An experiment conducted at the RIKEN Nishina Center RI Beam Factory, using a 238U beam with an energy of 345 MeV/nucleon, led to the discovery of seven new neutron-rich isotopes in the rare-earth region: 152Cs, 155Ba, 158La, 159Ce, 160Ce, 173Gd, and 175Tb. These isotopes were produced through in-flight fission of a 238U beam and were subsequently separated and identified in-flight using the BigRIPS fragment separator. For the identification, the mass-to-charge ratio and atomic number of the isotopes were determined by measuring the time-of-flight, magnetic rigidity, and energy loss. The observations of all seven new isotopes were statistically confirmed based on a significance test with pvalues. The observed counts for the new isotopes were consistent with a systematic decrease in production cross sections with increasing mass number, supporting their discovery.

According to quantum chromodynamics, vacuum is not an empty space, because it is filled with quark–antiquark pairs. The pair has the same quantum numbers as the vacuum and forms a condensate because the strong interaction of the quantum chromodynamics is too strong to leave the vacuum empty. This quark–antiquark condensation, the chiral condensate, breaks the chiral symmetry of the vacuum. The expectation value of the chiral condensate is an order parameter of the chiral symmetry, which is expected to decrease at high temperatures or high matter densities where the chiral symmetry is partially restored. Head-on collisions of nuclei at ultra-relativistic energies have explored the high-temperature regime, but experiments at high densities are rare. Here we measure the spectrum of pionic 121Sn atoms and study the interaction between the pion and the nucleus. We find that the expectation value of the chiral condensate is reduced at finite density compared to the value in vacuum. The reduction is linearly extrapolated to the nuclear saturation density and indicates that the chiral symmetry is partially restored due to the extremely high density of the nucleus. In quantum chromodynamics, the condensation of quark–antiquark pairs breaks the chiral symmetry of vacuum. Experiments with pionic tin atoms demonstrate that the symmetry is partially restored at high densities.

Spectroscopy of an unbound nucleus $$^{17}$$ C was performed using the SAMURAI spectrometer at RIBF of RIKEN. Six resonances were observed for the $$^{16}$$ C+n system with relative energies of 0.52, 0.77, 1.36, 1.91, 2.22 and 3.20 MeV. The excitation energies ( $$E_x$$ ) of the observed resonances were deduced, by taking into account the states of the $$^{16}$$ C fragments identified by coincident $$\gamma $$ rays, as $$E_x$$ =(3.02), 1.51, (3.86), 2.65, (4.72) and 3.94 MeV. The orbital angular momenta of the two observed states in $$^{17}$$ C at $$E_x$$ =2.65 and 3.94 MeV were determined as 1 by comparing parallel momentum distributions with theoretical predictions.

Modern theories of physics tell that the vacuum is not an empty space [1, 2]. Hidden in the vacuum is a structure of anti-quarks ¯ q and quarks q . The ¯ q and q pair has the same quantum number as the vacuum and condensates in it since the strong interaction of the quantum chromo-dynamics (QCD) is too strong to leave it empty. The ¯ qq condensation breaks the chiral symmetry of the vacuum. The expectation value (cid:104) ¯ qq (cid:105) is an order parameter [3]. For higher temperature or higher matter-density, | (cid:104) ¯ qq (cid:105) | decreases reflecting the restoration of the symmetry [4]. In contrast to these clear-cut arguments, experimental evidence is so far limited. First of all, the ¯ qq is noth-ing but the vacuum itself. It is neither visible nor perceptible. In this article, we unravel this invis-ible existence by high precision measurement of pionic atoms, π − -meson–nucleus bound systems. Using the π − as a probe, we demonstrate that | (cid:104) ¯ qq (cid:105) | is reduced in the nucleus by a factor of 58 ± 4% compared with that in the vacuum. This reduction indicates that the chiral symmetry is partially restored due to the extremely high density of the nucleus. The present experimental result clearly exhibits the existence of the hidden structure, the chiral condensate, in the vacuum.

The search for new isotopes using the in-flight fission of a U-238 beam has been conducted concurrently with decay measurements, during the so-called EURICA campaigns, at the RIKEN Nishina Center RI Beam Factory. Fission fragments were analyzed and identified in flight using the BigRIPS separator. We have identified the following 36 new neutron-rich isotopes: Rb-104, Zr-113, Nb-116, Mo-118,Mo-119, Tc-121,Tc-122, Ru-125, Rh-127,Rh-128, Pd-129,Pd-130,Pd-131, Ag-132, Cd-134, In-136,In-137, Sn-139,Sn-140, Sb-141,Sb-142, Te-144,Te-145, I-146,I-147, Xe-149,Xe-150, Cs-149,Cs-150,Cs-151, Ba-153,Ba-154, and La-154,La-155,La-156,La-157.

