Gas-filled Recoil Ion Separator Research Articles

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.

α -decay-correlated mass measurement of Ra206,207g,m using an α -TOF detector equipped multireflection time-of-flight mass spectrograph system

The atomic masses of the isotopes Ra206,207 have been measured via decay-correlated mass spectroscopy using a multireflection time-of-flight mass spectrograph equipped with an α-TOF detector. The Ra isotopes were produced as fusion-evaporation products in the V51+Tb159 reaction system and delivered by the gas-filled recoil ion separator GARIS-II at RIKEN. The α-TOF detector provides for high-accuracy mass measurements by correlating time-of-flight signals with subsequent α-decay events. The masses of Ra206 and Ra207g,m were directly measured using a multireflection time-of-flight mass spectrograph equipped with an α-TOF detector. The mass excesses of Ra206,207g and the excitation energy of Ra207m were determined to be ME=3540(50)keV/c2, 3538(15)keV/c2, and Eex=552(42) keV, respectively. The α-decay branching ratio of Ra207m, bα=0.26(20), was directly determined from decay-correlated time-of-flight signals, and the reduced α width of Ra207m was calculated to be δ2=43−34+68 keV from the branching ratio. The spin-parity of Ra207m was confirmed to be Jπ=13/2+ from decay correlated mass measurement results.1 MoreReceived 20 June 2021Accepted 28 September 2021Corrected 29 October 2021DOI:https://doi.org/10.1103/PhysRevC.104.044617©2021 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasAlpha decayBinding energy & massesNuclear reactionsProperties190 ≤ A ≤ 219Nuclear Physics

First high-precision direct determination of the atomic mass of a superheavy nuclide

We present the first direct measurement of the atomic mass of a superheavy nuclide. Atoms of Db257 (Z=105) were produced online at the RIKEN Nishina Center for Accelerator-Based Science using the fusion-evaporation reaction Pb208(V51, 2n)Db257. 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 Db257 ions to a helium gas-filled ion stopping cell wherein they were thermalized. Thermalized Db3+257 ions were then transferred to a multireflection 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 Db3+257 time-of-flight detection events from background by means of correlation with characteristic α decays. The extreme sensitivity of this technique allowed a precision atomic mass determination from 11 events. The mass excess was determined to be 100063(231)stat(132)syskeV/c2. 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.Received 4 June 2020Revised 3 March 2021Accepted 30 July 2021DOI:https://doi.org/10.1103/PhysRevC.104.L021304©2021 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasBinding energy & massesNuclear fusionPropertiesA ≥ 220MassNuclear mass rangesTechniquesAtom & ion trapping & guidingMass spectrometryTime-of-flight mass spectrometryNuclear Physics

Reduction of contaminants originating from primary beam by improving the beam stoppers in GARIS-II

Two independent beam stoppers have been developed for improving the beam separation of the gas-filled recoil ion separator GARIS-II. Performance of these supplemental beam stoppers was investigated 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.

Production of Bh266 in the Cm248(Na23,5n)Bh266 reaction and its decay properties

The nuclide Bh266 was produced in the Cm248(Na23,5n)Bh266 reaction at beam energies of 125.9, 130.6, and 135.3 MeV. Decay properties of Bh266 were investigated with a rotating wheel apparatus for α and spontaneous fission (SF) spectrometry under low background conditions attained by a gas-jet transport system coupled to the RIKEN gas-filled recoil ion separator. Based on genetically correlated α−α and α-SF decay chains, a total of 23 chains were assigned to Bh266 and its daughter nuclide Db262 and granddaughter Lr258. The half-life of Bh266 was measured to be T1/122=10.0−1.7+2.6s which is an order of magnitude longer than the literature data. The α-particle energies of Bh266 disperse widely in the range of Eα=8.62–9.40MeV. The maximum production cross section for the Cm248(Na23,5n)Bh266 reaction was determined to be σ=57±14 pb at 130.6 MeV, whereas the upper limit for the Cm248(Na23,4n)Bh267 reaction was σ≤14 pb at 121.2 MeV. These cross sections are discussed by comparing with the literature data as well as the theoretical calculations.1 MoreReceived 11 June 2020Accepted 30 July 2020DOI:https://doi.org/10.1103/PhysRevC.102.024625©2020 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasAlpha decayFissionLow & intermediate energy heavy-ion reactionsNuclear fusionNuclear reactionsPropertiesA ≥ 220Nuclear Physics

Study of Quasielastic Barrier Distributions as a Step towards the Synthesis of Superheavy Elements with Hot Fusion Reactions.

