive Isotope Beam Factory (RIBF) in RIKEN. The first application of mass measurement using the R3 mass spectrometer at RIBF is reported. Rare isotopes produced at RIBF-^{127}Sn, ^{126}In, ^{125}Cd, ^{124}Ag, ^{123}Pd-were injected in R3. Masses of ^{126}In, ^{125}Cd, and ^{123}Pd were measured whereby the mass uncertainty of ^{123}Pd was improved. This is the first reported measurement with a new storage ring mass spectrometry technique realized at a heavy-ion cyclotron and employing individual injection of the preidentified rare nuclei. The latter is essential for the future mass measurements of the rarest isotopes produced at RIBF. The impact of the new ^{123}Pd result on the solar r-process abundances in a neutron star merger event is investigated by performing reaction network calculations of 20 trajectories with varying electron fraction Y_{e}. It is found that the neutron capture cross section on ^{123}Pd increases by a factor of 2.2 and ß-delayed neutron emission probability, P_{1 n}, of ^{123}Rh increases by 14%. The neutron capture cross section on ^{122}Pd decreases by a factor of 2.6 leading to pileup of material at A=122, thus reproducing the trend of the solar r-process abundances. The trend of the two-neutron separation energies (S_{2n}) was investigated for the Pd isotopic chain. The new mass measurement with improved uncertainty excludes large changes of the S_{2n} value at N=77. Such large increase of the S_{2n} values before N=82 was proposed as an alternative to the quenching of the N=82 shell gap to reproduce r-process abundances in the mass region of A=112-124.

ß-Delayed One and Two Neutron Emission Probabilities Southeast of

The ß-delayed one- and two-neutron emission probabilities (P_{1n} and P_{2n}) of 20 neutron-rich nuclei with N≥82 have been measured at the RIBF facility of the RIKEN Nishina Center. P_{1n} of ^{130,131}Ag, ^{133,134}Cd, ^{135,136}In, and ^{138,139}Sn were determined for the first time, and stringent upper limits were placed on P_{2n} for nearly all cases. ß-delayed two-neutron emission (ß2n) was unambiguously identified in ^{133}Cd and ^{135,136}In, and their P_{2n} were measured. Weak ß2n was also detected from ^{137,138}Sn. Our results highlight the effect of the N=82 and Z=50 shell closures on ß-delayed neutron emission probability and provide stringent benchmarks for newly developed macroscopic-microscopic and self-consistent global models with the inclusion of a statistical treatment of neutron and γ emission. The impact of our measurements on r-process nucleosynthesis was studied in a neutron star merger scenario. Our P_{1n} and P_{2n} have a direct impact on the odd-even staggering of the final abundance, improving the agreement between calculated and observed Solar System abundances. The odd isotope fraction of Ba in r-process-enhanced (r-II) stars is also better reproduced using our new data.

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.

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.