Measurement of the heaviest Beta-delayed 2-neutron emitter: 136Sb

EPJ Web of Conferences 146, 01005 (2017) DOI: 10.1051/epjconf/201714601005 ND2016 Measurement of the heaviest β-delayed 2-neutron emitter: 136Sb R. Caballero-Folch1,2,a, I. Dillmann2,3,b,J.L.Ta ´ ın4, J. Agramunt4, C. Domingo-Pardo4, A. Algora4,J. ¨ Ayst¨ o5, F. Calvi˜ no2, L. Canete6,G.Cort ` es2, T. Eronen6, E. Ganioglu7, W. Gelletly4,8,D.Gorelov 6, V. Guadilla4, J. Hakala6, A. Jokinen6, A. Kankainen6, V. Kolhinen6, J. Koponen6,M.Marta 3, E. Mendoza9, A. Montaner-Piz´ a4,I.Moore 6, Ch. Nobs10, S. Orrigo4, H. Penttil¨ a6, I. Pohjalainen6, J. Reinikainen6,A.Riego 2, S. Rinta-Antila6, B. Rubio4, P. Salvador-Casti˜ neira2, V. Simutkin6, and A. Voss6 1TRIUMF, Vancouver BC, V6T 2A3, Canada 2Universitat Polit` ecnica de Catalunya, 08028 Barcelona, Spain 3GSI Helmholtz Center for Heavy Ion Research, 64291 Darmstadt, Germany 4IFIC, CSIC-Universitat de Val` encia, 46071 Val` encia, Spain 5University of Helsinki, Helsinki, Finland 6University of Jyvaskyla, Dept. of Physics, PO Box 35 YFL, 40014 University of Jyvaskyla, Finland 7University of Istanbul, 34134 Vezneciler, Turkey 8University of Surrey, Guildford GU2 7XH, UK 9CIEMAT, 28040 Madrid, Spain 10 University of Brighton, BN2 4AT Brighton, UK Abstract.The β-delayed neutron emission probability, Pn, of very exotic nuclei is crucial for the understanding of nuclear structure properties of many isotopes and astrophysical processes such as the rapid neutron capture process (r-process). In addition β-delayed neutrons are important in a nuclear power reactor operated in a prompt sub-critical, delayed critical condition, as they contribute to the decay heat inducing fission reactions after a shut down. The study of neutron-rich isotopes and the measurement of β-delayed one-neutron emitters (β1n) is possible thanks to the Rare Isotope Beam (RIB) facilities, where radioactive beams allow the production of exotic nuclei of interest, which can be studied and analyzed using specific detection systems. This contribution reports two recent measurements of β-delayed neutron emitters which allowed the determination of half-lives and the neutron branching ratio of isotopes in the mass region above A = 200 and N >126, and a second experiment which confirmed 136Sb as the heaviest double neutron emitter (β2n) measured so far. 1. Introduction The study of the decay modes of unstable nuclei is an important input to nuclear databases. It provides information needed to develop applications, improve simulations and calculations for theoretical models, and to understand the properties of atomic nuclei experimentally. In the neutron-rich region, β−-decay is the dominant mode, however β-delayed one-neutron emission, discovered by Roberts et al. [1] in 1939, becomes the dominant decay channel for very neutronrich isotopes. The importance in measuring the decays of neutron-emitting nuclei in certain regions of the chart of nuclides has been emphasized by international data evaluators experts [2,3] and by theoreticians, who need more realistic inputs for levels above the neutron separation energy, Sn, for their nuclear structure models, and to reproduce astrophysical environments where the r-process nucleosynthesis could take place [4–8]. In order to obtain new experimental β-delayed neutron emission probabilities, Pn-values, a new high efficiency 4πneutron detector named BELEN [9,10] has been designed and ae-mail: [email protected] be-mail: [email protected] developed. Since 2009, BELEN has allowed the study of the β-delayed neutron emission of many species, some of them in regions with very little nuclear data information. This study presents two measurements performed at the GSI Helmholtz Center for Heavy Ion Research using the Fragment Separator (FRS) in Darmstadt (Germany) [11] and at the JYFL Accelerator Laboratory of the University of Jyv¨ askyl¨ a (Finland) using the JYFLTRAP Penning Trap [12] at the IGISOL-4 facility. At GSI it was possible to produce the heaviest β1n emitters measured so far in the region of Hg and Tl with N>126, and in Jyv¨ askyl¨ a the first β2n emitter measured above A =100, 136Sb, was extracted without isobaric contamination. Both experiments provided new P1n- and P2n-values for the isotopes studied. 2. β-delayed one neutron emission beyond A >200 and N >126 Apart from a single measurement of 210Tl in the sixties [13,14], the available information concerning P1n-emitters reached only to masses below A =150 [15–17]. In this nuclear data conference we presented the results of a measurement in which we obtained, c The Authors, published by EDP Sciences. This is an Open Access article distributed under the terms of the Creative Commons Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/). EPJ Web of Conferences 146, 01005 (2017) DOI: 10.1051/epjconf/201714601005 ND2016 for the first time, the P1n-values in the region beyond A =200 and with N >126 for several isotopes [18]. These included the half-lives of 18 isotopes (204–206Au,208–211Hg,211–216Tl,215–218Pb,218–220Bi), nine of them for the first time, and the neutron branching ratios for those which are energetically allowed neutron