First high-precision direct determination of the atomic mass of a superheavy nuclide
P. Schury, T. Niwase, M. Wada, P. Brionnet, S. Chen, T. Hashimoto, H. Haba, H. Hirayama, D.S. Hou, S. Iimura, H. Ishiyama, S. Ishizawa, Y. Ito, D. Kaji, S. Kimura, H. Koura, J.J. Liu, H. Miyatake, J.-Y. Moon, K. Morimoto, K. Morita, D. Nagae, M. Rosenbusch, A. Takamine, Y.X. Watanabe, H. Wollnik, W. Xian, S.X. Yan
FFirst high-precision direct determination of the atomic mass of a superheavy nuclideevinces a new means to unambiguously determine atomic numbers
P Schury, ∗ T. Niwase,
2, 3, 1
M. Wada, P. Brionnet, S. Chen,
4, 1
T. Hashimoto, H. Haba, H. Hirayama, D.S. Hou,
6, 7, 8
S. Iimura,
9, 3, 1
H. Ishiyama, S. Ishizawa,
10, 3
Y. Ito,
11, 3, 1
D. Kaji, S. Kimura, H. Koura, J.J. Liu,
4, 1
H. Miyatake, J.-Y. Moon, K. Morimoto, K. Morita,
2, 12
D. Nagae, M. Rosenbusch, A. Takamine, Y.X. Watanabe, H. Wollnik, W. Xian,
4, 1 and S.X. Yan KEK Wako Nuclear Science Center, Wako, Saitama 351-0198, Japan Department of Physics, Kyushu University, Nishi-ku, Fukuoka 819-0395, Japan RIKEN Nishina Center for Accelerator-Based Science, Wako, Saitama 351-0198, Japan Department of Physics, The University of Hong Kong, Pokfulam, 999077, Hong Kong Institute for Basic Science, 70, Yuseong-daero 1689-gil, Yusung-gu, Daejeon, Korea Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, China University of Chinese Academy of Sciences, Beijing 100049, China School of Nuclear Science and Technology, Lanzhou University, Lanzhou 730000, China Department of Physics, Osaka University, Osaka, Japan Graduate School of Science and Engineering, Yamagata University, Yamagata, Japan Advanced Science Research Center, Japan Atomic Energy Agency (JAEA), Tokai, Ibaraki 319-1195, Japan Research Center for SuperHeavy Elements, Kyushu University, Nishi-ku, Fukuoka 819-0395, Japan New Mexico State University, Las Cruces, NM 88001, USA Institute of Mass Spectrometer and Atmospheric Environment, Jinan University, Guangzhou, 510632, China (Dated: June 5, 2020)Atoms of the superheavy element isotope
Db ( Z =105) were produced online in the fusion-evaporation reaction Pb( V, 2n)
Db. The gas-filled recoil ion separator GARIS-II was used tosuppress both the unreacted primary beam and transfer products, prior to delivering the energeticbeam of
Db ions to a helium gas-filled ion stopping cell wherein they were thermalized. Ther-malized Db ions were then transferred to a multi-reflection time-of-flight mass spectrographfor mass analysis. An alpha-particle detector embedded in the ion time-of-flight detector alloweddisambiguation of the rare Db time-of-flight detection events from background by means ofcorrelation with characteristic alpha-decays. Using 11 events where α -decay and time-of-flight couldbe correlated to Db, a mass excess of 100 063(231) stat (2) sys keV/c could be determined. Com-paring to several mass models, we show the technique can be used to unambiguously determine theatomic number as Z =105. The precise identification of superheavy nuclei is alongstanding issue that has largely been achieved throughcross-bombardment experiments in recent years [1–3].For the nuclei in the island of hot fusion superheavy nu-clides (SHN), however, cross-bombardment does not fullyresolve the question as all such nuclides thus far producedexhibit decay chains which terminate in spontaneous fis-sion prior to reaching well-known nuclides. Results fromefforts to unambiguously determine Z using characteris-tic X-rays [4] have yet to garner widespread acceptance,either. The Provisional Report of the 2017 Joint Work-ing Group of IUPAC and IUPAP [5] suggested that di-rect determination of the atomic mass with a precision δm (cid:46) could, in many cases, be a valid meansto fully determine the A and Z of an uncertain nuclide,particularly if decay information were simultaneously ob-tained. A first effort in this direction has recently shownsome promise by directly verifying the mass number of asuperheavy nuclide [6], however without reaching a levelof mass precision needed to confirm the atomic number.Beyond identification of superheavy nuclei, the precisedetermination of atomic masses is vital to understandingthe nature of the heaviest elements. Proper evaluation of the possible production – both in the laboratory [7, 8]and in the cosmos [9, 10] – of nuclides in the island ofstability requires accurate atomic masses in the heavyand superheavy region, in addition