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.

Interaction cross sections σ I for 58–77 Ni on a carbon target at 280 MeV / nucleon were measured at the RIKEN Radioactive Isotope Beam Factory (RIBF) using the BigRIPS fragment separator. The aim of this work is to deduce matter radii from the measured values of σ I through Glauber-model analysis and, by combining them with the charge radii that have been precisely determined by the laser spectroscopy, to determine the neutron-skin thickness r np of Ni isotopes over a wide range of neutron excess δ = ( N Z )/ A from 0.03 to 0.27. The present data of σ I constitute the first systematic set along the isotopic chain in the Ni mass region, providing a consistent basis for investigating the evolution of nuclear size and neutron-skin with increasing neutron excess. The slope of the neutron-skin thickness r np as a function of the neutron excess δ is expected to provide an important constraint on the L parameter of the nuclear matter equation of state (EOS), which represents the first-order term in the density dependence of the symmetry energy in nuclear matter. The EOS is essential for understanding not only the structure of atomic nuclei but also the mechanisms of supernova explosions and the properties of neutron stars. The results of this study are therefore expected to provide valuable insights into the elucidation of the EOS.

The charge-changing cross sections σ_CC have emerged as a promising observable for deducing nuclear charge radii of unstable nuclei. In this study, we measured σ_CC for neutron-rich nickel isotopes ^58–77 Ni at 250 MeV/u. The experimental results were analyzed by using a Glauber model including charged-particle evaporation effects, and the consistency between the measured σ _CC and existing charge radii was investigated. The results demonstrate that the present approach provides a reliable description of σ _CC systematics in the Ni isotopic chain. These findings support the applicability of σ _CC measurements as an alternative probe of charge radii, particularly for nuclei where direct measurements are not available.

We have developed a new experimental technique to determine the gamma emission probability from unbound states populated by the (d, p) reaction at 20 MeV/nucleon under the inverse kinematics. This was achieved using the decelerating and focusing device OEDO at RIBF. Due to the inverse kinematics, the outgoing reaction residues can be identified by a magnetic spectrometer. By combining the missing spectroscopy with the identification of the residual ions, the γ emission channel was determined event-by-event as a function of the excitation energy, without detecting γ ray from the unbound states. As the first application, the method was used to study the neutron capture reaction on 79 Se via the (d, p) reaction as a surrogate reaction for the neutron energies up to 5 MeV. Additionally, the technique was employed to investigate the compound reaction channel of (n, γ) cross sections on 130 Sn.

An ionization chamber (IC) is employed in nuclear experiments to measure energy deposits and provide crucial information for identifying nuclei in flight after undergoing nuclear reactions or decay. In ICs using P-10 (Ar+CH4) gas, the atomic number (Z) identification capability deteriorates significantly when Z>70 at an energy range of 150-300 MeV/u. This degradation is caused by the substantial energy straggling caused by charge-state fluctuation when the beams pass through a gaseous medium. This study used a xenon-based gas (Xe+CH4), with a large charge-state changing cross section, to improve the Z identification. The responses of ICs with P-10 and xenon-based gases were examined using( 92)(238)U beams and cocktail radioactive isotope (RI) beams with Z=40-90, which were produced in the BigRIPS separator at the RI Beam Factory. For U-238 beams at 344 MeV/u, incident-charge dependence was observed in the energy deposits in the P-10 gas IC, whereas it was not observed in the xenon-based gas IC. When using cocktail RI beams, the P-10 gas IC failed to separate ions with the same mass-to-charge ratio but differing by one in Z within Z=70-84 at 210-220 MeV/u. However, the xenon-based gas IC successfully separated these ions. The xenon-based gas IC demonstrated Z identification with separations better than 3 sigma across a wide Z range (Z=40-90) and emerged as a practical solution for the Z identification of heavy-ion beams.

A search for new neutron-rich isotopes near the $N=60$ isotones $^{92}\mathrm{Ge}$ and $^{93}\mathrm{As}$ has been performed using a 345 MeV/nucleon $^{238}\mathrm{U}$ beam at the RIKEN Nishina Center RI Beam Factory. Fission fragments produced were analyzed and identified in flight using the large-acceptance two-stage separator BigRIPS. We have observed a total of 15 new neutron-rich isotopes: $^{84}\mathrm{Cu}, ^{86,87}\mathrm{Zn}, ^{88,89}\mathrm{Ga}, ^{91,92}\mathrm{Ge}, ^{93,94,95}\mathrm{As}, ^{96,97}\mathrm{Se}, ^{99,100}\mathrm{Br}$, and $^{103}\mathrm{Kr}$.

