We report on the observation of 67Kr that has been produced in an experiment

performed at the RIKEN/BigRIPS facility. The two-proton decay

of 67Kr has been evidenced and this nucleus is thus the fourth observed long

lived ground-state two-proton emitter, after 45Fe, 48Ni and 54Zn. In addition,

the decay of several isotopes in the mass region has been investigated.

While for previous cases of two-proton radioactivity, the theoretical models

could reproduce the measured data, this is not the case anymore for

67Kr. Two interpretations have been proposed to explain this discrepancy:

a transition between real two-proton and sequential decay or the influence

of deformation. These hypotheses will be tested in future experiments by

measuring the angular and energy correlations of the emitted proton

Blank, B.

Giovinazzo, J.

Goigoux, T.

Kiss, G. G.

English

Repository of the Academy's Library

Vol. 51 (2020) Acta Physica Polonica B No 3TWO-PROTON RADIOACTIVITY: THE INTERESTINGCASE OF 67Kr AND FURTHER STUDIES∗J. Giovinazzoa, T. Goigouxa, B. Blanka, P. AscheraM. Gerbauxa, S. Grévya, T. Kurtukian-Nietoa, C. MagronaP. Doornenbalb, N. Fukudab, N. Inabeb, G.G. Kissb,†, T. KubobS. Kubonob, S. Nishimurab, H. Sakuraib, Y. Shimizub, C. SidongbP.-A. Söderströmb, T. Sumikamab, H. Suzukib, H. TakedabP. Vib, J. Wub, D.S. Ahnc, J. Agramuntc, A. Algorac,dV. Guadillac, A. Montaner-Pizac, A.I. MoralescS.E.A. Orrigoc, B. Rubioc, Y. Fujitae, M. TanakaeW. Gelletlyc,f , P. Aguilerag, F. Molinag, F. Dielh, D. LubosiG. de Angelisj, D. Napolij, C. Borceak, A. Bosol, R.B. CakirlimE. Ganioglum, J. Chiban, D. Nishimuran, H. Oikawan, Y. TakeioS. Yagio, K. Wimmero, G. de Francep, S. Goq, B.A. BrownraCentre d’Études Nucléaires de Bordeaux Gradignan, FrancebRIKEN Nishina Center, Wako, Saitama, JapancInstituto de Física Corpuscular, Valencia, SpaindMTA Atomki, Debrecen, HungaryeDepartment of Physics, Osaka University, JapanfDepartment of Physics, University of Surrey, United KingdomgComisión Chilena de Energía Nuclear, Santiago, ChilehInstitute of Nuclear Physics, University of Cologne, GermanyiPhysik Department E12, Technische Universität München, GermanyjLaboratori Nazionali di Legnaro dell’INFN, ItalykNIPNE IFIN-HH, Bucharest-Măgurele, RomanialINFN Sezione di Padova and Dipartimento di Fisica, Universitá di Padova, ItalymDepartment of Physics, Istanbul University, Vezneciler, Istanbul, TurkeynDepartment of Physics, Tokyo University of Science, JapanoDepartment of Physics, University of Tokyo, JapanpGrand Accélérateur National d’Ions Lourds, Caen, FranceqDepartment of Physics and Astronomy, University of Tennessee, USArDepartment of Physics and Astronomy, NSCL, MSU, USA(Received January 7, 2020)∗ Presented at the XXXVI Mazurian Lakes Conference on Physics, Piaski, Poland,September 1–7, 2019.† Present address: Institute for Nuclear Research (ATOMKI), 4001 Debrecen, Hungary.(577)578 J. Giovinazzo et al.We report on the observation of 67Kr that has been produced in an ex-periment performed at the RIKEN/BigRIPS facility. The two-proton decayof 67Kr has been evidenced and this nucleus is thus the fourth observed longlived ground-state two-proton emitter, after 45Fe, 48Ni and 54Zn. In addi-tion, the decay of several isotopes in the mass region has been investigated.While for previous cases of two-proton radioactivity, the theoretical mod-els could reproduce the measured data, this is not the case anymore for67Kr. Two interpretations have been proposed to explain this discrepancy:a transition between real two-proton and sequential decay or the influenceof deformation. These hypotheses will be tested in future experiments bymeasuring the angular and energy correlations of the emitted protons.DOI:10.5506/APhysPolB.51.5771. IntroductionThe two-proton radioactivity is a very exotic decay mode that occursfor nuclei beyond the proton drip-line, with an even atomic number Z.Together with 1-proton radioactivity (for odd-Z nuclei), the phenomenonwas suggested in the 1960s by theoretical predictions [1]. The process hasbeen evidenced in the decay of 45Fe in experiments at GANIL/LISE3 [2]and GSI/FRS [3] in 2002. For candidate nuclei, the last two protons arenot bound by strong interaction, but the ground-state energy is below