to understanding howfission-recycling in the astrophysical r-process [11] affectsproduction of elements up to and beyond uranium. Thisphenomenon strongly determines whether the r-processcould produce long-lived superheavy elements (SHE) inthe predicted island of stability. It also impacts the pro-duction cross-sections for creating such nuclei in the lab-oratory. In addition to informing the process of fission-recycling, accurate atomic masses provide a means tostudy the evolution of shell-structure in high- Z nuclei,such as the degree to which the N =152 subshell closurepersists [12–14] with increasing Z , which could impacttheoretical predictions of the location of the island ofstability and the half-lives of the nuclides in its vicinity.However, among isotopes of transuranium elements,directly determined atomic masses are rare [15–17]; forSHE ( Z ≥ a r X i v : . [ nu c l - e x ] J un Q1 D1 Q2 Q3 D2
Projectile Evaporation residues Gas-filled region Ta beam dump
Target Wheel Rotatable energy degradersInsertableSi-detector array0.5 um Mylar windowsCryogenic helium gas stopping cellRF carpet
QPIGAnalyte linear trap Reference linear trapFlat trapSteerersInjection mirrorEjection mirror Thermal ion source α -TOF detector Incoming ionsTrajectory of secondary electrons α -decay signalTOF signal M R T O F - M S FIG. 1. Sketch (not to scale) of the apparatus used inthe measurement. Dubnium atoms are produced via fusion-evaporation reactions. Ions of the fusion-evaporation prod-ucts are separated from the primary beam using the gas-filledrecoil ion separator GARIS-II. The ions are stopped in thehelium gas cell and subsequently stored in an RF ion trap be-fore being sent to a multi-reflection time-of-flight mass spec-trograph (MRTOF-MS) for analysis. The ion detector at theend of the MRTOF-MS can detect ion implantation and sub-sequent α -decay. Z =101 and Z =103, while providing the first anchorpoints from which to indirectly determine the massesof several SHE nuclides, including Db. In this let-ter we report the first direct measurement of the atomicmass of
Db by multi-reflection time-of-flight massspectrograph (MRTOF-MS) utilizing a newly developed“ α -TOF” detector [23] that improves the sensitivity ofthe MRTOF-MS by providing correlational data betweentime-of-flight (ToF) and subsequent α -decay. This mea-surement represents the first direct mass measurementof a superheavy nuclide, and the first online mass mea-surement to utilize α -decay correlated time-of-flight massspectroscopy. It serves as both a cross-check for our pre-vious indirect measurement of the mass of Db [15] anda proof-of-principle for future efforts to measure nuclidesin the hot-fusion superheavy island in which the nuclidesdo not connect to well-known nuclei via α -decay, andwhere typical yields will be on the order of a few per day.Atoms of Db were produced in the fusion-evaporation reaction
Pb( V, 2n)
Db. The RIKENRing Cyclotron (RRC) provided a 306 MeV beam of Vwith maximum intensity of ≈
500 pnA. The beam im-pinged upon a rotating target wheel comprised of alu-minum energy degraders and
Pb targets. The targetswere made from
Pb enriched to 99.6% and depositedon a 30 µ g/cm carbon backing, with a typical leadthickness of 360 µ g/cm . Aluminum energy degraders of12 um thickness were utilized to reduce the beam energyto 243 MeV at target center. A detector angled 45 ◦ tothe beam axis was located near the target wheel to mea-sure the rate of elastic recoils from the target, providinga means to measure the effective primary beam dose andprovide some indication of a failed target segment.To separate the desired beam of Db from the pri- mary beam and various transfer products, the gas-filledrecoil ion separator GARIS-II [24] was utilized. The sep-arator was filled with dilute helium gas at 70 Pa. Fromprevious experience with
Db [25] the selective dipole(D1 in Fig. 1) of GARIS-II was set to 14.2 kG.As shown in Fig. 1, after exiting GARIS-II the beampassed through rotatable Mylar energy degraders prior toentering a helium-filled gas stopping cell. The gas cell wascryogenically cooled to 60 K and pressurized to 200 mbarroom temperature equivalent. The thickness of the My-lar degraders was chosen to reduce the energy of