A search for new isotopes in the neutron-rich rare-earth region has been carried out using a 345 MeV/nucleon 238U beam at the RIKEN Nishina Center RI Beam Factory. Fragments produced were analyzed and identified using the BigRIPS in-flight separator. We observed a total of 29 new neutron-rich isotopes: 153Ba, 154,155,156La, 156,157,158Ce, 156,157,158,159,160,161Pr, 162,163Nd, 164,165Pm, 166,167Sm, 169Eu, 171Gd, 173,174Tb, 175,176Dy, 177,178Ho, and 179,180Er.

A new isomer with ms half-life was observed in 160Nd at RIBF, RIKEN Nishina Center by using in-flight fission of 238U beam and a cluster-type Ge detector array, EURICA. The experimental results and a PSM calculation indicate that the isomer in 160Nd is a 2 quasi-particle excitation of neutrons with a configuration of n1/2[521]x7/2[633] as the case in other N = 100 isotones. The E(4+)/E(2+) ratio of the ground-state band, 3.29, shows the 160Nd is well deformed and the ground band has a rotational nature. The 4- excitation of 160Nd showed an increase in energy by ~100 keV compared to that of 162Sm as predicted by the PSM calculation. A PSM calculation was performed by changing input beta_4 value and confirmed that the large hexadecupole deformation in Nd was responsible for the increase of the isomer energy.

The spectroscopic structure of 19C, a prominent one-neutron halo nucleus, has been studied with a 20C secondary beam at 290 MeV/nucleon and a carbon target. Neutron-unbound states populated by the one-neutron knockout reaction were investigated by means of the invariant mass method. The preliminary relative energy spectrum and parallel momentum distribution of the knockout residue, 19C*, were reconstructed from the measured four momenta of the 18C fragment, neutron, and beam. Three resonances were observed in the spectrum, which correspond to the states at Ex = 0.62(9), 1.42(10), and 2.89(10) MeV. The parallel momentum distributions for the 0.62-MeV and 2.89-MeV states suggest spin-parity assignments of 5/2+ and 1/2−, respectively. The 1.42-MeV state is in line with the reported 5/22+ state.

Interaction cross sections (sigma(I)) and reaction cross sections(sigma(R)) are physical quantities which are strongly related to the nuclear size. In our previous study of sigma(I) for Ne isotopes, the deformation features of neutron-rich Ne isotopes in the so-called "island of inversion" region have been successfully observed, and also the formation of the deformed halo structure in Ne-31 has been indicated. In this study, sigma(I) for F19-27 up to the vicinity of the island of inversion have been measured at around 240A MeV using BigRIPS at RIBF, RIKEN. Our preliminary results are slightly larger than A(1/3) systematics and some of the data could be explained by nuclear deformation.

A spectroscopic study of 17C was performed via the one-neutron knockout reaction of 18C on a carbon target at RIKEN-RIBF. Three unbound states at excitation energies of 2.66(2), 3.16(5), and 3.97(3) MeV (preliminary) were observed. The energies are compared with shell-model calculations and existing measurements to deduce their spin-parities. From the comparison, the states at 2.66(2) and 3.97(3) MeV are suggested to be 1/2− and 3/2−, respectively. From its decay property, the state at 3.16(5) MeV is indicated to be 9/2+.

We conduct an experimental project to make spectroscopy of deeply bound pionic atoms systematically over wide range of nuclei. We aim at studying the strong interaction in the low energy region, which has close connection to spontaneous chiral symmetry breaking and its partial restoration in nuclear matter. First experimental results show improved spectral resolution and much better statistical sensitivity than previous experiments. Present status of the experiment is reported.

The spectroscopic structure of 19C, a prominent one-neutron halo nucleus, has been studied with a 20C secondary beam at 290 MeV/nucleon and a carbon target. Neutron-unbound states populated by the one-neutron knockout reaction were investigated by means of the invariant mass method. The preliminary relative energy spectrum and parallel momentum distribution of the knockout residue, 19C*, were reconstructed from the measured four momenta of the 18C fragment, neutron, and beam. Three resonances were observed in the spectrum, which correspond to the states at Ex = 0.62(9), 1.42(10), and 2.89(10) MeV. The parallel momentum distributions for the 0.62-MeV and 2.89-MeV states suggest spin-parity assignments of 5/2+ and 1/2−, respectively. The 1.42-MeV state is in line with the reported 5/22+ state.