theCoulomb barrier, which gives the life-time of the emitter. In addition, dueto pairing, sequential emission of the protons is energetically forbidden, andthe correlated protons have to be emitted simultaneously. After the emis-sion by tunnel effect through the barrier, the sub-system (p–p, or 2He) isunbound.Today, 4 long-lived emitters (in the ms range) are known: 45Fe [2–5],48Ni [6, 7], 54Zn [8] that have been produced and studied at GSI, GANIL andNSCL, and recently 67Kr [9–11], in an experiment at the RIKEN/BigRIPSfacility that is reported in this paper. These nuclei have been producedat fragmentation facilities, and the first experiments were based on theimplantation-decay correlation technique in silicon detectors. This allowedfor an indirect observation of the two-proton radioactivity based on globalquantities: the half-life, the branching ratio and the energy of the decay.Calculations performed in a theoretical 3-body framework [12, 13] showedthat the p–p correlation pattern is sensitive to the orbital configuration ofthe nuclei. New devices have been developed, in order to measure the tracksof the individual particles in time projection detectors (TPC) and deducethe angular and energy correlations of the emitted protons [4, 5, 14].In this paper, we report some results from the experiment performed atRIKEN to produce nuclei at the proton drip-line in the 67Kr mass region.The exotic nuclei have been produced in the fragmentation of a 78Kr beamTwo-proton Radioactivity: the Interesting Case of 67Kr and Further Studies 579(250 nA, 350AMeV) on a beryllium target, and selected by the BigRIPS andthe ZDS separators [15]. They were implanted in the WAS3ABi [16] deviceconsisting of 3 DSSSDs (each detector is 1mm thick, with 60 vertical and 40horizontal strips of 1mm width). The charged particles emitted during theradioactive decay of implanted nuclei were detected in the silicon detectorsthat are surrounded by the EURICA [17] array of germanium detectors forthe coincident γ detection.New isotopes have been identified, and the decay of several nuclei havebeen investigated. In the case of 67Kr, we could conclude on two-proton emis-sion, in competition with β+/electron capture decay. For previous cases oftwo-proton radioactivity, the experimental data could be fairly well under-stood in available theoretical frameworks. In the case of 67Kr, these modelscould not reproduce the measured half-life, and this triggered new theoreticaldevelopments [18, 19] in two-proton radioactivity modeling. Further exper-imental studies should be carried out to confirm the proposed hypotheses,by measuring p–p correlations for this decay.2. New isotopesThe fragments were identified with the BigRIPS standard detection, fromenergy loss, time-of-flight and magnetic rigidity determination [11]. Theresult is presented in Fig. 1. 63Se, 68Kr and 67Kr have been observed forthe first time [9], with respectively 348, 477 and 82 counts at the F7 focalplane of the separator. Only a fraction of these nuclei is transmitted throughthe ZDS spectrometer to the implantation/decay detectors. This fraction isabout 40% for the most exotic fragments.Fig. 1. Fragment identification matrix from the BigRIPS detection. The circlesindicate the isotopes that have been observed for the first time.580 J. Giovinazzo et al.3. Beta-decay resultsFor ions implanted in the WAS3ABi detectors, the correlation of subse-quent decay events allows to determine the charged particles energy distribu-tion (with coincident γ rays) and the time distribution between implantationand decay events from which the decay half-life is estimated. The details ofthe analysis can be found in Ref. [11].TABLE IComparison of the half-lives and total beta-delayed proton emission ratios measuredfor the decay of several isotopes produced in the experiment. The results are takenfrom Ref. [11].Isotope Isospin Half-life T1/2 [ms] Total β-protonTZ branching [%]Previous Present Previous Present63Se −5/2 13.2± 3.9 89± 1159Ge 13.3± 1.7 93± 768Kr −2 21.6± 3.3 89+11−1069Kr −3/2 28± 1 [20] 27.8± 1.6 93+7−627± 3 [21] 99+1−11 [21]65Se 33± 4 [21] 34.2± 0.7 88+12−13 [21]61Ge 45± 6 [22] 40.7± 0.4 62± 4 [22] 87± 340± 15 [23]57Zn 48± 3 [22] 45.7± 0.6 78± 17 [22]40± 10 [24]55Cu 57± 3 [25] 55.5± 