Dbto be commensurate with the stopping power of the he-lium in the gas cell. A static electric field transportedstopped ions to a traveling-wave radio-frequency (RF)ion carpet [26, 27] with a 0.74 mm diameter exit orifice.After exiting the gas cell, ions were transported through adifferentially pumped region by use of quadrupole radio-frequency ion guides and trapped in a segmented linearPaul trap, which is part of a three-trap suite used toprepare analyte and reference ions for analysis by theMRTOF-MS using the concomitant referencing method[15, 28] that allows analyte ions to be accumulated withnearly 100% duty cycle. The traps were cryogenicallycooled to ≈
150 K to minimize the probability of storedions charge exchanging with residual background gases.The MRTOF-MS, a device finding widespread accep-tance in recent years [29–39], is composed of a pair ofion mirrors separated by a field-free drift region. Theoutermost electrode of each mirror is switched to allowions to enter and exit. Ions are stored in the MRTOF-MS for a time sufficient to allow the ions to reflect aspecific number of times and achieve a time focus. Dur-ing the measurement reported herein, the mass resolvingpower at the time focus was typically R m ≈
250 000, asdemonstrated in Fig. 2, with flight times of t ∼
10 ms for
A/q ≈
85 ions. To preclude detector dead time leading toundercounting that could affect the reference peak shape,the reference ion source was adjusted so as to detect onereference ion ( Rb + or Cs + ) per cycle on average.Stable molecular ions produced in the gas cell or trans-fer products not removed by GARIS-II may have mass-to-charge ratios significantly differing from the analyteion and will make fewer or more reflections than theanalyte ions and may, by happenstance, appear at thesame ToF as the analyte ions. As such, erroneous at-tribution is a concern with MRTOF-MS measurements[28, 30]. Precluding erroneous attributions requires mul-tiple measurements at various numbers of reflections. Inthe case of analyte ions detected at a rate of a few per day,however, confidence in the ability to exclude backgroundnoise (dark counts, cosmic rays, and α - or β -decay from e .g. transfer product ions), or even extremely low-yieldmolecular ions with mass-to-charge ratio nearly identicalto the analyte, becomes an issue of concern.To overcome these issues we have developed a novel“ α -TOF” detector [23], featuring a silicon PIN diode em- R e l a t i ve I n t e n s i t y (t-t c )/t c [ppm] -4 -2 0 2 4 6 8 10 Rb +185 Au ++ FIG. 2. Observed ToF spectra for singly-charge Rb + ref-erence ions and doubly-charged Au ++ analyte ions. Spec-tra are plotted in terms of relative time with respect to thepeak position t c . The spectra were measured concomitantlyand each ion species made 325 laps in the MRTOF reflec-tion chamber. The Rb + and Au ++ had flight times of t c ≈ t c ≈ R m =275,000 wasachieved, while the singly-charged ions exhibit lower resolvingpower and greater asymmetry in the peak shape. bedded in the impact plate of a commercial MagneToFion detector. The detector has an energy resolution of ≈
140 keV FWHM. High-confidence measurements canbe achieved when using this detector with low-yield, α -decaying species by excluding candidate ToF events forwhich subsequent α -decay events are not observed.Since the α -TOF detector’s location precludes α -particle energy calibration by offline sources, Hg wasproduced via the
La( V, 5n) reaction prior to produc-tion of
Db. The 5653 keV and 5372 keV α -particlesfrom the α -decay of Hg [40, 41] were used to calibratethe α -TOF’s silicon PIN diode.Separately, the incoming rate of Hg was measuredon an insertable silicon PIN diode array located betweenGARIS-II and the gas cell. Using the measured rate of Hg in MRTOF-MS time-of-flight spectra, the effi-ciency from gas cell through to α -TOF was determinedto be between 4% and 5% for ToF detection.Ions are implanted on the detector with K ≈ q .The implantation depth is only a few angstroms and thedetection efficiency is geometrically limited to 45%. Sincethe recoil from a detected α -particle is sufficient to ejectthe atom from the detector surface, sequential α -particlesfrom decay chains cannot be observed. Fortunately, whenan undetected α particle is not emitted at an overly shal-low angle, the decay product atom is not removed fromthe surface and there is a similar 45% probability for “sec-ond chance” detection of the daughter’s α -decay. Thelifetimes of nuclides in the Db decay chain allow theevaluation to extend out four decays, through
