The KEK Isotope Separation System (KISS) at RIKEN has been successfully implemented to investigate nuclear structure of the nuclei in the vicinity of neutron magic number N= 126 and ^238 U from the astrophysical perspective. The KISS facility provides a low-energy (20 keV) ion beam for nuclear spectroscopy utilizing multinucleon transfer (MNT) reactions to produce neutron-rich nuclei in combination with an argon gas-cell system. These nuclei are re-ionized by applying in-gas-cell laser ionization technique, enabling laser ionization spectroscopy even for refractory elements with the atomic number Z= 70–78, which are inaccessible at other facilities. We have successfully reported the experimental results using in-gas-cell laser ionization with a limited laser resolution of about 10 GHz. To achieve significantly higher precision in laser spectroscopy, with a resolution of a few hundred MHz at KISS, we plan to install a CLARIS (Collinear LAser Resonance Ionization Spectroscopy) beamline.

Precise mass measurements of refractory nuclei produced by multinucleon transfer reactions of a 136Xe beam and natIr target were conducted at the KISS (KEK Isotope Separation System) facility, using a multireflection time-of-flight mass spectrograph. The study encompasses ions with mass numbers A = 188, 189, 190, 192, and atomic numbers Z = 74-79. Our findings show good agreement between the evaluated masses, excluding 192Re, and known values, within the specified error margins. The assessment includes the reevaluation of the rejected mass value for 189W in the AME2020 atomic mass evaluation. The determined atomic mass of 189W aligns closely with previously reported values obtained through direct mass measurements using the GSI storage ring ESR and beta-decay Q-value measurements. The flat S2n pattern between 189W and 190W was interpreted as a result of a shape transition at N = 116 according to Hartree-Fock-Bogoliubov calculations with the SV-min parametrization. Additionally, the updated mass value for 192Re also suggests a shape transition at N = 117, consistent with previous studies.

We investigated the energy dependence of the number of triggered pixels, or cluster size, when charged particles are detected using the TimePix3 detector with a silicon sensor. We measured protons in the range of 1.5~3.3 MeV from a Pelletron accelerator at RIKEN using a TimePix3 detector with a 500 um-thick silicon sensor. We determined from the experimental results a cluster size comprised between 30 and 80 pixels. To understand the physical process that produces large cluster images and its energy dependence, we simulated the charge carrier drifts in the sensor, assuming the incidence of a proton in the detector. The cluster sizes estimated in the simulation were smaller than those observed in the experiment, and remained constant across the entire energy range, when thermal diffusion and charge carriers self-repulsion were considered as the factors of the cluster image formation. In addition, we discovered that the size of the cluster image and its energy dependence observed in the experiment could be well explained when considering that the TimePix3 detector is sensitive to the transient induced charges, allowing even pixels that do not collect the charge carriers to trigger. We conclude that the cluster size measurement is a promising method for evaluating the energy deposited by a charged particle in the TimePix3 detector.

We are developing a laser spectroscopic method to study the nuclear structure of radioactive isotopes utilizing superfluid helium (He II) as an efficient stopper for highly energetic ion beams and as an in-situ laser spectroscopic environment. Recently, we conducted an ion stopping experiment for the ^84 Rb ^37+ beams with the energy of approximately 350 A MeV at QST-HIMAC with a cryostat system used for 66 A MeV at RIKEN-RIPS experiment (Appl. Phys. Express 12, 016502 (2019)). The radioactive ^84 Rb ions were produced via a projectile fragmentation reaction using accelerated ^84 Kr ^36+ beams and a 12-mm thick Be target. As a first step, we measured the stopping range distribution of the injected ion beams in liquid N _2 to estimate the stopping range distribution in He II and the spot size of the injected ion beams using a plastic scintillator. Then, a laser-induced fluorescence (LIF) detection experiment was performed using He II. We successfully observed the LIF from ^84 Rb atoms. We estimated the longitudinal range straggling of ^84 Rb ions in superfluid helium from the obtained results. The details of experiment at QST-HIMAC and results are given in this report.

To detect and track structural changes in atomic nuclei, the systematic study of nuclear levels with firm spin-parity assignments is important. While linear polarization measurements have been applied to determine the electromagnetic character of gamma-ray transitions, the applicable range is strongly limited due to the low efficiency of the detection system. The multi-layer Cadmium-Telluride (CdTe) Compton camera can be a state-of-the-art gamma-ray polarimeter for nuclear spectroscopy with the high position sensitivity and the detection efficiency. We demonstrated the capability to operate this detector as a reliable gamma-ray polarimeter by using polarized 847-keV gamma rays produced by the ^56Fe ( p, p'γ ) reaction. By combining the experimental data and simulated calculations, the modulation curve for the gamma ray was successfully obtained. A remarkably high polarization sensitivity was achieved, compatible with a reasonable detection efficiency. Based on the obtained results, a possible future gamma-ray polarimetery is discussed.

