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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.

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.

A new multi-reflection time-of-flight mass spectrograph for nuclear mass measurements with high precision has been constructed at the SLOWRI facility at RIKEN-RIBF. We aim to couple this new device to a cryogenic gas cell operated at the BigRIPS facility of RIBF. It is part of the SLOWRI project for slowing down and stopping intense radioactive ion beams provided downstream of RIKEN’s superconducting ring cyclotron accelerator. In particular, this device is dedicated to be located at the end of the BigRIPS beam line to access ions that are presently guided to beam dumps during in-beam experiments. In this article the apparatus is described in detail.

We developed KEK Isotope Separation System (KISS) in RIKEN Nishina center to produce neutron-rich isotopes around N = 126 for the study of β-decay properties from astrophysical interests. The KISS is an argon-gas-cell-based on laser ion source with ISOL, and, therefore, allows us to extract heavy refractory elements and investigate neutron-rich isotopes around N = 126. We extracted the laser-ionized unstable nuclei in the vicinity of 198Pt, and performed β-γ spectroscopy and in-gas-cell laser ionization spectroscopy successfully.

We developed a multi-segmented proportional gas counter to perform β-decay spectroscopy of neutron-rich nuclei extracted from KEK Isotope Separation System (KISS). The 32 segmented gas counters allow us two-dimensional tracking of the events to identify β-ray events by discriminating background events. The detectors being by low-density material of thin aluminized Mylar foils and P10 gas enabled us to detect low energy electrons down to 100 keV. The absolute detection efficiency was 45% at Q β = 1 MeV, and the measured background event rate was 0.11 cps. The detector was applied to perform β-decay spectroscopy and hyperfine structure measurements of neutron-rich isotopes of Pt, Ir, and Os. We started to develop the gas counter with three-dimensional tracking by replacing the material of anode wire from BeCu to resistive thin carbon to reduce the background event rate below 0.01 cps.

Deexcitation $\ensuremath{\gamma}$ rays associated with an isomeric state of $^{186}\mathrm{Ta}$ were investigated. The isomers were produced in multinucleon transfer reactions between a $^{136}\mathrm{Xe}$ beam and a natural W target and were collected and separated by the KEK Isotope Separation System. Two $\ensuremath{\gamma}$ transitions with energies of 161.1(2) and 186.8(1) keV associated with an isomeric decay were observed for the first time. The half-life of the isomeric state of the neutral atom $^{186\mathrm{m}}\mathrm{Ta}$ was deduced as 17(2) s. Based on the comparison with the previous measurements of the isomeric state using the Experimental Storage Ring at GSI Darmstadt and the coupling of angular momenta of individual particle orbitals in odd-odd nuclei, a decay scheme of $^{186\mathrm{m}}\mathrm{Ta}$ was proposed.

The $\ensuremath{\beta}\text{\ensuremath{-}}\ensuremath{\gamma}$ spectroscopy of $^{195}\mathrm{Os}$ nucleus was performed at the KEK isotope separation system (KISS). The radioactive isotope of $^{195}\mathrm{Os}$ was produced by the multinucleon transfer reactions of $^{136}\mathrm{Xe}$ beams and $^{198}\mathrm{Pt}$ target. The isotope of interest was separated and extracted by using KISS, which is an argon-gas-cell based laser ion source with a mass separator. Those extracted isotopes are implanted into an aluminized mylar tape, which is surrounded by the multisegmented proportional gas counter (MSPGC) and four clover-type high-purity germanium (HPGe) detectors for the $\ensuremath{\beta}\text{\ensuremath{-}}\ensuremath{\gamma}$ spectroscopy. 27 $\ensuremath{\gamma}$-ray peaks were observed in coincidence with internal conversion electrons and x rays detected by the MSPGC. 21 of them were identified as $\ensuremath{\beta}$-delayed $\ensuremath{\gamma}$ rays of the $^{195}\mathrm{Os}$ ground state, where the energies of 19 $\ensuremath{\gamma}$-ray peaks agree with the literature values of transitions observed in $^{195}\mathrm{Ir}(n,\ensuremath{\gamma})$ reactions and two $\ensuremath{\gamma}$-ray peaks were newly found. The $\ensuremath{\beta}$-decay branching ratios of the $^{195}\mathrm{Os}$ ground state were deduced. The obtained log-$ft$ values range from $5.99(4)$ to $7.56(22)$, indicating all $\ensuremath{\beta}$-decay channels are the first forbidden transitions. It suggests that the spin-parity of the $^{195}\mathrm{Os}$ ground state is $3/{2}^{\ensuremath{-}}$, which is consistent with the systematical expectation. The $\ensuremath{\beta}$-decay half-life of the $^{195}\mathrm{Os}$ ground state was evaluated to be 6.5(4) min, which agrees well to the previously known value 6.5(11) min.