Es. Ac-counting for the α -decay branching ratios of each nuclide[45, 46] the total likelihood to detect one of the α -decaysin the Db decay chain would be 65%.Identifying correlated events began with applying a E α ≥ α -decay singles. If a ToFsingle with t ∈ t c ±
50 ns, where t c is the expected ToF of the Db ion based on AME16 [43], is observed withinthe 120 s prior to a gated α -decay single, the events weredetermined to constitute a correlated event candidate.Based on previous experiences [15, 22, 42] the variousRF ion guides were initially set to transport Db and Cs + reference ions. However, after ≈
36 hours,with a dose on target of 4.7 × particles, no correlatedevent candidates were observed. In light of the NISTAtomic Spectra Database [44] showing a third ionizationpotential of 23.1 ± Db and Rb + reference ions. Multiple correlated α -ToF eventsfrom Db were then observed within 24 hours. Therate was consistent with expectations based on targetthickness, primary beam intensity, and the known 4%system efficiency. In total 14 correlated event candidateswere observed during 105 hours of measurement. Figure3 plots the correlated events observed in the course of thiswork in terms of detected α -decay energy and time fromimplantation to subsequent α -decay; events were namedin the order they were observed.Evaluating which nuclide produced the observed α -decay can help exclude false correlations. To this end,the right panel of Fig. 3 shows the anticipated decay timeprobability distributions [47] for each nuclide; multiplecurves are shown for nuclides with known isomers. Simi-larly, the upper panel shows the detector response curvesfor each α -decay which could be observed in the Dbdecay chain. The events appear to fall into two clusterscorresponding to
Db or
Lr and
Md or
Es. Ifwe include consideration of Po α -decay, we find thatevents E4, E7, and E10 are more consistent with Pothan with any nuclide in the
Db decay chain.The ToF-singles events accumulated over the entirecourse of the Db mass measurements are shown inFig. 4(b). So as to simplify the evaluation by compari-son to Fig. 2, the ToF singles are plotted in terms of theToF ratio ρ (see Eq. 1). Based on Fig. 2, the Db ToF peak should span less than 7 ppm, however the cor-related α -ToF candidates span 12.5 ppm. If events E4,E7 and E10 (previously noted to be consistent with Po α -decay) were to be excluded, the span reduces to 6 ppm.Figure 4(a) shows the α -decay singles events with E α ranging from 7.0 MeV up to 11.5 MeV, accumulatedover the entire course of the Db mass measure-ments. Correlated ToF- α -decay events are shown in redand blue. Centered near 7.5 MeV, a large peak of α -singles events can be seen. These are presumed to befrom Po ( T / =516 ms) resulting from the β -decay of At ( T / =7.214 h) of which we observed ≈ α -decay singles is consistent with Po α -decay based onthe detector efficiency and the β -decay branching ratioof At. Based on the 120 s coincidence window and235 observed α -decays commensurate with Po in the
E [MeV] α Po Es Md Lr Db Lr Db Md Es E4E10 E8 E11E3 E2 E12 E9E6E13E14E5 E1E7 D e c a y t i m e [ s ] FIG. 3. Distribution of ToF-correlated α -decay events interms of α -decay energy and decay time. For each nuclidein the decay chain of Db, the probability distribution interms of decay time [47] are shown at right. The detectorresponse function for each decay is shown at top, overlayingthe α -decay singles spectrum with correlated event candidatesdenoted by colored marks. The multiple α -decay channelswhich exist for Lr and
Db are shown. course of 105 hours of data accumulation, during which37 ToF singles events in the vicinity of Db were ob-served, we could expect to observe ≈ Po and e.g. ToF dark counts. Onthis basis, we exclude events E4, E7, and E10 from ouranalysis of the atomic mass of
Db.After excluding events E4, E7, and E10 there are 11correlated events and 13 uncorrelated ToF singles withinthe span of the Db ToF peak, along with 13 ToFsingles shown outside that span. If we assume the 13 un-correlated ToF singles outside the Db ToF peak tobe dark counts, then we can infer that ≈ Db ToF peak range are also darkcounts ( ∼ ≈ α -decays. From that we can in-fer a 61% overall efficiency to detect one of the α -decaysin the Db decay chain, in good agreement with theestimated 65% likelihood. Moreover, despite the verylow dark count rate, 39% of the total ToF singles in the Db range are dark counts, highlighting the impor-tance of α -decay correlations in such low-yield