1.8 0 [25]27± 8 [26] 15.0± 4.3 [26]64As −1 72± 6 [27] 69.0± 1.418+43−7 [28]60Ga 76± 3 [29] 70.8± 2.070± 13 [28]70± 15 [30] 1.6± 0.7 [30]56Cu 80± 2 [29] 80.2± 0.782± 9 [28]93± 3 [31] 0.40± 0.12 [31]78± 15 [32]65As −1/2 126± 5 [27] 130.3± 0.6128± 16 [33]63Ge 156± 11 [29] 153.6± 1.1149± 4 [27]150± 9 [34]Two-proton Radioactivity: the Interesting Case of 67Kr and Further Studies 581The decay of 59Ge, 63Se and 67,68Kr has been measured for the first time.59Ge and 63Se were candidates for two-proton radioactivity, but no evidenceof such a decay branch could be observed in the data. In addition, the decayof several β-delayed proton or β emitters has been analyzed. The results aresummarized in Table I.When previous measurements are available, the half-lives are all in goodagreement with literature values. For the only proton emission branchingratio that was previously measured (61Ge decay), our value is 5 standarddeviations away from the previous value. The reason for this discrepancyis that in previous measurement, the proton emission at an energy belowthe main peak at 3.2MeV is not observed, resulting in an underestimatedbranching ratio. When considering only the proton emission from this peak,both measurements are in good agreement.4. Two-proton radioactivity of 67KrThe charged particle energy and time distributions for the decay of 67Krare presented in Fig. 2. The peak at 1690 keV is assigned to the two-protonemission branch (details in Ref. [10]). We obtained a half-life of 7.4± 2.9msfor the nucleus and a 2P branching ratio of 37±14%. The partial half-life isthus 10 ± 6ms for the β decay, in agreement with the value from the grosstheory [35], and 20± 11ms for two-proton emission.The two-proton half-life has been compared to a hybrid model [36] thatcombines the good dynamics of a 3-body model [37, 38] with the partialemission spectroscopic amplitudes from the configuration interaction model(details in Ref. [36]). While the calculated half-lives are in fairly good agree-ment (within a factor of 2) with the experimental values for the previouslystudied precursors (see also Refs. [14, 39]), in the case of 67Kr, the differ-ence is in the order of a factor 20. This result has triggered new theoreticalcalculations aiming to explain this large discrepancy.A first hypothesis is related to the decay mechanism. According to arecent semi-analytical R-matrix calculation [18], depending on the positionof the 66Br+p resonance, the decay could take place in a transition regionbetween a direct and a sequential two-proton emission. In such a case,the energy sharing between the emitted particles must be sensitive to thesequential component, with a contribution that is not centered on an equalsharing of the energy.Another possible reason of the discrepancy is that 67Kr is most likely lo-cated in a deformation region [40]. A calculation in the three-body Gamowcoupled-channel (GCC) framework resulted in a half-life of 24+10−7 ms [19],in agreement with the experiment, when considering a deformation of β2 '−0.3 due to the g9/2 orbital. The GCC calculation also predicts proton–proton angular correlations with different distributions whether the g9/2 or-bital is considered or not.582 J. Giovinazzo et al.Fig. 2. (Color online) The upper plot shows the charged particle energy distributionin the decay of 67Kr, measured in the detector where the implantation occurred.The black/blue histogram corresponds to the events with no coincident signal froma β particle in neighboring detectors. The peak at 1690 keV corresponds to the two-proton transition. The lower plot represents the time distribution of events afterimplantation. The fit (gray/red curve), taking into account the daughter decay ofthe different decay branches (two-proton and β), gives a half-life of 7.4± 2.9ms.5. Further studiesThe experimental perspectives for ground-state two-proton radioactivitystudies follow two main directions. The first one is to improve the knowl-edge on already identified emitters such as 48Ni, 54Zn and 67Kr for whichvery little or nothing is known about the correlations of emitted protons.The second direction is to search for new candidates, which implies to pro-duce and identify new isotopes and observe their decay modes. The protondrip-line