Mass-separated ${^{187}\mathrm{Ta}}_{114}$ in a high-spin isomeric state has been produced for the first time by multinucleon transfer reactions, employing an argon gas-stopping cell and laser ionization. Internal $\ensuremath{\gamma}$ rays revealed a ${T}_{1/2}=7.3\ifmmode\pm\else\textpm\fi{}0.9\text{ }\text{ }\mathrm{s}$ isomer at $1778\ifmmode\pm\else\textpm\fi{}1\text{ }\text{ }\mathrm{keV}$, which decays through a rotational band with perturbations associated with the approach to a prolate-oblate shape transition. Model calculations show less influence from triaxiality compared to heavier elements in the same mass region. The isomer-decay reduced $E2$ hindrance factor ${f}_{\ensuremath{\nu}}=27\ifmmode\pm\else\textpm\fi{}1$ supports the interpretation that axial symmetry is approximately conserved.

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 $^{198}\mathrm{Ir}$.

We performed in-gas-cell laser ionization spectroscopy of $^{194,196}\mathrm{Os}$, which were produced by multinucleon transfer reactions at the KEK Isotope Separation System (KISS). The nuclei of interest were identified through $\ensuremath{\beta}$-decay spectroscopy and a newly applied multireflection time-of-flight mass spectrograph (MRTOF-MS). From the study of laser resonance spectra, isotope shifts (ISs) of the transition ${\ensuremath{\lambda}}_{1}=247.758$ nm for $^{194}\mathrm{Os}$ and $^{196}\mathrm{Os}$ were measured for the first time. The change in the mean-square charge radius $\ensuremath{\delta}{\ensuremath{\langle}{r}^{2}\ensuremath{\rangle}}^{192,A}$ and the quadrupole deformation parameter ${\ensuremath{\langle}{\ensuremath{\beta}}_{2}^{2}\ensuremath{\rangle}}^{1/2}$ were deduced from the IS measurements. The results were compared with the theoretical calculations and the predicted shape transition at $^{194}\mathrm{Os}$ is discussed.

Deexcitation $\ensuremath{\gamma}$ rays associated with an isomeric state of the $^{195}\mathrm{Os}$ were measured for the first time. The half-life of the isomeric state of the neutral atom $^{195\mathrm{m}}\mathrm{Os}$ was deduced. The isomers were produced in multinucleon transfer reactions between $^{136}\mathrm{Xe}$ beams and $^{198}\mathrm{Pt}$ targets, and were collected and separated by KEK Isotope Separation System. Analysis of $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ correlation and comparison with previous measurement of the isomeric state using the ESR at GSI Darmstadt suggests a possible decay scheme of $^{195\mathrm{m}}\mathrm{Os}$.

A procedure is introduced to extract the time-of-flight probability density distribution for ions of one type using an accumulated ion peak as a template for use in the fitting of time-of-flight peaks of co-accumulated ions. We demonstrate the use of logarithmic splines and discuss the statistical criteria for the selection of the spline smoothing parameter to obtain a smooth function for approximating such an ion peak. Furthermore, a “Bootstrap” method is proposed to determine the uncertainty of the fitted time-of-flight values, which is calculated using the extracted peak shape via a repeated statistical experiment. The developed procedure is validated against time-of-flight data obtained in a previously reported experiment. It is shown that atomic masses derived from our newly proposed methodology are in good agreement with those from the 2016 Atomic Mass Evaluation and the values determined using an analytically defined function (exponential-Gaussian hybrid), approximately matching the observed peak shape. In contrast to the previously used approach, our newly proposed methodology is a much more flexible, robust, and universal method for peak shape approximation, which can be applied to any peak shape.

