Publisher：Interactions  

We have developed the KEK Isotope Separation System (KISS) at RIKEN to study the nuclear structure of the nuclei in the vicinity of neutron magic number N= 126 from the astrophysical perspective. These neutron-rich nuclei have been produced by using multinucleon transfer (MNT) reactions with combinations of the low-energy 136Xe beam and the production targets of W, Ir, and Pt. At the KISS facility, radioisotopes are ionized by applying in-gas-cell laser ionization technique. In this process, we can perform laser ionization spectroscopy of the refractory elements with the atomic number Z= 70–78 such as Hf, Ta, W, Re, Os, Ir, and Pt, which cannot be performed in other facilities. Laser spectroscopy can effectively investigate nuclear structure through the measured magnetic moments, isotope shifts (IS) Δν, changes in the mean-square charge radii δ<r2>, and quadrupole deformation parameters |<β22>|1/2. We have studied the ionization schemes of these elements through offline tests and performed in-gas-cell laser ionization spectroscopy of these refractory neutron-rich nuclei produced at KISS.

DOI： 10.1007/s10751-024-01886-1

Scopus

Publisher：Physical Review C  

We report the mass measurements of neutron-rich isotopes produced via spontaneous fission of Cf252 using a multireflection time-of-flight mass spectrograph. The mass of Ce155 is determined experimentally for the first time. A discrepancy between the experimental and literature values was found for the mass of Sb127, which was previously deduced through indirect measurements. In comparison with several theoretical predictions, both the values and the trend of the mass excesses of Ce152-155 cannot be consistently explained. The wide-range and simultaneous mass measurements of the multireflection time-of-flight mass spectrograph enable us to crosscheck the existing mass data, and the conflict between the measured time-of-flight ratio and the extracted mass would imply the necessity of re-examining them.

DOI： 10.1103/PhysRevC.110.045810

Web of Science

Scopus

Publisher：Physical Review C  

Atomic masses of the neutron-rich isotopes Ga83,84, Ge82-86, As82-89, Se82,84-91, Br85,86,89-92, Kr89,91,92, and Rb91 have been measured with high precision and accuracy using the multi-reflection time-of-flight mass spectrograph at the RIBF facility. The masses of As88,89 were measured for the first time and the mass uncertainties of Ge86 and Se90,91 were significantly reduced from several hundred keV to below the 10 keV scale. Investigation of shell evolution and potential subshell closures are proceeded by analysis of two-neutron separation energy systematics. As a result of the precise mass measurements, no prominent change on the slope of the two-neutron separation energy beyond N=56 is observed in Se isotopes, indicating the subshell closure effect at N=56 of Se does not exist. Also, various leading mass models are compared with the measurements. The impact of the more precisely known data on r-process nucleosynthesis calculations is investigated in light of these new mass measurements, showing a remarkable reduction of reaction-rate uncertainties.

DOI： 10.1103/PhysRevC.109.035804

Web of Science

Scopus

Language：English   Publisher：Scientific Reports  

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.

DOI： 10.1038/s41598-024-52692-2

Web of Science

Scopus

PubMed

Publisher：Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment  

We have developed radio-frequency (RF) wire carpets and installed them in a cryogenic helium gas cell to increase the extraction yields of target-like fragments (TLFs) produced in multi-nucleon transfer (MNT) reactions at the KEK Isotope Separation System (KISS). KISS is an on-line isotope separator based on a gas cell system, and has been implemented for nuclear spectroscopy of isotopes approaching the N=126 neutron shell closure below lead and neutron-rich actinides. We optimized the design of the RF wire carpet for efficient ion transport of TLFs from the simulations. We then performed experiments both off-line using rubidium ions from an alkali thermal ion-source in the gas cell, and on-line using TLFs produced by a 136Xe beam impinging upon a 198Pt target system, to confirm the performance expected of the design. We confirmed an increase in the extraction yields of more than an order of magnitude when using the cryogenic He gas cell with RF wire carpets as compared to our typical argon gas cell system with laser resonant ionization. The techniques and knowledge acquired in the development of these RF wire carpets will be applied to new RF wire carpets for use in a large-volume helium gas cell which will be installed at an upgraded KISS facility in the near future.