In conventional ionization chambers (ICs) using P-10 (Ar+CH4) gas, as the atomic number (Z) of the ion beams increases in the energy region of 200-300 MeV/u, the Z resolution deteriorates rapidly when Z>70. This degradation is attributed to substantial energy loss straggling caused by charge state fluctuation when the beams traverse a gas medium. The energy loss straggling intensifies when the beams cannot attain charge state equilibrium in the IC gas. In this study, a xenon-based gas (Xe+CH4), presenting a larger charge state changing cross section, was employed in the IC to reach charge state equilibrium. The responses of ICs with P-10 and the xenon-based gases were examined using 238U beams and cocktail radioactive isotope (RI) beams with Z=40-90 at the RI Beam Factory (RIBF). For 238U beams at 165-344 MeV/u, the P-10 gas IC yielded an energy resolution of 1.9-3.0 maximum (FWHM), which proved inadequate for Z identification in the uranium region. In contrast, the xenon-based gas IC demonstrated a satisfactory energy resolution of 1.4-1.6 and 0.74 was achieved by the P-10 and the xenon-based gas ICs, respectively, for beams with Z=84-88 at 200 MeV/u. The contrast in Z resolutions between the P-10 and the xenon-based gas ICs was effectively elucidated by the energy loss straggling model, incorporating collisional straggling and straggling due to charge state changes in the IC gases. The xenon-based gas IC, with more than 3sigma Z separation across a broad Z range (Z=40-90), emerged as a practical solution for Z identification of heavy ion beams.

New isotopes Si-45,Si-46, which extend beyond the most neutron-rich Si isotopes confirmed thus far, have been discovered at the RI Beam Factory at the RIKEN Nishina Center. The isotopes were produced by the in-flight fragmentation reactions of Zn-70 at 345 MeV/nucleon on a Be target. All the projectile fragments were analyzed and identified using the large-acceptance two-stage separator BigRIPS. Consequently, six Si-45 events and one Si-46 event were confirmed. The neutron-bound nature of odd-N Si-45 may have a significant impact on the nuclear stability of neutron-rich proton sd-shell nuclei at approximately N=40.

Nuclear transmutation is emerging as a promising approach for reprocessing high-level waste, specifically treating long-lived nuclides like Zr-93 from spent fuel. It is essential to accumulate reaction data for these nuclei to advance this prominent treatment and to build a comprehensive understanding of reaction mechanisms. In this study, the residual production cross-sections resulting from proton-induced reactions on Zr-93 were measured at 27 MeV/nucleon in inverse kinematics. At the RI Beam Factory (RIBF), the Optimized Energy Degrading Optics beamline was used to deduce production cross-sections for isotopes Nb91-93, Zr-91,Zr-92, and Y-88,Y-89. Comparing the results from this study and prior research with calculated excitation functions, a moderate agreement is found with theoretical predictions derived from TALYS and CCONE. The measured cross-sections offer valuable insights for future considerations in nuclear-waste treatment facilities. This is particularly relevant for facilities exploring innovative methods, such as accelerator-driven systems.

The deuteron is a loosely bound system that can easily break up into its constituent proton and neutron whilst in the presence of Coulomb and nuclear fields. Previous experimental studies have shown that this breakup process has a significant impact on residual-nucleus production from deuteron bombardment in the high-energy range of 50-210 MeV/nucleon. However, there remains a lack of cross-section data at energies below 50 MeV/nucleon. The current study determined Zr-93 + d reaction cross sections under inverse kinematics at approximately 28 MeV/nucleon using the BigRIPS separator, OEDO beamline, and SHARAQ spectrometer. Cross sections from this research were compared with previous measurements and theoretical calculations. The experimental results show a large enhancement of the production cross sections of residual nuclei, especially those produced from a small number of particle emissions, compared to the proton-induced reaction data at similar bombarding energy. The DEURACS calculation, which quantitatively takes deuteron-breakup effects into account, reproduces the data well. As a long-lived fission product, Zr-93 remains a challenge for nuclear-waste disposal and treatment. This study's low-energy data may assist future consideration of nuclear-waste treatment facilities, where Zr-93 + d may feasibly transmute the waste into short-lived/stable nuclei.

We have measured the differential cross section and analyzing power for (p, 2p) reactions that lead to discrete states of residual nuclei at an incident energy of 197 MeV for five kinds of target nuclei, C-12, O-16, Ca-40,Ca-48, and Zr-90. The data are compared with distorted-wave impulse approximation calculations using two kinds of global optical potentials. The spectroscopic factors obtained from these calculations agree with those determined in (e, e ' p) studies within 20%, except for one orbital of the Zr-90 nucleus where the distortion effect is much greater than that for the other orbitals of Zr-90 and for other target nuclei. We also observed a clear and visibly distinct j-dependence in the analyzing power.