between the known cases and the tin isotopes is of special interestbecause if other emitters are found, this would map a region between theclosed shells Z = 28 and Z = 50, crossing a deformation zone. This region ofthe table of isotopes should be reachable in a near future with the SuperFRSat GSI/FAIR.Two-proton Radioactivity: the Interesting Case of 67Kr and Further Studies 583The GCC calculation is performed for 67Kr, but also for the spherical case48Ni as a benchmark for the model. In both cases, the correlation pattern isnot measured, but this can be achieved using ACTAR TPC [41] as trackingdetector, in order to measure the angle and energy of individual protons.This device has been used in a recent experiment [42] at GANIL/LISE3where the proton radioactivity of isomers in 53Co and 54Ni has been suc-cessfully observed. An experimental program for the correlation measure-ment is proposed at GANIL/LISE3 for 48Ni [43] and at RIKEN/BigRIPSfor 67Kr [44].In the decay of 67Kr, the angular distribution will allow to discriminatebetween GCC predictions with or without the g9/2 orbital, but also to testthe sequential decay hypothesis by measuring the energy sharing of the pro-tons. It is also necessary to measure the angular correlations of the protonsin the decay of 48Ni because the predictions from the GCC model differ fromthe one that we can extrapolate from the 3-body model, available only for45Fe [4, 12] and for 54Zn [14].On the experimental side, in addition to the measurements of the proton–proton correlation pattern that will be performed for 48Ni and 67Kr, a higherstatistics would also be of great interest in the case of 54Zn for a detailed com-parison with available calculations. With the development of intense beamsand new facilities (such as FAIR/SuperFRS), the search for new emittersshould allow to reach the next closed shell at Z = 50.6. ConclusionDuring the experiment performed at the RIKEN/BigRIPS facility, weproduced nuclei at the proton drip-line in the region Z ≤ 36 and stud-ied their radioactive decay. From the analysis of the 67Kr decay data, wemeasured a 7.4 ± 2.9ms half-life with a two-proton radioactivity branch of37 ± 14%. The experimental results are in disagreement with expectationfrom available calculations. This lead to new theoretical works suggest-ing either a transitional situation between direct and sequential two-protonemission or the influence of deformation in order to reproduce the measuredhalf-life. These hypotheses will be addressed in further experiments foreseento be carried out with the tracking detector ACTAR TPC to measure theangular correlation and the energy sharing of the emitted protons.This experiment was carried out under programs No. NP0702-RIBF4R1-01, No. NP1112-RIBF82, and No. NP1112-RIBF93 at the RIBF operated byRIKEN Nishina Center, RIKEN and CNS, University of Tokyo. We wouldlike to thank the whole RIBF accelerator staff for the support during theexperiment and for maintaining excellent beam conditions with record in-584 J. Giovinazzo et al.tensities. This work was supported by the French–Japanese “Laboratoireinternational associé” FJ-NSP, by JSPS KAKENHI grant No. 25247045,by the Istanbul University Scientific Project Unit under project No. BYP-53195, by the Max-Planck Partner group, by the NSF grant No. PHY-1404442, by the UK Science and Technology Facilities Council (STFC) grantNo. ST/F012012/1, by the Japanese Society for the Promotion of Scienceunder grant No. 26 04808, by MEXT Japan under grant No. 15K05104,and by the Spanish Ministerio de Economia y Competitividad under grantsNo. FPA2011- 24553 and No. FPA2014-52823-C2-1-P, Centro de ExcelenciaSevero Ochoa del IFIC SEV-2014-0398; Junta para la Ampliación de Estu-dios Programme (CSIC JAE-Doc contract) cofinanced by FSE. This workwas supported by NKFIH (NN128072), by the New National Excellence Pro-gram of the Ministry for Innovation and Technology (UNKP-19-4-DE-65).G.G. 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TWO-PROTON RADIOACTIVITY: THE INTERESTING

CASE OF 67Kr AND FURTHER STUDIES

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TWO-PROTON RADIOACTIVITY: THE INTERESTING CASE OF 67Kr AND FURTHER STUDIES

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