DOI： 10.1016/j.nima.2023.168838

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Physical Review C  

The atomic masses of Ag111,113, Pd111-113, Rh111-113, Ru111-113, and Mo111,112 have been measured during the online commissioning experiments of the ZeroDegree multi-reflection time-of-flight Mass Spectrograph (ZD MRTOF-MS) at the RIKEN RI beam factory. The mass of Mo112 has been determined. For the previously known masses, a good agreement between our results and the 2020 Atomic Mass Evaluation has been observed in most cases. The determined two-neutron separation energies for Mo isotopes up to N=70 show a smooth trend. In this work, the performed experiment and analysis procedure are presented. The theoretical interest in the measured region is highlighted, and the results are discussed in terms of the various mass surface formulas including the new mass data. Furthermore, a comparison between our results and global theoretical mass models is given, and we provide a benchmark for results from a Bayesian machine learning algorithm for future mass extrapolation.

DOI： 10.1103/PhysRevC.108.054312

Web of Science

Scopus

Publisher：Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms  

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.

DOI： 10.1016/j.nimb.2023.05.031

Web of Science

Scopus

Publisher：Progress of Theoretical and Experimental Physics  

Previously, we developed an α-TOF detector for correlated measurements of atomic masses and decay properties of radioactive isotopes using a multi-reflection time-of-flight mass spectrograph, and successfully demonstrated α-decay-correlated mass measurements of heavy and superheavy nuclides. In this work, we develop a β-TOF detector, an improvement upon the α-TOF detector, to extend this technology to β-decaying nuclides. Online commissioning with 81Ga yielded a measured half-life of 1.44(31) s, in agreement with the previous literature, and confirmed the efficiency of detecting a decay-correlated event to be consistent with the solid angle limitation.

DOI： 10.1093/ptep/ptad039

Web of Science

Scopus

Publisher：Physical Review C  

The ground-state magnetic dipole moment of the neutron-rich O21 isotope has been measured via β-ray-detected nuclear magnetic resonance (β-NMR) spectroscopy by using a spin-polarized secondary beam of O21 produced from the Ne22 primary beam. From the present measurement, the g factor |gexp(Og.s.21)|=0.6036(14) has been determined. Based on the comparison of this value with Schmidt values, we unambiguously confirm the νd5/2 configuration with spin and parity assignments Iπ=5/2+ for the O21 ground state, suggested by previously reported studies. Consequently, the magnetic moment has been determined as μexp(Og.s.21)=(-)1.5090(35)μN. 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 O21 nucleus in its ground state does not manifest any anomalous structure and is not influenced by the proximity of the drip line.

DOI： 10.1103/PhysRevC.107.024306

Web of Science

Scopus

Publisher：Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment  

A newly assembled multi-reflection time-of-flight mass spectrograph (MRTOF-MS) at RIKEN's RIBF facility became operational for the first time in spring 2020; further modifications and performance tests using stable ions were completed in early 2021. By using a pulsed-drift-tube technique to modify the ions’ kinetic energy in a wide range, we directly characterize the dispersion function of the system for use in a new procedure for optimizing the voltages applied to the electrostatic mirrors. Thus far, a mass resolving power of Rm>1000000 is reached within a total time-of-flight of only 12.5ms, making the spectrometer capable of studying short-lived nuclei possessing low-lying isomers. Detailed information about the setup and measurement procedure is reported, and an alternative in-MRTOF ion selection scheme to remove molecular contaminants in the absence of a dedicated deflection device is introduced. The setup underwent an initial on-line commissioning at the BigRIPS facility at the end of 2020, where more than 70 nuclear masses have been measured. A summary of the commissioning experiments and results from a test of mass accuracy will be presented.