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.

The very proton-rich 71 Kr isotope was produced through the in-flight fragmentation of 78 Kr on a beryllium target at RIKEN – Nishina Center in order to study its β-decay properties. A stack of double-sided silicon strip detectors, called WAS3ABi, was used as the decay station, where the detection of ion implants, β-decays and β-delayed protons took place. Beta-delayed γ-rays were measured using a system of 84 HPGe detectors, called EURICA, surrounding the decay station. The main goal of the present study was the precise measurement of the half-life of 71 Kr, as in the literature there is an almost 10 σ difference between the most precise independent results. Implant–β time correlations, implant–proton time correlations and implant–β–γ time correlations were all used to derive the half-life value, followed by a thorough investigation of systematic uncertainties for each method. As these values were found to be consistent, the weighted average t 1 / 2 = 94.40 +19 ms is reported as a new half-life value in this work. Furthermore a total of 26 previously unreported γ following the β-decay of 71 Kr were also identified in the analysis.

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 $\ensuremath{\beta}$-decay scheme of $^{138}\mathrm{Te}$ and the level structure of $^{138}\mathrm{I}$ is reported for the first time. The experiment was performed at the Radioactive Isotope Beam Factory of RIKEN, as one of the EUROBALL-RIKEN Cluster Array campaigns. Secondary radioactive ions, including $^{138}\mathrm{Te}$ and $^{138}\mathrm{Sb}$, were produced by the in-flight fission of a $^{238}\mathrm{U}$ beam with the energy of 345 MeV per nucleon. From the $\ensuremath{\beta}$ decay of $^{138}\mathrm{Te}$, the level scheme of $^{138}\mathrm{I}$ was supplemented with new spin and parity assignments, such as the low-lying negative-parity states and a positive-parity ${1}^{+}$ state. This ${1}^{+}$ state can be interpreted as being associated with the $\ensuremath{\pi}0{h}_{11/2}\ensuremath{\bigotimes}\ensuremath{\nu}0{h}_{9/2}$ partner orbital configuration populated by the Gamow-Teller transition between a neutron in the $0{h}_{9/2}$ orbital and a proton in the $0{h}_{11/2}$ orbital. Details of the structure of $^{138}\mathrm{I}$ are discussed in terms of the proton-neutron interactions and Gamow-Teller transition strength within the theoretical context of shell-model calculations.

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The magnetic moment of the isomeric state of the neutron-rich ^75Cu nucleus was measured using a highly spin-aligned beam produced via a two-step reaction scheme. In the experiment carried out at the BigRIPS at RIBF, we achieved to produce spin alignment reaching 30% by employing the one-proton removal from ^76Zn to produce ^75Cu. In the magnetic moment measurement, a method of time-differential perturbed angular distribution (TDPAD) was employed. Precession of the isomeric state with spin parity of 3/2^− was clearly observed with significance larger than 5σ in the TDPAD spectrum. The magnetic moment of the isomeric state of ^75Cu was determined to be μ = 1.40(6)μ_N.

We have measured the differential cross-sections and analyzing powers for (p, 2p) reactions at an incident energy of 392 MeV on C-12, O-16, Ca-40, and Pb-208 nuclei, leading to discrete states of the residual nuclei. The data are compared with two kinds of distorted-wave impulse approximation (DWIA) calculations, a standard calculation using a global optical potential and a calculation using wave functions generated in a relativistic Hartree model. The spectroscopic factors deduced from these two calculations agree with those determined in (e, e' p) studies mostly within 15% in the case of the lighter three target nuclei. However, those for the Pb-208 target are overestimated compared with the (e, e' p) results. In the heavy target case, the DWIA results are very sensitive to the radius parameter of the bound-state potential and thus a careful treatment is required. Regarding the analyzing powers of the present measurement, we confirmed that the j-dependence is sufficient for practical spectroscopic use.

Spallation reaction for the long-lived fission product 107 Pd has been studied for the purpose of nuclear waste transmutation. The isotopic-distribution cross sections on both proton and deuteron were obtained at 118 MeV/nucleon in inverse kinematics at the RIKEN Radioactive Isotope Beam Factory. A large cross-section difference was found between the proton and deuteron results for the light-mass products. The data were compared with the SPACS semi-empirical parameterization and the PHITS calculations including both the intranuclear cascade and evaporation processes. In addition, the potential of spallation reaction for transmutation of 107 Pd is discussed.