studies.To determine the atomic mass, we make use of a single-reference method [48] to evaluate the mass of an analyteion using only one species of reference ion. The mass-to-charge ratio of the analyte can then be related to thatof the reference by ( A/q ) analyte = ρ · ( A/q ) reference . Thevalue ρ is the actual experimental data, given by ρ = (cid:18) t analyte − t t reference − t (cid:19) , (1)where t represents some inherent delay between the ionsstarting their movement in the analyzer and the start ofthe clock, while t analyte and t reference are the times-of- (a)(b) c oun t s / . pp m ( ρ - )/ [ppm] -10 -5 0 5 10 Uncorrelated ToF singlesCorrelated ToF events C oun t s / ke V Decay energy [MeV]
All α singlesCorrelated α events E4, E10 E8 E3 E11 E2 E12 E5E13E14E1E6, E9E10 E9E2, E4E3 E5E6, E11 E14E12 E13 E1E7 E8E7 Db Coincidental α events Db Coincidental ToF events
FIG. 4. Singles spectra accumulated over the course of themeasurements for (a) α -decays above 7 MeV and (b) ToF inthe vicinity of Db in terms of ρ (see Eq. 1). The ToFregion spans ∆ m = ± . . Red and blue denoted sin-gles events represent Db decay chain correlated and Po α -decay coincident events, respectively. flight of the analyte and reference, respectively. Basedon ρ ( Rb + / Pb ++ ) measured at the end of theonline experiment, it was determined that t =75(4) ns.To exclude confusing an ion with significantly different A/q for our intended analyte ion, multiple spectra aretypically made for at two different numbers of oscillationsin the MRTOF-MS reflection chamber [28]. The times-of-flight t analyte and t reference would typically be determinedby fitting the analyte and reference ions’ spectra with aresponse function known to well-reproduce the data. Inthis work, however, it was not possible to perform suchfittings on the analyte spectral peaks as the number ofevents at any given number of laps did not exceed three.How to properly evaluate an atomic mass from suchdata is a non-trivial question. The asymmetry of the re-sponse function complicates things. However, as can beseen in Fig. 2, doubly-charged atomic ions exhibit higherresolving power and less rightward skew, likely an effectof Wiley-Mclaren optics [49] that we may assume to befurther enhanced in triply-charged ions. As such, thesimplest solution to determining an atomic mass from asmall number of single-ion ToF data would be to calcu-late the algebraic weighted average of each ion’s ρ -value.The uncertainty in the atomic mass could then be esti-mated by renormalizing the weighted average uncertaintyusing the Birge ratio [51] of the set.To test the feasibility of this, we used the data fromFig. 2 and calculated the weighted average ρ for everyconsecutive set of 10 Au ++ analyte ions, 3 358 sets in TABLE I. Summary of correlated ToF- α events, showing thenumber of times the Db reflects back-and-forth in theMRTOF-MS (laps), the observed α -particle energy and thetime between implantation and decay ( E α and ∆ t α , respec-tively), and the best estimate as to the nuclide which emit-ted the detected α -particle in each correlated event (Nuclide).The ρ column provides the evaluated ratio of mass-to-chargeratios for Db and Rb + , see text for details.Event laps E α [MeV] ∆ t α [s] Nuclide ρ E1 300 9.19 3.54
Db 1.009 314964(90)E2 300 8.14 105.00
Md 1.009308647(157)E3 300 8.02 18.50
Md 1.009309454(237)E5 325 9.00 0.70
Db 1.009309712(91)E6 325 9.35 1.30
Db 1.009309926(119)E8 324 7.81 44.00
Es 1.009319206(155)E9 324 9.35 0.36
Db 1.009307610(173)E11 327 8.08 43.40
Md 1.009309949(156)E12 327 8.77 4.30
Lr 1.009313092(150)E13 331 9.06 0.15
Db 1.009314345(148)E14 331 9.16 1.20
Db 1.009310844(144)Weighted Average 1.009311901(40)AME 2016 Value 1.00931286(84)Birge ratio 24.4Reweighted Average 1.009311901(973) total. The ToF of each Rb + reference was determinedby fitting a spectrum composed of reference ions accu-mulated 7.5 s before and after a given analyte ion. Itwas found that the most probable uncertainty for any10 ion set was δ ( ρ )=5.5 × − . A histogram of devia-tions from AME16 values, meanwhile, exhibited a highlyGaussian profile centered at ρ − ρ =2.9 × − with σ =6.7 × − . Based on this evaluation, we adopt thissingle-ion analysis methodology, which may be discussedmore fully in a future publication.The result of such an analysis for the Db is shown in Table I. After renormalizing the un-certainties, the weighted average A/q ratio for Db compared to Rb + was