RI Beam Factory Research Articles

Discovery of Neutron-Rich Silicon Isotopes 45,46Si

Abstract New isotopes $^{45,46}$Si, 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 $^{70}$Zn 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 $^{45}$Si events and one $^{46}$Si event were confirmed. The neutron-bound nature of odd-N$^{45}$Si may have a significant impact on the nuclear stability of neutron-rich proton $sd$-shell nuclei at approximately $N=40$.

Production Cross Sections of Residual Nuclides from 93Zr + p at 27 MeV/Nucleon

Abstract Nuclear transmutation emerges as a promising approach for reprocessing high-level waste, specifically treating long-lived nuclides like 93Zr 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 93Zr were measured at 27 MeV/nucleon in inverse kinematics. At the RI Beam Factory (RIBF) the OEDO beamline was used to deduce production cross sections for isotopes, 91 − 93Nb, 91, 92Zr, and 88, 89Y. 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.

First Exploration of Monopole-Driven Shell Evolution above the N=126 Shell Closure: New Millisecond Isomers in ^{213}Tl and ^{215}Tl.

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.

Gamma-ray spectroscopy of 55Sc

The N = 34 isotope 55Sc has been investigated using in-beam γ-ray spectroscopy at the RIKEN Radioactive Isotope Beam Factory. Spectra from the direct (p, pn) reaction as well as indirect reaction channels have been investigated. γ rays with energies 496(10), 570(12), 682(14), 1510(30), 1780(36), 2345(57) and 2470(50) keV have been observed. A level scheme was constructed based on γ γ coincidence analysis and relative intensities. The results have been compared to the level scheme already reported in literature, as well as to large-scale shell model calculations in the sd − pf model space. A new level at 1510keV, decaying directly to the ground state, has been proposed and spin-parity J π = 7/2− was tentatively assigned. The effect of including the ν g 9/2 orbital is discussed. It can be concluded that the main low-energy properties of 55Sc seem to be included in the original sd − pf model space.

Excited-State Half-Lives in ^{130}Cd and the Isospin Dependence of Effective Charges.

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.

Position of the single-particle 3/2− state in 135Sn and the N=90 subshell closure

The decay of excited states of the nucleus 135Sn, with three neutrons outside the doubly-magic 132Sn core, was studied in an experiment performed at the Radioactive Isotope Beam Factory at RIKEN. Several γ rays emitted from excited 135Sn ions were observed following one-neutron and one-neutron-one-proton removal from 136Sn and 137Sb beams, respectively, on a beryllium target at relativistic energies. Based on the analogy to 133Sn populated via one-neutron removal from 134Sn, an excitation energy of 695(15) keV is assigned to the 3/2− state with strongest single-particle character in 135Sn. This result provides the first direct information about the evolution of the neutron shell structure beyond N=82 and thus allows for a crucial test of shell-model calculations in this region. The experimental findings are in full agreement with calculations performed employing microscopic effective two-body interactions derived from CD-Bonn and N3LO nucleon-nucleon potentials, which do not predict a pronounced subshell gap at neutron number N=90. The occurrence of such a gap in 140Sn, i.e., when the 1f7/2 orbital is completely filled, had been proposed in the past, in analogy to the magicity of 48Ca, featuring a completely filled 0f7/2 orbital one harmonic oscillator shell below.

Development of the segmented plastic scintillation detector GARi for studying $r$-process nucleosynthesis at RIBF

\(\beta\)-decay properties of very neutron-rich nuclei, such as the half-life and the \(\beta\)-delayed neutron emission probabilities, play an essential role in the astrophysical rapid neutron capture process (r-process), where elements heavier than iron may be synthesized. Investigating those crucial properties has been one of the main objectives of the experimental programs at the RIKEN RI Beam Factory (RIBF). Recently, the development of a fast-timing detector system comprising a highly segmented plastic scintillation detector GARi (Gas-cell Active detector for Radioisotope decay), a neutron time-of-flight detector array, and a LaBr\textsubscript{3} detector array is being conducted for the measurement of \(\beta\)-delayed neutron emission and other related \(\beta\)-decay properties of neutron-rich nuclei at the F11 decay station of RIBF. The GARi detector has been constructed from a fast, segmented plastic scintillator coupled with nine multi-anode photomultiplier tubes (PMTs), resulting in enhanced position sensitivity. Therefore, this detector can be employed as an implantation detector and can also serve as a trigger signal in neutron time-of-flight experiments. This work details the development of the GARi detector and the results tested with the radioactive sources.

The SCRIT electron scattering facility at RIKEN RI Beam Factory

The SCRIT (Self-Confining Radioactive-isotope Ion Target) electron scattering facility has been constructed at RIKEN RI Beam Factory. After the commissioning experiment, the development towards electron scattering with short-lived unstable nuclei has been carried out. Details of the ion dynamics inside the SCRIT system were studied with stable nuclei, and the achieved luminosity was 3.2 × 1027 cm−2s−1 with an electron–beam current of 250 mA and the injection of 108 Xe ions. Electron scattering with online-produced 137Cs ions has been recently commissioned. In the near future, electron scattering with short-lived unstable nuclei will be performed.