R. Lozeva,1,* A. Odahara,2 C.-B. Moon,3 S. Nishimura,4 P. Doornenbal,4 H. Naı̈dja,1,5,6 F. Nowacki,1 P.-A. Söderström,4 T. Sumikama,7 G. Lorusso,4 J. Wu,8,4 Z. Y. Xu,9 H. Baba,4 F. Browne,10,4 R. Daido,2,4 J.-M. Daugas,11 F. Didierjean,1 Y. Fang,2,4 T. Isobe,4 I. Kojouharov,5 N. Kurz,5 Z. Patel,12,4 S. Rice,12,4 H. Sakurai,4,9 H. Schaffner,5 L. Sinclair,13,4 H. Watanabe,14 A. Yagi,2,4 R. Yokoyama,15 T. Kubo,4 N. Inabe,4 H. Suzuki,4 N. Fukuda,4 D. Kameda,4 H. Takeda,4 D. S. Ahn,4 D. Murai,16 F. L. Bello Garrote,17 E. Ideguchi,18 T. Ishigaki,2,4 H. S. Jung,19 T. Komatsubara,20 Y. K. Kwon,20 S. Morimoto,2,4 M. Niikura,9 H. Nishibata,2,4 I. Nishizuka,7,4 T. Shimoda,2 and K. Tshoo20 1IPHC, CNRS, IN2P3 and University of Strasbourg, F-67037 Strasbourg Cedex 2, France 2Department of Physics, Osaka University, Osaka 560-0043 Toyonaka, Japan 3Department of Display Engineering, Hoseo University, Chung-Nam 336-795, Republic of Korea 4RIKEN Nishina Center, Wako-shi, Saitama 351-0198, Japan 5GSI Helmholtzzentrum für Schwerionenforschung GmbH, D-64291 Darmstadt, Germany 6University of Constantine, 25000 Constantine, Algeria 7Department of Physics, Tohoku University, Miyagi 980-8578, Japan 8School of Physics and State key Laboratory of Nuclear Physics and Technology, Peking University, Beijing 100871, China 9Department of Physics, University of Tokyo, Tokyo 113-0033, Japan 10School of Computing, Engineering and Mathematics, University of Brighton, Brighton BN2 4GJ, United Kingdom 11CEA, DAM, DIF, F-91297 Arpajon Cedex, France 12Department of Physics, University of Surrey, Guildford GU2 7XH, United Kingdom 13Department of Physics, University of York, Heslington, York YO10 5DD, United Kingdom 14School of Physics and Nuclear Energy Engineering, Beihang University, Beijing 100191, China 15CNS, University of Tokyo, Wako, Saitama 351-0198, Japan 16Department of Physics, Rikkyo University, Toshima, Tokyo 171-8501, Japan 17Department of Physics, University of Oslo, N-0316, Oslo, Norway 18RCNP, Osaka University, Ibaraki, Osaka 567-0047, Japan 19Department of Physics, University of Notre Dame, Notre Dame, Indiana 46556, USA 20Rare Isotope Science Project, Institute for Basic Science, Daejeon 305-811, Republic of Korea (Received 1 May 2015; revised manuscript received 3 July 2015; published 3 August 2015; corrected 5 August 2015)

Unbound states in 17C were investigated via one-neutron knockout of 18C. The experiment was performed using SAMURAI spectrometer in RIBF at RIKEN. By invariant mass spectroscopy, three resonances were measured at excitation energies of 3.03(12), 2.74(3), and 4.03(6) MeV as preliminary results. For the excited state at 2.74(3) MeV, the parallel momentum distribution was satisfactorily described by the distribution calculated for p-wave knockout from 18C.

The present work aims at exploring neutron-unbound states of 19C via the one-neutron knockout reaction. The invariant mass measurement in inverse kinematics was carried out with a carbon target and a 20C secondary beam at 290 MeV/nucleon. The preliminary relative energy spectrum and the parallel momentum distribution of the system of 18C + n were reconstructed from the measured momenta of the 18C fragment and decayed neutron. A new resonance was observed at Erel = 2.3 MeV, which corresponds to the unbound state of 19C at Ex = 2.9 MeV. The parallel momentum distribution for this resonance suggests a spin-parity assignment of 1/2−. Additionally, two known states were seen at Ex = 0.6 and 1.4 MeV, consistent with the 5/2+1 and 5/2+2 states, respectively.