Achieving the highest possible mass resolving power in a multi-reflection time-of-flight mass spectrometer requires very high-stability power supplies. To this end, we have developed a programmable high-voltage power supply that can achieve long-term stability on the order of parts-per-million. Herein we present the design of the stable high-voltage system and bench-top stability measurements up to 1~kV; the stabilization technique can, in principle, be applied up to 15~kV or more.. We demonstrate that in the $\le$1~Hz band the output stability is on the level of 1~part per million (ppm) during one hour, with only slightly more output variation across 3 days. We further demonstrate that the output is largely free of noise in the 1~Hz -- 200~Hz band. We also demonstrate settling to the ppm level within one minute following a 100~V step transition. Finally, we demonstrate that when these power supplies are used to bias the electrodes of a multi-reflection time-of-flight mass spectrograph the measured time-of-flight is stable on the ppm-level for at least one hour., Comment: 10 pages, 12 figures, Report of electronics development

Two independent beam stoppers have been developed for improving the beam separation of the gas-filled recoil ion separator GARIS-II. Performance evaluation of these supplemental beam stoppers was performed by using the 208Pb (18O,3n)223Th reaction. A160-fold enhancement of the signal-to-noise ratio at the GARIS-II focal plane was observed., Comment: 4 pages, 4 figures

We present the first direct measurement of the atomic mass of a superheavy nuclide. Atoms of $^{257}$Db ($Z$=105) were produced online at the RIKEN Nishina Center for Accelerator-Based Science using the fusion-evaporation reaction $^{208}$Pb($^{51}$V, 2n)$^{257}$Db. The gas-filled recoil ion separator GARIS-II was used to suppress both the unreacted primary beam and some transfer products, prior to delivering the energetic beam of $^{257}$Db ions to a helium gas-filled ion stopping cell wherein they were thermalized. Thermalized $^{257}$Db$^{3+}$ ions were then transferred to a multi-reflection time-of-flight mass spectrograph for mass analysis. An alpha particle detector embedded in the ion time-of-flight detector allowed disambiguation of the rare $^{257}$Db$^{3+}$ time-of-flight detection events from background by means of correlation with characteristic $\alpha$-decays. The extreme sensitivity of this technique allowed a precision atomic mass determination from 11 events. The mass excess was determined to be $100\,063(231)_\textrm{stat}(132)_\textrm{sys}$~keV/c$^2$. Comparing to several mass models, we show the technique can be used to unambiguously determine the atomic number as $Z$=105 and should allow similar evaluations for heavier species in future work., Comment: Submitted to PRC as Rapid Comm., 5 pages, 4 figures

We introduce a new concomitant referencing mode for operating a multi-reflection time-of-flight mass spectrograph (MRTOF-MS), wherein the reference and analyte ions are interleaved on a cycle by cycle bases. Using this mode, we demonstrate an improved technique for performing wide bandwidth mass measurements via MRTOF-MS. This new technique offers a simplified analysis and high-accuracy., 8 pages, 6 figures, follow-up for DOI:10.1016/j.ijms.2013.11.005

A multi-segmented proportional gas counter (MSPGC) with high detection efficiency and low-background event rate has been developed for β -decay spectroscopy. The MSPGC consists of two cylindrically aligned layers of 16 counters (32 counters in total). Each counter has a long active length and small trapezoidal cross-section, and the total solid angle of the 32 counters is 80% of 4 π . β -rays are distinguished from the background events including cosmic-rays by analyzing the hit patterns of independent counters. The deduced intrinsic detection efficiency of each counter was almost 100%. The measured background event rate was 0.11 counts per second using the combination of veto counters for cosmic-rays and lead block shields for background γ -rays. The MSPGC was applied to measure the β -decay half-lives of 198Ir and 199mPt. The evaluated half-lives of T 1 ∕ 2 = 9 . 8 ( 7 ) s and 12.4(7) s for 198Ir and 199mPt, respectively, were in agreement with previously reported values. The estimated absolute detection efficiency of the MSPGC from GEANT4 simulations was consistent with the evaluated efficiency from the analysis of the β - γ spectroscopy of 199Pt, saturating at approximately 60% for Q β > 4 MeV.