The excitation functions for quasielastic scattering of ^{22}Ne+^{248}Cm, ^{26}Mg+^{248}Cm, and ^{48}Ca+^{238}U are measured using a gas-filled recoil ion separator. The quasielastic barrier distributions are extracted for these systems and are compared with coupled-channel calculations. The results indicate that the barrier distribution is affected dominantly by deformation of the actinide target nuclei, but also by vibrational or rotational excitations of the projectile nuclei, as well as neutron transfer processes before capture. From a comparison between the experimental barrier distributions and the evaporation residue cross sections for Sg (Z=106), Hs (108), Cn (112), and Lv (116), it is suggested that the hot fusion reactions take advantage of a compact collision, where the projectile approaches along the short axis of a prolately deformed nucleus. A new method is proposed to estimate the optimum incident energy to synthesize unknown superheavy nuclei using the barrier distribution.

The study of rhenium pentacarbonyl complexes using single-atom chemistry in the gas phase.

A gas-phase chemical study of rhenium carbonyls was carried out using short-lived radioisotopes produced at a heavy-ion accelerator. The Re isotopes produced in the nuclear reactions of natGd(23Na,xn)172-177Re were pre-separated with a gas-filled recoil ion separator and their carbonyls were synthesized in a mixture of inert gas and carbon monoxide. Using a low temperature isothermal chromatography apparatus, the adsorption enthalpies of Re carbonyls were derived to be ΔHads = -42 ± 2 kJ mol-1 on a Teflon® surface by fitting the external chromatograms with a Monte Carlo simulation program. A chemical yield of 25% relative to that of the transport yield for Re by a He/KCl gas-jet was achieved. The laser-ablation time-of-flight mass-spectrometric technique was employed to identify the species of Re carbonyls produced in the gas phase. The most stable species was deduced to be Re(CO)5 based on the mass-spectrometric analysis as well as quantum chemistry calculations.

New mass anchor points for neutron-deficient heavy nuclei from direct mass measurements of radium and actinium isotopes

The masses of the exotic isotopes $^{210-214}$Ac and $^{210-214}$Ra have been measured with a multi-reflection 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 $\alpha$-$\gamma$ 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 $Z=92$ sub-shell 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.

Atomic masses of intermediate-mass neutron-deficient nuclei with relative uncertainty down to 35-ppb via multireflection time-of-flight mass spectrograph

High-precision mass measurements of Cu63, Zn64−66, Ga65, Ge65−67, As67, Br78,81, Rb80, and Sr79 were performed utilizing a multireflection time-of-flight mass spectrograph combined with the gas-filled recoil ion separator GARIS-II. In the case of Ga65, a mass uncertainty of 2.1keV, corresponding to a relative precision of δm/m=3.5×10−8, was obtained and the mass value is in excellent agreement with the 2016 Atomic Mass Evaluation. For Ge67 and Br81, where masses were previously deduced through indirect measurements, discrepancies with literature values were found. The feasibility of using this device for mass measurements of more neutron-deficient nuclei, which have significant impact on the rp-process pathway, is discussed.

β -delayed fission of Am230

The exotic decay process of $\ensuremath{\beta}$-delayed fission ($\ensuremath{\beta}\mathrm{DF}$) has been studied in the neutron-deficient isotope $^{230}\mathrm{Am}$. The $^{230}\mathrm{Am}$ nuclei were produced in the complete fusion reaction $^{207}\mathrm{Pb}(^{27}\mathrm{Al},4n)^{230}\mathrm{Am}$ and separated by using the GARIS gas-filled recoil ion separator. A lower limit for the $\ensuremath{\beta}\mathrm{DF}$ probability ${P}_{\ensuremath{\beta}\text{DF}}(^{230}\mathrm{Am})g0.30$ was deduced, which so far is the highest value among all known $\ensuremath{\beta}\mathrm{DF}$ nuclei. The systematics of $\ensuremath{\beta}\mathrm{DF}$ in the region of $^{230}\mathrm{Am}$ will be discussed.

Decay Measurement of283Cn Produced in the238U(48Ca,3n) Reaction Using GARIS-II

The production of 283Cn in the hot fusion reaction of 238U(48Ca,3n) was confirmed using a new gas-filled recoil ion separator GARIS-II. At the excitation energies of the compound nucleus E* = 34.5 MeV, two decay events from 283Cn were observed, resulting in the cross section of \(\sigma _{3n} = 2.0_{ - 1.3}^{ + 2.7}\) pb. One is the spontaneous fission with decay time τ = 14.4 s. The other is the 9.45 MeV α-decay with τ = 5.4 s, followed by the spontaneous fission of 279Ds (154 ms).