The magnetic dipole moment and the electric quadrupole moment are the nuclear moments that provide us with key information about the proton and neutron configurations in a nucleus and the shape of a nucleus, respectively. In the study of nuclear structure through the measurement of the nuclear moments, a technique to produce spin orientation of rare-isotope beams has played important roles. Recently, a scheme of the two-step projectile fragmentation was developed to produce high spin alignment in RI beams and was applied to the frontier of the study for nuclear structure of neutron-rich nuclei, such as ^75 Cu and ^99 Zr. The recent activities of the nuclear-moment measurements using highly spin-aligned beams at RIKEN RIBF are reported.

In the traditional view, heavy deformed nuclei are like axially-symmetric prolate ellipsoids, rotating about one of the short axes. In the present picture, their shapes may be triaxial. The triaxial shape yields complex rotations, which actually well reproduce experimental data, as confirmed by state-of-the-art Configuration Interaction calculations. Two origins are suggested for the triaxiality: (i) binding-energy gain by the symmetry restoration for triaxial shapes, and (ii) another gain by specific components of the nuclear force, like tensor force and high-multipole (e.g. hexadecupole) central force. While the origin (i) produces basic modest triaxiality for virtually all deformed nuclei, the origin (ii) produces more prominent triaxiality for a certain class of nuclei. An example of the former is 154Sm, a typical showcase of axial symmetry but is now suggested to depict a modest yet finite triaxiality. The latter, prominent triaxiality, is discussed from various viewpoints for some exemplified nuclei including 166Er, and experimental findings. Many-body structures of the gamma band and the double-gamma band are clarified. Regarding the general features of rotational states of deformed many-body systems including triaxial ones, the well-known J(J+1) rule of rotational excitation energies is derived, within the quantum mechanical many-body theory, without resorting to the quantization of a rotating classical rigid body. This derivation is extended to finite K. The present picture of the rotation is robust and can be applied to various shapes or configurations, including clusters and molecules. Thus, two long-standing open problems, (i) occurrence and origins of triaxiality and (ii) quantum many-body derivation of rotational energy, are resolved. Their possible relations to Davydov's rigid-triaxial-rotor model are mentioned.

Collinear laser spectroscopy on isotopic chains of refractory elements is under preparation at the SLOWRI facility in the radioactive isotope (RI) beam factory of RIKEN. Towards online measurement on RI beams, we have prepared an offline setup and performed test measurements using singly charged barium isotopes. Then we measured spectra of singly charged zirconium ions as a proof of principle for refractory elements.

The ground-state magnetic dipole moment of the neutron-rich 21O isotope has been measured via beta-ray-detected nuclear magnetic resonance (beta-NMR) spectroscopy by using a spin-polarized secondary beam of 21O produced from the 22Ne primary beam. From the present measurement, the g factor |gexp(21Og.s.)| = 0.6036(14) has been determined. Based on the comparison of this value with Schmidt values, we unambiguously confirm the nu d5/2 configuration with spin and parity assignments I pi = 5/2+ for the 21O ground state, suggested by previously reported studies. Consequently, the magnetic moment has been determined as mu exp(21Og.s.) = (-)1.5090(35)mu N. The obtained experimental magnetic moment is in good agreement with the predictions of the shell-model calculations using the USD, YSOX, and SDPF-M interactions as well as random phase approximation (RPA) calculations. This observation indicates that the 21O nucleus in its ground state does not manifest any anomalous structure and is not influenced by the proximity of the drip line.

A long-standing crucial question with atomic nuclei is whether or not α clustering occurs there. An α particle (helium-4 nucleus) comprises two protons and two neutrons, and may be the building block of some nuclei. This is a very beautiful and fascinating idea, and is indeed plausible because the α particle is particularly stable with a large binding energy. However, direct experimental evidence has never been provided. Here, we show whether and how α (-like) objects emerge in atomic nuclei, by means of state-of-the-art quantum many-body simulations formulated from first principles, utilizing supercomputers including K/Fugaku. The obtained physical quantities exhibit agreement with experimental data. The appearance and variation of the α clustering are shown by utilizing density profiles for the nuclei beryllium-8, -10 and carbon-12. With additional insight by statistical learning, an unexpected crossover picture is presented for the Hoyle state, a critical gateway to the birth of life.

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.

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.

published or not.The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

The ion source combining laser ablation of solid targets in helium gas and an RF ion guide system with an RF carpet was constructed for a reference measurement of isotope shifts. It is important for performing planned collinear laser spectroscopy of RI beams from the SLOWRI facility at RIKEN, especially for medium-mass nuclei of refractory elements. They are difficult to produce via the ISOL method owing to the chemical properties of the elements. The laser-ablated ions of Ni, Zr, Ag, Ta, W, and Ba were transported by the ion guide and mass spectra consistent with the natural abundance were successfully observed corresponding to 105–107 ions per laser pulse.