We have developed a doughnut-shaped argon gas cell with a rotating target for the KEK Isotope Separation System (KISS). The new gas cell increased extraction yields for target-like fragments (TLFs) produced in multi-nucleon transfer (MNT) reactions. KISS is an on-line isotope separator, which makes use of a laser ion source combined with an argon gas cell as a gas catcher for TLFs. This system has been implemented for nuclear spectroscopy of neutron-rich isotopes in the region of N = 126 . A gas flow simulation was performed to optimize the design of the gas cell for efficient and fast transport of TLFs. To confirm the performance expected at the design stage, we performed off- and on-line experiments by using iridium atoms evaporated from a filament in the gas cell and TLFs produced by a 136 Xe beam and 198 Pt target system respectively. We could increase the extraction yield by more than an order of magnitude by applying a doughnut-shaped gas cell with a rotating target. Thus, we could perform nuclear spectroscopy in this heavy region with the use of KISS.

We present visible spectra of highly charged holmium ions, which is a potential candidate ion for constructing a precise atomic clock. A compact electron beam ion trap is used for producing and trapping holmium ions. By observing electron energy dependence of the spectra, the charge state is assigned for each emission line. We also present theoretical transition frequency and amplitude obtained with two different methods.

Various isotopes of Ac, Ra, Fr, and Rn were produced by fusion-evaporation reactions using a 48Ca beam. The energetic ions were stopped in and extracted from a helium gas cell. The extracted ions were identified using a multi-reflection time-of-fight mass spectrograph. In all cases, it was observed that the predominant charge state for the extracted ions, including the alkali Fr, was 2+.

This paper describes the laser and optical system for the Parasitic radioactive isotope (RI) beam production by Laser Ion-Source (PALIS) in the RIKEN fragment separator facility. This system requires an optical path length of 70 m for transporting the laser beam from the laser light source to the place for resonance ionization. To accomplish this, we designed and implemented a simple optical system consisting of several mirrors equipped with compact stepping motor actuators, lenses, beam spot screens and network cameras. The system enables multi-step laser resonance ionization in the gas cell and gas jet via overlap with a diameter of a few millimeters, between the laser photons and atomic beam. Despite such a long transport distance, we achieved a transport efficiency for the UV laser beam of about 50%. We also confirmed that the position stability of the laser beam stays within a permissible range for dedicated resonance ionization experiments.

We present an experimental observation of the visible magnetic dipole transition between the fine structure levels in the ground term of ${\mathrm{Ba}}^{6+}$ ($5s{\phantom{\rule{0.16em}{0ex}}}^{2}5{p}^{2}$: $^{3}P_{0}\ensuremath{-}^{3}P_{1}$) with a compact electron beam ion trap. The wave number in vacuum of the transition is determined to be 15505.506(37) ${\mathrm{cm}}^{\ensuremath{-}1}$ by simultaneously observing the reference lines of neutral Ne. The present measurement reduces the uncertainty in the previously reported value obtained from observation of the electric dipole transitions to the ground terms in the extreme ultraviolet range [Phys. Scr. 46, 403 (1992)].

We report on the performance of a recently developed high-efficiency, low-background, three-dimensional tracking multi-segmented proportional gas counter (3DT-MSPGC), intended for β -decay spectroscopy of neutron-rich nuclei in the vicinity of neutron magic number N =126 at the KEK Isotope Separation System (KISS) facility. The 3DT-MSPGC uses two concentric cylindrical layers of proportional gas counters (16 per layer, 32 in total) featuring resistive anode wires (3 k Ω /cm). By independently measuring the charge deposition from both ends of each counter, we constrain β particle trajectories and determine where they intersect with the central axis of the 3DT-MSPGC. New compact charge-sensitive pre-amplifier modules, along with shaping amplifier and fast-timing amplifier modules, were developed to deduce the hit position by precisely measuring the charge deposition and timing. We have studied the performance of the 3DT-MSPGC in on- and off-line experiments. A longitudinal position resolution of ≤ 3.5 mm (FWHM) was achieved for each counter. The determination of the event trajectories allowed for suppression of background events, resulting in the background-event rate reaching 0.020(3) count-per-second (cps) while still maintaining a high-detection efficiency. This reduction of background is shown to improve the signal-to-noise ratio in determination of the half-life of 196Ir by a factor of 2.3(6), resulting in a significant improvement in achievable precision.