Study of the Reaction48Ca +248Cm →296Lv*at RIKEN-GARIS

The fusion reaction 48Ca + 248Cm → 296Lv* was investigated using the gas-filled recoil ion separator GARIS at RIKEN. The reaction was studied at excitation energies of 41.3 and 38.2 MeV. A total of seven decay chains were observed. Three of the chains were assigned to the decay of 292Lv and three to the decay of 293Lv. A possible observation of an α-decay branch of 284Cn populating the new spontaneously fissioning nucleus 280Ds is discussed. The assignment of one of the decay chains to the 3n evaporation channel, resulting in a spontaneous fission branch of 285Cn, or to the 2n channel, resulting in the population of the new fissioning nucleus 286Cn, is ambiguous. Measured cross sections are in agreement with previously published values.

First online multireflection time-of-flight mass measurements of isobar chains produced by fusion-evaporation reactions: Toward identification of superheavy elements via mass spectroscopy

Using a mulit-reflection time-of-flight mass spectrograph (MRTOF-MS) located after a gas cell coupled with the gas-filled recoil ion separator GARIS-II, the masses of several heavy nuclei have been directly and precisely measured. The nuclei were produced via fusion-evaporation reactions and separated from projectile-like and target-like particles using GARIS-II before being stopped in a helium-filled gas cell. Time-of-flight spectra for three isobar chains, 205Fr-205Rn-205At-205Po, 206Fr-206Rn-206At and 201Rn-201At-201Po-201Bi, were observed. Precision atomic mass values were determined for 205,206Fr, 201At, and 201Po.

Status of the low-energy super-heavy element facility at RIKEN

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.

Superheavy element chemistry at GARIS

A gas-jet transport system has been installed to the RIKEN GAs-filled Recoil Ion Separator, GARIS to start up SuperHeavy Element (SHE) chemistry. This system is a promising approach for exploring new frontiers in SHE chemistry: background radioactivities from unwanted by-products are suppressed, a high gas-jet transport yield is achieved, and new chemical reactions can be investigated. Useful radioisotopes of 261 Rfa ,b , 262 Db, and 265 Sga ,b for chemical studies were produced in the reactions of 248 Cm(18 O,5n )261 Rfa ,b , 248 Cm(19 F,5n )262 Db, and 248 Cm(22 Ne,5n )265 Sga ,b , respectively. They were successfully extracted to a chemistry laboratory by the gas-jet method. Production and decay properties of 261 Rfa ,b , 262 Db, and 265 Sga ,b were investigated in detail with the rotating wheel apparatus for α- and spontaneous fission spectrometry. Present status and perspectives of the SHE chemistry at GARIS are also briefly presented.

Production of 88Nb and 170Ta for chemical studies of element 105, Db, using the GARIS gas-jet system

A technology to utilize radioisotopes of Nb and Ta was developed for chemical studies of element 105, Db, by coupling a gas-jet transport system to the RIKEN gas-filled recoil ion separator (GARIS). 88Nb and 170Ta were generated via the natGe(19F, xn)88Nb and natGd(19F, xn)170Ta reactions, respectively. They were separated by GARIS and extracted by the gas-jet method to a chemistry laboratory under the almost same experimental conditions for 262Db produced in the 248Cm(19F, 5n)262Db reaction. Yields of 88Nb and 170Ta at the chemistry laboratory were optimized by changing the setting parameters of the GARIS gas-jet system, such as magnetic field of GARIS, energy degrader and shutter for recoil ions, and volume of the recoil transfer chamber.

Startup of a new gas-filled recoil separator GARIS-II

A new gas-filled recoil ion separator GARIS-II was on-line tested by using two 40Ar-induced fusion reactions of 169Tm(40Ar,4n)205Fr and 208Pb(40Ar,3n)245Fm, respectively. The products were collected to a focal plane with high efficiency of 47 ± 1 and 63 ± 9 % for 205Fr and 245Fm under low background condition.

Production ofDb262in theCm248(F19,5n)Db262reaction and decay properties ofDb262andLr258