determined to be ρ =1.009 311 901(973) stat (7) sys , where the systematic un-certainty is derived from δt =4 ns. From this A/q ratiowe derive a mass excess of 100 063(231) stat (2) sys keV/c ,a 171(231) keV/c reduction in the binding energy ascompared to AME 2016 and in good agreement with ourprevious indirect mass determination.The determination of the atomic number from pre-cise mass measurements depends on predictions fromtheory, or extrapolations as available from the AMEframework [43]. In Fig. 5 we compare our result withglobal mass models obtained with various techniques andinclude models discussed previously for the Md mea-surements [15] among others: the shell model basedDZ10 [52, 53], macroscopic-microscopic models FRDM12[54] and WS4RBF [55], self-consistent mean-field modelsHFB21 [56] and HFB32 [57], Weizsacker-Bethe [58], andKTUY05 [59]. The models and the extrapolation fromAME16 cover wide bands of binding energies among the Mass Excess [MeV/c ]
96 98 100 102 104 106 108 Rf Db Sg DZ [52,53]WB03 [58]HFB21 [56]AME16 [43]KTUY05 [59]
WS4-RBF [55]
HFB32 [57]FRDM12 [54]This Work
FIG. 5. Mass excess determined in this work comparedwith mass excess values for A =257 isobars from various massmodels [52–59] and AME16 extrapolations [43]. It is visuallyclear that the measured nuclide is uniquely consistent with Z =105. different isobars. However, as the bands do not overlapacross adjacent isobars, these models allow us to deter-mine the atomic number Z =105 for Db with high sig-nificance using our new data. Similar distributions in themodels are found among sets of isobars near
Mc, mak-ing the present work a proof-of-principle demonstrationfor future efforts in the island of hot-fusion SHN.In this letter we have presented a new technique tomass analyze extremely low-yield species. Over thecourse of 5 days we observed ∼ Db correlated α -ToFevents, from which it was possible to determine the massof Db with a relative precision of δm/m =9.7 × − ,well beyond the level of precision required to determine Z by comparison with mass models. The basic tech-nique used to determine the atomic mass from a smallset of independent single-ion detections – arithmetic av-eraging with Birge ratio renormalization – was validatedby Au ++ data accumulated under the same condi-tions as the Db . After excluding spurious corre-lations between Po α -decay and ToF dark counts,we could determine the mass excess for Db to be100 063(231) stat (2) sys keV/c . This value is in goodagreement with our previous indirect mass determina-tion. Additionally, as it was observed that Db wasdominantly delivered from the gas cell as a triply-chargedion, we conclude that the third ionization potential ofdubnium must be less than 24.5 eV.The same techniques presented here will be used infuture measurements of nuclides in the hot-fusion super-heavy island to directly confirm their identities. Addi-tionally, we will change the target material to PbS infuture Db studies to allow an order of magnitude higherprimary beam intensity and allow for direct determina-tion of the atomic masses of , Db, which will providea direct analysis of the N = 152 sub-shell closure. In themore distant future, the technique may be applied toidentification of multi-nucleon transfer products. Such areaction may populate both sides of the valley of stability,making identification solely by mass spectroscopy moredifficult. However, by utilizing ToF correlated α -decayenergy measurements, it will be possible to distinguishbetween neutron-rich and neutron-deficient isobars.To better resolve isomeric states in future measure-ments, efforts are underway to improve the α -detectorenergy resolution. Similarly, an anticipated doubling ofthe mass resolving power of the MRTOF will allow a sim-ilar precision as presented herein to be achieved with asfew as 3 correlated α -ToF events in future measurements.We wish to express gratitude to the Nishina Centerfor Accelerator-Based Science at RIKEN and the Cen-ter for Nuclear Study at the University of Tokyo fortheir support of online measurements. This work wassupported by the Japan Society for the Promotion ofScience KAKENHI (Grant Numbers 2200823, 24224008,24740142, 15H02096, 17H06090, 19K03899, 18H03711,and 15K05116). ∗ [email protected][1] Yu. Ts. Oganessian, F. Sh. Abdullin, S. N. Dmitriev,J. M. Gostic, J. H. Hamilton, R. A. Henderson, M. G.Itkis, K. J. Moody, A. 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