Offline development for collinear laser spectroscopy at the SLOWRI facility

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.

First spectroscopic study of odd-odd Cu78

Nuclei in the vicinity of Ni78 are important benchmarks for nuclear structure, which can reveal changes in the shell structure far from stability. Spectroscopy of the odd-odd isotope Cu78 was performed for the first time in an experiment with the EURICA setup at the Radioactive Isotope Beam Factory at RIKEN Nishina Center. Excited states in the neutron-rich isotope were populated following the β decay of Ni78 produced by in-flight fission and separated by the BigRIPS separator. A level scheme based on the analysis of γ−γ coincidences is presented. Tentative spin and parity assignments were made when possible based on the β-decay feeding intensities and γ-decay properties of the excited states. Time correlations between β and γ decay show clear indications of an isomeric state with a half-life of 3.8(4) ms. Large-scale Monte Carlo shell-model calculations were performed using the A3DA-m interaction and a valence space comprising the full fp shell and the 1g9/2 and 2d5/2 orbitals for both protons and neutrons. The comparison of the experimental results with the shell-model calculations allows interpreting the excited states in terms of spin multiplets arising from the proton-neutron interaction. The results provide further insight into the evolution of the proton single-particle orbitals as a function of neutron number, and quantitative information about the proton-neutron interaction outside the doubly magic Ni78 core.Received 6 October 2022Revised 3 January 2023Accepted 9 January 2023DOI:https://doi.org/10.1103/PhysRevC.107.044301©2023 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasBeta decayEnergy levels & level densitiesIsomer decaysNuclear spin & parityNuclear structure & decaysNucleon-nucleon interactionsShell modelProperties59 ≤ A ≤ 89Nuclear Physics

Status update of the Charge Stripper Rings project

The RIKEN RI (radioactive isotope) Beam Factory (RIBF) provides the world's most intense heavy ion beams exceeding 345 MeV/u and is the primary facility for in-flight RI beam generation. The RIBF has steadily improved its performance. In particular, the intensity of the uranium beam, which is critical for producing in-flight fission RI beams, has been dramatically increased by a factor of 240 compared to 2008. To further increase the intensity of the uranium beam, a new acceleration scheme using charge stripper rings (CSRs) has been proposed as a cost-effective way to increase charge stripping efficiency. In this paper, we discuss the key design issues of CSR and required upgrades of existing ring cyclotrons with the introduction of CSR and space-charge effects.

Gamow-Teller decay of Te142 to I142

The β decay of Te90142 to I89142 was investigated for the first time. The parent nucleus was produced by the in-flight fission of a U238 beam with an energy of 345 MeV per nucleon, impinging on a Be9 target at the Radioactive Isotope Beam Factory of RIKEN. Excited states in I142 were established by β-delayed γ-ray spectroscopy. The observed (1+) states in I142 could be interpreted to be predominantly the ν0h9/2⊗π0h11/2 configuration formed by a Gamow-Teller transition between a neutron in the 0h9/2 orbital and a proton in the 0h11/2 orbital. Additional features of the (1+) states are discussed by comparing with neighboring heavier isotones, such as Cs144 and La146. In the context of deformed shell-model calculations, the (11+) state is closely related to the ν[5,3,2]3/2⊗π[5,5,0]1/2 configuration, which may be related to the weak Gamow-Teller transition strength.1 MoreReceived 30 August 2022Revised 16 November 2022Accepted 11 January 2023DOI:https://doi.org/10.1103/PhysRevC.107.014311©2023 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasBeta decayEnergy levels & level densitiesNuclear structure & decaysNucleon-nucleon interactionsShell modelProperties90 ≤ A ≤ 149TechniquesRadiation detectorsRadioactive beamsSpectrometers & spectroscopic techniquesNuclear Physics

Study of the N=32 and N=34 Shell Gap for Ti and V by the First High-Precision Multireflection Time-of-Flight Mass Measurements at BigRIPS-SLOWRI.

The atomic masses of ^{55}Sc, ^{56,58}Ti, and ^{56-59}V 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 ^{56,58}Ti and ^{56-59}V, the mass uncertainties have been reduced down to the order of 10keV, 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 ^{58}Ti and ^{59}V. 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 νp_{3/2} orbit is identified as a feature unique to Ca. We perform new MonteCarlo shell model calculations including the νd_{5/2} and νg_{9/2} orbits and compare the results with conventional shell model calculations, which exclude the νg_{9/2} and the νd_{5/2} 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.