We developed KEK Isotope Separation System (KISS) for the nuclear spectroscopy of the nuclei in the vicinity of N = 126. The spectroscopy is important to identify the explosive astrophysical environment for the formation of the third peak in the observed solar r-abundance pattern. We report the experimental results of in-gas-cell laser ionization spectroscopy and β-decay spectroscopy for the nuclei in the vicinity of 198Pt.

In order to investigate nuclei produced via fusion–evaporation reactions, especially super-heavy elements (SHE), we have begun construction of a facility for conversion of fusion–evaporation residues (EVR) to low-energy beams. At the base of this facility is a small cryogenic gas cell utilizing a traveling wave RF-carpet, located directly following the gas-filled recoil ion separator GARIS-II, which will thermalize EVRs to convert them into ion beams amenable to ion trapping. We present here the results of initial studies of this small gas cell.

KEK Isotope Separation System (KISS) has been developed at RIKEN to produce neutron rich isotopes with N = 126 to study the β -decay properties for application to astrophysics. The KISS is an element-selective mass-separation system which consists of an argon gas cell-based on laser ion source for atomic number selection and an ISOL mass-separation system. The argon gas cell of KISS is a key component to stop and collect the unstable nuclei produced in a multi-nucleon transfer reaction, where the isotopes of interest will be selectively ionized using laser resonance ionization. We have performed off- and on-line experiments to study the basic properties of the gas cell as well as of the KISS. We successfully extracted the laser-ionized stable 56 Fe (direct implantation of a 56 Fe beam into the gas cell) atoms and 198 Pt (emitted from the 198 Pt target by elastic scattering with a 136 Xe beam) atoms from the KISS during the commissioning on-line experiments. We furthermore extracted laser-ionized unstable 199 Pt atoms and confirmed that the measured half-life was in good agreement with the reported value.

In applied forms of time-of-flight mass spectrometry utilizing ion storage devices prior to an analysis device, a non instantaneous electric ejection pulse applied in the region of ion storage is used to accelerate ions into the time-of-flight analyzer. The calculated mass value of the ions from the time-of-flight is dependent on the duration of the field transition up to full strength. For novel applications dedicated to precision measurements, such as multi-reflection time-of-flight mass spectrometry of short-lived isotopes, the goal is to continuously decrease the measurement uncertainty while providing a mass accuracy on the same order. Even though dynamic-field models for time-of-flight mass spectrometry have been considered in the past for technological advances, it is important to study the accuracy of the measured mass in this context. Using a simplified linear model for the field transition, we provide a basic investigation of the scenario, and discuss the deviation from the classical "mass-over-charge" dependency of the ions' time-of-flight, which becomes violated. The emerging mass discrepancy depends on the distance between the mass of the ion used for calibration and that of the ion of interest and, in extreme cases, can increase to about one percent for systems with short times-of-flight. However, for typical conditions in single-reference multi-reflection time-of-flight mass spectrometry, mass deviations caused by this effect typically remain below the 1 ppm level. If a mass calibration using two or more ion species is possible during the measurement, the effect becomes negligible for appropriate choices of reference masses., 14 pages, 9 figures

The masses of charged atoms and molecules were first investigated by laterally dispersive sector field mass analyzers, which early on already achieved high mass resolving powers. Equally, high mass resolving powers were achieved by time-of-flight mass analyzers during the last decades. These measurements became possible when fast and precise electronic circuitries became available. Such techniques have been developed and used extensively for the mass analysis of short-lived nuclei, whose mass values reveal insight in processes that describe the formation of elements in star explosions. Precise mass determinations of short-lived ions have been performed for energetic ions in large accelerator storage rings as well as for low-energy ions in time-of-flight mass spectrographs with long flight paths. Similarly, precise mass measurements can also be performed for molecular ions that help to reveal the structure of molecules. In case of very high mass resolving powers, the mass determination of molecular ions can be so high that the measured ion mass directly reveals the molecule’s sum formula.