The fundamental symmetries, charge conjugation (C), parity (P) and time reversal (T), play a significant role in the Standard Model (SM) of elementary particle physics. Of these, T symmetry and the combined CP symmetry are the least well understood, and they hold valuable clues for unraveling the secrets of nature. All subatomic particles are postulated to possess an intrinsic property known as a permanent electric dipole moment (EDM). The EDM of an atom is a combination of those of each constituent particle and also CP-violating interactions between the particles. Being many-particle systems, atoms and molecules arc ideal candidates for probing a rich variety of both T- and CP-violating interactions. Paramagnetic atoms, which have a single valence electron in their outer shell, are sensitive to subtle signals associated with CP violations in the leptonic sector, i.e., the EDM of the electron. At present, we are developing a high-intensity laser-cooled Fr factory at RIKEN accelerator facility in an attempt to evaluate the EDM of Fr to an accuracy of 10(-30) ecm. Laser cooling is important for achieving highly accurate EDM measurements, since it allows long interaction times using an optical lattice. The current status of the laser-cooled Fr EDM experiments is presented in this paper.

The cross sections for the production of three most neutron-deficient zinc isotopes, Zn54-56, in the projectile fragmentation of Kr-78 at 345 MeV/nucleon on a beryllium target, were measured. Although the results are smaller by over an order of magnitude from those obtained with a Ni-58 beam at 75 MeV/nucleon on a nickel target, the rates of reaction products are found to be larger in the case of high-energy fragmentation of Kr-78. The experimental cross sections for the most neutron-deficient isotopes of even-Z nuclei between zinc and krypton were analyzed in the framework of the abrasion-ablation model. A very good agreement with the data was found when the nuclear masses predicted by a particular variant of the Hartree-Fock-Bogoliubov mass model were used. The prospects for the observation of more exotic isotopes in this region are discussed in context of 2p radioactivity.

We propose a method to measure the electron electric dipole moment (eEDM) using ultracold entangled francium (Fr) atoms trapped in an optical lattice, yielding an uncertainty below the standard quantum limit. Among the alkali atoms, Fr offers the largest enhancement factor to the eEDM. With a Fr based experiment, quantum sensing using quantum entangled states could enable a search for the eEDM at a level below 10 −30 e cm. We estimate statistical and systematic errors attached to the proposed measurement scheme based on this quantum sensing technique. A successful quantum sensing of the eEDM could enable the exploration of new physics beyond the standard model of particle physics.

BigRIPS is a powerful two-stage in-flight separator for the research with exotic nuclei studied in frontier experiments since more than a decade. The ion-optical system is very versatile due to the multi-stage structure of BigRIPS combined with the ZeroDegree spectrometer or the Superconducting Ring Cyclotron (SRC). Various optical modes can be flexibly realized according to the purpose of experiments. Two categories of developments are presented here. One is a new operating mode of BigRIPS aiming at higher ion-optical resolving power. BigRIPS itself has a two-stage structure. Spatial isotope separation is made at both the first and second stages. In the standard operating mode of BigRIPS, at the second stage the two spatial separations with energy degraders are subtractive in their resolving powers. Here, we present the additive mode. With the resulting increased spatial separation power, the isotopic background can be substantially reduced. Higher ion-optical resolving powers of the first and second BigRIPS degrader stages are also investigated with the goals to reduce further the background and to yield access to new isotopes of heavier elements. The other development is a dispersion-matched system with BigRIPS for high-resolution spectrometer experiments. The BigRIPS and ZeroDegree spectrometer are presently two independent, coupled achromatic systems. A new dispersion-matched mode of BigRIPS and ZeroDegree will enable novel experiments. For high-resolution spectroscopy experiments with high-intensity light projectiles, SRC and BigRIPS can be operated as a dispersion-matched system. The described different ion-optical developments are a base for a new category of experiments exploring exotic nuclei and mesic atoms. Characteristic future experiments with these new ion-optical developments are exemplified in this report.

Hyperfine structure (HFS) measurements of neutron-rich iridium isotopes $^{196,197,198}\mathrm{Ir}$ ($Z=77,\phantom{\rule{0.16em}{0ex}}N=119$--121) were performed via in-gas-cell laser resonance ionization spectroscopy at the KEK Isotope Separation System. Magnetic dipole moments $\ensuremath{\mu}$ and isotope shifts were determined from the HFS spectra. The variation of mean-square charge radii and quadrupole deformation parameters of these isotopes were evaluated from the isotope shifts. The $\ensuremath{\mu}$ value of $^{197}\mathrm{Ir}$ agreed with a theoretical value based on the strong coupling model, and the Ir nucleus was interpreted as prolately deformed by the theoretical calculations. The $\ensuremath{\mu}$ values of $^{196,198}\mathrm{Ir}$ were also compared with semiempirical values calculated based on the strong coupling model. From the comparison, we can suggest the possible spin values of ${I}^{\ensuremath{\pi}}=1,{2}^{\ensuremath{-}}$ for $^{196}\mathrm{Ir}$ and ${I}^{\ensuremath{\pi}}={1}^{\ensuremath{-}}$ for $^{198}\mathrm{Ir}$.

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.