DOI： 10.1016/j.nima.2022.167824

Web of Science

Scopus

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Physical Review Letters  

The atomic masses of Sc55, Ti56,58, and V56-59 have been determined using the high-precision multireflection time-of-flight technique. The radioisotopes have been produced at RIKEN's Radioactive Isotope Beam Factory (RIBF) and delivered to the novel designed gas cell and multireflection system, which has been recently commissioned downstream of the ZeroDegree spectrometer following the BigRIPS separator. For Ti56,58 and V56-59, the mass uncertainties have been reduced down to the order of 10 keV, shedding new light on the N=34 shell effect in Ti and V isotopes by the first high-precision mass measurements of the critical species Ti58 and V59. With the new precision achieved, we reveal the nonexistence of the N=34 empirical two-neutron shell gaps for Ti and V, and the enhanced energy gap above the occupied νp3/2 orbit is identified as a feature unique to Ca. We perform new Monte Carlo shell model calculations including the νd5/2 and νg9/2 orbits and compare the results with conventional shell model calculations, which exclude the νg9/2 and the νd5/2 orbits. The comparison indicates that the shell gap reduction in Ti is related to a partial occupation of the higher orbitals for the outer two valence neutrons at N=34.

DOI： 10.1103/PhysRevLett.130.012501

Web of Science

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PubMed

Publisher：Journal of Physics: Conference Series  

We developed a laser frequency stabilization and an optical fiber transmission system for the the francium electric dipole moment search. The absolute accuracy of a laser frequency stabilization scheme using a state-of-the-art commercial wavelength meter was 0.48 MHz at ±2 nm and -1.33 MHz at ±200 nm from calibration wavelength, respectively, and the frequency instability is below 10-9 with a standard deviation of 0.56 MHz over 60 hours. We also demonstrated that a 400 m long fiber laid between laboratories can transmit 30 mW of trapping laser light, which is sufficient for a magneto-optical trapping of francium. The polarization crosstalk in the fiber was stable at -25 dB over 12 hours of measurement.

DOI： 10.1088/1742-6596/2249/1/012010

Scopus

Language：English   Publisher：Physical Review Letters  

We have measured the 3d→2p transition x rays of kaonic He3 and He4 atoms using superconducting transition-edge-sensor microcalorimeters with an energy resolution better than 6 eV (FWHM). We determined the energies to be 6224.5±0.4(stat)±0.2(syst) eV and 6463.7±0.3(stat)±0.1(syst) eV, and widths to be 2.5±1.0(stat)±0.4(syst) eV and 1.0±0.6(stat)±0.3(stat) eV, for kaonic He3 and He4, respectively. These values are nearly 10 times more precise than in previous measurements. Our results exclude the large strong-interaction shifts and widths that are suggested by a coupled-channel approach and agree with calculations based on optical-potential models.

DOI： 10.1103/PhysRevLett.128.112503

Web of Science

Scopus

PubMed

Publisher：Optics InfoBase Conference Papers  

We installed a 400-m-long polarization-maintaining fibers for magneto-optical trapping of Francium atoms. Fiber polarization stability of ~9×10-4 was achieved with an averaging time of 10 seconds, which even allows fluorescence observation at 20 atoms.

DOI： 10.1364/CLEOPR.2022.CTuP7C_03

Scopus

Publisher：2022 Conference on Lasers and Electro-Optics Pacific Rim, CLEO-PR 2022 - Proceedings  

We installed a 400-m-long polarization-maintaining fibers for magneto-optical trapping of Francium atoms. Fiber polarization stability of ~9×10-4 was achieved with an averaging time of 10 seconds, which even allows fluorescence observation at 20 atoms.

DOI： 10.1109/CLEO-PR62338.2022.10432223

Scopus

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1103/PhysRevLett.112.162502

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1063/1.2090290