The masses of ^{246}Es, ^{251}Fm, and the transfermium nuclei ^{249-252}Md and ^{254}No, produced by hot- and cold-fusion reactions, in the vicinity of the deformed N=152 neutron shell closure, have been directly measured using a multireflection time-of-flight mass spectrograph. The masses of ^{246}Es and ^{249,250,252}Md were measured for the first time. Using the masses of ^{249,250}Md as anchor points for α decay chains, the masses of heavier nuclei, up to ^{261}Bh and ^{266}Mt, were determined. These new masses were compared with theoretical global mass models and demonstrated to be in good agreement with macroscopic-microscopic models in this region. The empirical shell gap parameter δ_{2n} derived from three isotopic masses was updated with the new masses and corroborates the existence of the deformed N=152 neutron shell closure for Md and Lr.

The masses of the exotic isotopes $^{210\ensuremath{-}214}\mathrm{Ac}$ and $^{210\ensuremath{-}214}\mathrm{Ra}$ have been measured with a multireflection time-of-flight mass spectrograph. These isotopes were obtained in flight as fusion-evaporation products behind the gas-filled recoil ion separator GARIS-II at RIKEN. The new direct mass measurements serve as an independent and direct benchmark for existing $\ensuremath{\alpha}\text{\ensuremath{-}}\ensuremath{\gamma}$ spectroscopy data in this mass region. Further, new mass anchor points are set for U and Np nuclei close to the $N=126$ shell closure for a future benchmark of the $Z=92$ subshell for neutron-deficient heavy isotopes. Our mass results are in general in good agreement with the previously indirectly determined mass values. Together with the measurement data, reasons for possible mass ambiguities from decay-data links between ground states are discussed.

Ions that are moved by electric fields in gases follow quite exactly the electric field lines since these ions have substantially lost their kinetic energies in collisions with gas atoms or molecules and so carry no momenta. Shaping the electric fields appropriately the phase space such ion beams occupy can be reduced and correspondingly the ion density of beams be increased.

We performed classical molecular dynamics (MD) simulations in order to search the conditions for efficient sympathetic cooling of highly charged ions (HCIs) in a linear Paul trap. Small two-component ion Coulomb crystals consisting of laser-cooled ions and HCIs were characterized by the results of the MD simulations.We found that the spatial distribution is determined by not only the charge-to-mass ratio but also the space charge effect. Moreover, the simulation results suggest that the temperature of HCIs do not necessarily decrease with increasing the number of laser-cooled ions in the cases of linear ion crystals. We also determined the cooling limit of sympathetically cooled 165Ho14+ ions in small linear ion Coulomb crystals. The present results show that sub-milli-Kelvin temperatures of at least 10 Ho14+ ions will be achieved by sympathetic cooling with a single laser-cooled Be+.

International audience; Precision mass measurements of ^63Cu, ^64–66Zn, ^65–67Ga, ^65–67Ge, ^67As, ^79,81Br, ^79Kr, ^80,81Rb, and ^79,80Sr were performed with a multireflection time-of-flight mass spectrograph. The masses of these nuclides were determined by the single reference method using isobaric references. In order to obtain precise results, time-of-flight drift compensations were performed and a phenomenological fit function was employed. Consequently, in the case of ^65Ga, a mass uncertainty of 2.1 keV, corresponding to a relative precision of \(\delta m/m = 3.5 \times 10^{ - 8}\), was obtained and the mass value is in excellent agreement with the 2016 Atomic Mass Evaluation.