The nuclide $^{262}\mathrm{Db}$ was produced in the $^{248}\mathrm{Cm}$($^{19}\mathrm{F}$,5$n$)$^{262}\mathrm{Db}$ reaction at beam energies of 103.1 and 97.4 MeV. Decay properties of $^{262}\mathrm{Db}$ were investigated with a rotating wheel apparatus for \ensuremath{\alpha} and spontaneous fission (SF) spectrometry under low background conditions attained by a gas-jet transport system coupled to the RIKEN gas-filled recoil ion separator. Based on genetically correlated \ensuremath{\alpha}-\ensuremath{\alpha} and \ensuremath{\alpha}-SF decay chains, a total of 76 chains were assigned to $^{262}\mathrm{Db}$ and its \ensuremath{\alpha}-decay daughter nuclide $^{258}\mathrm{Lr}$. No correlated decay chain was observed for Db produced in the $^{248}\mathrm{Cm}$($^{19}\mathrm{F}$,4$n$)Db reaction and its daughter Lr. The half-lives of $^{262}\mathrm{Db}$ and $^{258}\mathrm{Lr}$ were measured to be ${T}_{1/\phantom{\rule{0.0pt}{0ex}}2}=33.{8}_{\ensuremath{-}3.5}^{+4.4}$ and $3.{54}_{\ensuremath{-}0.36}^{+0.46}\mathrm{s}$, respectively, and they confirmed the literature data. The \ensuremath{\alpha}-particle energy $({E}_{\ensuremath{\alpha}})$ and intensity (${I}_{\ensuremath{\alpha}}$) of $^{262}\mathrm{Db}$ were revised to ${E}_{\ensuremath{\alpha}}$ = 8.46 \ifmmode\pm\else\textpm\fi{} 0.04 MeV (${I}_{\ensuremath{\alpha}}$ = 70 \ifmmode\pm\else\textpm\fi{} 5%) and ${E}_{\ensuremath{\alpha}}$ = 8.68 \ifmmode\pm\else\textpm\fi{} 0.03 MeV (${I}_{\ensuremath{\alpha}}$ = 30 \ifmmode\pm\else\textpm\fi{} 5%). The SF activity with ${T}_{1/2}$ = 30.2 \ifmmode\pm\else\textpm\fi{} 6.1 s was assigned to $^{262}\mathrm{Db}$, and the SF branch of $^{262}\mathrm{Db}$ was determined to be ${b}_{\mathrm{SF}}$ = 52 \ifmmode\pm\else\textpm\fi{} 4%. From an observation of two SF decays which correlated to the \ensuremath{\alpha} decay of $^{262}\mathrm{Db}$, an electron capture (EC) decay with ${b}_{\mathrm{EC}}$ = 2.6 \ifmmode\pm\else\textpm\fi{} 1.8% was suggested in $^{258}\mathrm{Lr}$. The production cross sections for the $^{248}\mathrm{Cm}$($^{19}\mathrm{F}$,5$n$)$^{262}\mathrm{Db}$ reaction were determined to be 2.1 \ifmmode\pm\else\textpm\fi{} 0.7 nb at 103.1 MeV and $0.{23}_{\ensuremath{-}0.11}^{+0.18}\mathrm{nb}$ at 97.4 MeV, whereas, those for the $^{248}\mathrm{Cm}$($^{19}\mathrm{F}$,4$n$)Db reaction were the upper limits of \ensuremath{\le}0.064 nb at 103.1 MeV and \ensuremath{\le}0.13 nb at 97.4 MeV. These cross sections are discussed by comparing with the literature data as well as the theoretical prediction.

Beam intensity monitor based on gas scintillation emitted from helium gas molecules in a gas-filled recoil ion separator

We have developed a new beam intensity monitor based on the gas scintillation emitted from helium gas molecules in a gas-filled recoil ion separator. The emission spectrum from the gas scintillation was measured using a monochromator over a wavelength range of 380–750nm. The emission strength as a function of the beam intensity was measured using a wavelength filter passing through the peak at 389nm. The emission strength was correlated as a function of the beam intensity within an intensity range of 0.006–0.68pμA.

Excitation functions for production of Rf isotopes in the248Cm+18O reaction

Production cross sections of Rf isotopes in the ${}^{248}$Cm$\phantom{\rule{0.16em}{0ex}}+\phantom{\rule{0.16em}{0ex}}$${}^{18}$O reaction were measured at the beam energy range of 88.2 to 101.3 MeV by use of a gas-filled recoil ion separator. The excitation functions of ${}^{260}$Rf, ${}^{261}$Rf${}^{a}$, and ${}^{262}$Rf were obtained together with those of spontaneously fissioning nuclides which have few-second half-lives and have been assigned to ${}^{261}$Rf${}^{b}$ and a longer-lived state of ${}^{262}$Rf. The excitation function of few-second spontaneously fissioning nuclide exhibited the maximum cross section at the ${}^{18}$O beam energy of 94.8 MeV. The shape of the excitation function was almost the same as that of ${}^{261}$Rf${}^{a}$, whereas it was quite different from those of ${}^{260}$Rf and ${}^{262}$Rf. A few-second spontaneously fissioning nuclide previously reported as ${}^{261}$Rf${}^{b}$ and ${}^{262}$Rf observed in ${}^{248}$Cm$\phantom{\rule{0.16em}{0ex}}+\phantom{\rule{0.16em}{0ex}}$${}^{18}$O reaction was identified as ${}^{261}$Rf${}^{b}$.