Facility upgrade for superheavy-element research at RIKEN

The RIKEN Nishina Center (RNC) executed an accelerator upgrade project for the heavy-ion linac (called RILAC). A superconducting RIKEN linear accelerator (SRILAC) and a new superconducting electron-cyclotron-resonance ion source (SC-ECRIS) to boost the final energy and intensity were constructed, aimed at synthesizing a new superheavy element, 119, through a hot fusion reaction. The project included the construction of a gas-filled recoil ion separator (GARIS-III) suitable for detecting the residues of the hot-fusion reaction. To avoid research interruption during the SRILAC construction period (2017–2019) and gain experience in hot-fusion reaction processes, GARIS-II located in the GARIS experimental hall in LINAC building was moved to the E6 experimental hall in Nishina building. Certain exploratory measurements were performed employing the beams accelerated by RILAC2 and the RIKEN ring cyclotron (RRC), which is a part of the existing accelerator complex of the radioactive isotope beam factory (RIBF). Further, commissioning experiments with the upgraded facility (SRILAC and GARIS-III) were performed. The upgrade project and its commissioning results are chronologically described in this article.

Discovery of ^{39}Na.

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.

Indirect method to estimate the deadtime of a data acquisition system as a function of the data size

An indirect method based on timestamp analysis is developed to estimate the deadtime of a data acquisition (DAQ) system as a function of data size (deadtime function). The performance of two types of time-to-digital converter (TDC) modules, the CAEN V1190 and the AMSC AMT-TDC, are studied by applying the proposed method to a test DAQ system built with the TDC modules. With the aid of this method, the DAQ system of the SAMURAI spectrometer at the RIKEN radioactive isotope beam factory (RIBF) is optimized. Through the optimization, the minimum deadtime of one event decreases from about 170 μs to about 45 μs.

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 β-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.

Superconducting Linac Booster for Super-Heavy Element Experiments at RIKEN Radioactive Isotope Beam Factory

The Rikagaku Kenkyūjo (RIKEN) heavy-ion linac was upgraded by introducing a new super-conducting linac-booster to advance the super-heavy elements synthesis program beyond nihonium at the RIKEN Radioactive Isotope Beam Factory (RIBF). The total acceleration voltage was upgraded from 25 MV with 12 room temperature drift-tube-linacs (DTLs) to 39 MV by introducing a superconducting linac booster, SRILAC. The upgrade of the beam intensity is realized by a newly constructed superconducting electron-cyclotron resonance ion source (SC-ECRIS). The construction of SRILAC and the SC-ECRIS started in FY2017. After the hardware installation and commissioning, the first beam acceleration test was successfully conducted at the end of FY2019, and user beam service was started in FY2020. This article provides an overview of the upgrade and its present performance.

First observation of the π0h11/2⊗ν0h9/2 partner orbital configuration in the odd-odd I138 nucleus

The β-decay scheme of Te138 and the level structure of I138 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 Te138 and Sb138, were produced by the in-flight fission of a U238 beam with the energy of 345 MeV per nucleon. From the β decay of Te138, the level scheme of I138 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 π0h11/2⊗ν0h9/2 partner orbital configuration populated by the Gamow-Teller transition between a neutron in the 0h9/2 orbital and a proton in the 0h11/2 orbital. Details of the structure of I138 are discussed in terms of the proton-neutron interactions and Gamow-Teller transition strength within the theoretical context of shell-model calculations.Received 5 December 2021Accepted 16 March 2022DOI:https://doi.org/10.1103/PhysRevC.105.034334©2022 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasBeta decayNuclear structure & decaysProperties90 ≤ A ≤ 149TechniquesRadiation detectorsRadioactive beamsShell modelSpectrometers & spectroscopic techniquesNuclear Physics

A neural network approach based on more input neurons to predict nuclear mass * *Supported by National Natural Science Foundation of China (12175170, 11675066), the Fundamental Research Funds for the Central Universities (lzujbky-2017-ot04), and Feitian Scholar Project of Gansu province

The study of nuclear mass is very important, and the neural network (NN) approach can be used to improve the prediction of nuclear mass for various models. Considering the number of valence nucleons of protons and neutrons separately in the input quantity of the NN model, the root-mean-square deviation of binding energy between data from AME2016 and liquid drop model calculations for 2314 nuclei was reduced from 2.385 MeV to 0.203 MeV. In addition, some defects in the Weizsäcker–Skyrme (WS)-type model were repaired, which well reproduced the two-neutron separation energy of the nucleus synthesized recently by RIKEN RI Beam Factory [Phys. Rev. Lett. 125, (2020) 122501]. The masses of some of the new nuclei appearing in the latest atomic mass evaluation (AME2020) are also well reproduced. However, the results of neural network methods for predicting the description of regions far from known atomic nuclei need to be further improved. This study shows that such a statistical model can be a tool for systematic searching of nuclei beyond existing experimental data.