Magnetic dipole moment and mean-square charge radius of $^{199}$Pt ($I^{\pi}=$ 5/2$^-$) have been evaluated for the first time from the investigation of the hyperfine splitting of the $\lambda_1=$ 248.792 nm transition by in-gas-cell laser ionization spectroscopy. Neutron-rich nucleus $^{199}$Pt was produced by multi-nucleon transfer reaction at the KISS where the nuclear spectroscopy in the vicinity of $N=$ 126 is planed from the aspect of an astrophysical interest as well as the nuclear structure. Measured magnetic dipole moment $+$0.63(13)$\mu_{\rm N}$ is consistent with the systematics of those of nuclei with $I^{\pi}=$ 5/2$^-$. The deformation parameter $| ^{1/2}|$ evaluated from the isotope shift indicates the gradual shape change to spherical shape of platinum isotopes with increasing neutron number toward $N=$ 126.

Using a multireflection time-of-flight mass spectrograph located after a gas cell coupled with the gas-filled recoil ion separator GARIS-II, the masses of several $\alpha$-decaying heavy nuclei were directly and precisely measured. The nuclei were produced via fusion-evaporation reactions and separated from projectilelike and targetlike particles using GARIS-II before being stopped in a helium-filled gas cell. Time-of-flight spectra for three isobar chains, $^{204}$Fr-$^{204}$Rn-$^{204}$At-$^{204}$Po, $^{205}$Fr- $^{205}$Rn-$^{205}$At-$^{205}$Po-$^{205}$Bi, and $^{206}$Fr-$^{206}$Rn-$^{206}$At, were observed. Precision atomic mass values were determined for $^{204-206}$Fr, $^{204,205}$Rn, and $^{204,205}$At. Identifications of $^{205}$Bi, $^{204,205}$Po, $^{206}$Rn, and $^{206}$At were made with N$\leq$10 detected ions, representing the next step toward use of mass spectrometry to identify exceedingly low-yield species such as superheavy element ions., 多反射時間飛行質量分析装置を理化学研究所設置の気体充填型反跳分離装置-II型(GARIS-II)と連結したガスセルに配置し、$\alpha$崩壊連鎖を伴う重原子核の質量の直接測定を実施した。融合-蒸発反応で生成された原子核をGARIS-IIを用いて分離し、ヘリウムガスセルで止めて目的の原子核を捕集した。本実験で$^{204}$Fr-$^{204}$Rn-$^{204}$At-$^{204}$Po, $^{205}$Fr-$^{205}$Rn-$^{205}$At-$^{205}$Po-$^{205}$Bi及び$^{206}$Fr-$^{206}$Rn-$^{206}$Atの3つの同重体鎖の時間飛行スペクトルが観測された。その結果、$^{204-206}$Fr, $^{204,205}$Rn、そして$^{204,205}$Atの原子質量値が精密に決定された、今回の解析において、$^{205}$Bi, $^{204,205}$Po, $^{206}$Rn、そして$^{206}$Atの同定は10個以下という少量のイオン数でなされた。この成果は 次のステップとして実施する予定である(生成量が極少量の)超重元素イオンの質量分析につながる成果となる。

High-precision mass measurements of $^{63}$Cu, $^{64-66}$Zn, $^{65}$Ga, $^{65-67}$Ge, $^{67}$As, $^{78,81}$Br, $^{80}$Rb, and $^{79}$Sr were performed utilizing a multireflection time-of-flight mass spectrograph combined with the gas-filled recoil ion separator GARIS-II. In the case of $^{65}$Ga, a mass uncertainty of 2.1 keV, corresponding to a relative precision of $\delta m / m = 3.5\times10^{-8}$, was obtained and the mass value is in excellent agreement with the 2016 Atomic Mass Evaluation. For $^{67}$Ge and $^{81}$Br, where masses were previously deduced through indirect measurements, discrepancies with literature values were found. The feasibility of using this device for mass measurements of nuclides more neutron-deficient side, which have significant impact on the $rp$-process pathway, is discussed., Comment: 15 pages, 6 figures, 1 table

A multi-reflection time-of-flight mass spectrograph (MRTOF-MS) has been implemented at RIKEN to provide high-precision mass measurements of very short-lived nuclei with half-lives down to ⩽10 ms. Such measurements are of particular interest for r-process nuclei and trans-Uranium nuclei. We provide a detailed description of the mechanical construction and electronics implementation of an MRTOF-MS at RIKEN. The resolving power is investigated by numerical simulation and these simulations are compared to experimental observations to understand the effects of voltage and thermal instabilities. The limits of precision and accuracy are investigated using offline ion sources, and the effects of non-isobaric contaminants are reported.

We demonstrated efficient two-color two-step laser ionization schemes in the combined use of λ1 ∼ 250 nm and λ2 = 307.9 nm, which are applicable to heavy refractory elements with an atomic number in the wide range of Z = 69–78. We investigated newly observed ionization schemes of tantalum and tungsten atoms in an argon-gas-cell-based laser ion source for the efficient ionization of atoms of unstable nuclei through the two-color two-step laser resonance ionization technique. We experimentally determined the ionization cross sections from the measured saturation curves by solving the rate equations for the ground, intermediate, and ionization continuum populations. Hyperfine structures of these elements were also studied to deduce the isotope-shift, pressure-shift, and pressure-broadening in the resonance spectra of the excitation transitions in the argon gas cell. The electronic factor F255 of the excitation transition λ1 = 255.2115 nm between the ground and intermediate states was deduced from the measured isotope shifts of stable 182,183,184,186W isotopes. The ionization schemes investigated here are applicable to extract any isotopes of these elements by considering the measured pressure shift and nuclear isotope shift in optimizing the wavelength λ1.

An in-flight separator, BigRIPS, at RIBF in RIKEN provides each experiment with specific nuclides separated from many nuclides produced by projectile fragmentation or in-flight fission. In this process, nuclides other than separated ones are discarded on the slits in BigRIPS, although they include many nuclides interested from the view point of nuclear structure. In order to extract these nuclides for parasitic experiments, we are developing a method using laser ion-source (PALIS). A test experiment with $^{78}$Se beam from RIBF has been performed by using a gas cell set in BigRIPS. Unstable nuclides around $^{67}$Se were stopped in the gas cell in accordance with a calculation using LISE code. The stopping efficiency has been estimated to be about 30\%. As a next step, we will establish the technique for extracting reaction products from the gas cell., 理化学研究所の不安定核ビーム施設(RIBF)では、入射核破砕反応や核分裂で生成される多くの核種からインフライト分離装置(BigRIPS)を用いて実験対象の核種を分離している。しかるに、分離された残りの核反応生成物の中にも核構造から興味深い多くの不安定核が含まれている。これらをBigRIPSから取り出して研究することができれば、RIBFの有効利用につながる。そこで、BigRIPS内に設置したガスセル中で核反応生成物を停止させてレーザーでイオン化して引き出す装置(PALIS)を開発中である。開発の一環として、RIBFの$^{78}$Krビームの破砕反応により生成する$^{67}$Se近傍の不安定核をガスセル中で停止させる実験を行なった。実験結果は破砕反応の模型計算の予測とよく一致し、ガスセル中での停止効率は約30\%と評価された。この結果を基に、次のステップとして、停止した核反応生成物をガスセルから引き出すことを行う。

Nuclear parameters such as lifetimes and masses of the waiting point nuclei of r-process nucleosynthesis are significant to investigate its astrophysical environment. However, the difficulty in the production of extremely neutron-rich nuclei at the 126 neutron closed shell makes their experimentalstudies unfeasible. Therefore, the theoretical nuclear models play crucial roles in the simulation of the r-process nucleosynthesis. The systematic nuclear spectroscopy of the neutron-rich nuclei around the 126 neutron closed shell provides significant inputs to those theoretical models to improve their predictability for the waiting point nuclei. We are developing KEK Isotope Separation System (KISS) to perform the systematic nuclear spectroscopy of those neutron-rich nuclei. The nuclei of interest are produced by multi-nucleon transfer reactions between 136Xe and 198Pt. The experimental study demonstrated its promising potential to produce them. We have successfully performed the β­γ spectroscopy and the laser ionization spectroscopy at KISS using the nuclear production by the multi-nucleon transfer reactions.

We are developing a new nuclear laser spectroscopic technique for the study of nuclear structure that can be applied to short-lived low-yield atoms with unstable nuclei. The method utilizes superfluid helium (He II) as a trapping medium for high-energy ion beams. A liquid helium cryostat with optical windows is a key apparatus for this type of experiment. We describe the design and the performance of the cryostat which is developed for the present project.