emitters (210–211Hg,211–216Tl). This experiment was performed in 2011 using the accelerator complex of GSI and the FRS facility with similar settings used previously in experiments around this region [19,20]. Nuclei from Pt (Z =78) to Fr (Z =87) were identified thanks to a precise time-of-flight measurement and ionization chambers on an event-by- event basis. This region comprises the aforementioned species of interest in the study with positive Qβn-values and candidates for neutron emission [21]. The version of the BELEN detector for this experiment consisted of two rings of 3He counters, 10 in the inner and 20 in the outer ring, embedded in a polyethylene matrix. The neutron detector was placed at the end of the FRS beamline surrounding the Silicon IMplantation Beta Absorber (SIMBA) detector [22,23]. SIMBA is a stack of stripped silicon detectors based on SSSD and DSSSD technology, and was used to detect the implants and the decays of the nuclei, allowing the correlation analysis between implanted ions, β-decays and neutrons. The results from this measurement are published in Ref. [18], and a more detailed publication has also been submitted. 3. β2n emission of 136Sb The BELEN detector has also been used several times at the IGISOL facility in Jyv¨ askyl¨ a (Finland) between 2009 and 2014 [10,24]. This facility can produce radioactive species of exotic nuclei by impinging a 25 MeV proton beam on a Uranium target. The reaction fragments are stopped in a buffer gas cell, extracted and accelerated towards a mass separator magnet, which separates nuclei according to the mass-over-charge, A/q, with a mass resolution of approximately 350. The specific isotope of interest is isolated using the JYFLTRAP Penning Trap system (see top of Fig. 1)[12,25]. The extraction of the isotopically pure beam, from the JYFLTRAP to the implantation onto a moving tape close to the β-decay plastic detector located at the end of the beam-line, was through a tube in vacuum, which was surrounded by BELEN (see bottom of Fig. 1). In this experiment, the high-efficiency version of the BELEN detector consisted of 48 3He proportional counters distributed in 3 rings. We emphasize the high and constant neutron efficiency around ε1n=60% (ε2n=36% for β2n emission) over the energy range of interest (see Fig. 2). In addition, an innovative self-triggered digital data acquisition system was specifically developed for BELEN [26]. This allows the registration of the neutrons coming after a β-decay, including multiple neutron emission (β2n) events. In this conference contribution we present a measurement of 136Sb with the aim of determining its double neutron emission probability (P2n-value). Apart of this isotope, in the latest experimental campaign at the IGISOL facility, many of the high-priority P1n- values from the IAEA list [3] were remeasured with high statistics [24]. In the particular case of 136Sb, the motivation for measuring this isotope relates to the fact Figure 1. Top view of the experimental setup: IGISOL JYFLTRAP at the top of the image. BELEN detector at the bottom. Figure 2. Simulation of the neutron efficiency along the energy range for BELEN. Curves represent the efficiency contribution of each ring and the upper one is the total efficiency. that it is present in the freeze-out of the r-process and contributes to the second abundance peak around N=82, in the A∼130 mass region. Furthermore it is present in the fission mass distribution of several actinides. A preceding measurement of the A =136 isobaric chain suggested this isotope to be a β2n emitter [27]. Although the contamination from other isobars, the double neutron emission branching ratio was determined to be P2n= 1.4±0.2%, far from the theoretical model predictions available from FRDM+QRPA which gives a branching ratio of P2n=6.2% [28]. In this experimental campaign the β1n branching ratios for the P1nstandards 95Rb and 137I were also measured, and they were used for the calibration of our system. We also confirmed the P1n-value of 136Te, which is the β-decay daughter. Since this isotope is present in the decay chain of 136Sb, an accurate determination of its neutron branching ratio is needed for the data analysis. Our preliminary value for 136Te is in agreement with the ones previously reported in Refs. [16,17]. Although the data analysis is ongoing, it 2 EPJ Web of Conferences 146, 01005 (2017) DOI: 10.1051/epjconf/201714601005 ND2016 s)µNeutron moderation time ( 40000−20000−0 20000 40000 Counts 0 1 2 3 4 5 6 First neutron Second neutron Figure 3. βnn correlation events. In blue is shown the correlation with the first neutron and in red the correlation with the second neutron emitted. Figure 4. Left: left downstream side of the DESCANT detector coupled with the GRIFFIN germanium array. Right: BRIKEN detector setup (July 2016). can be confirmed that a new P1n-value will be reported for 136Sb and that it is also a double neutron emitter. In Fig. 3 the β2n correlations observed during 6 days of beamtime are shown. A total of 26 βnn net correlated events yield a two neutron branching ratio of P2n<1%. This is a lower value than those mentioned above. 4. Upcoming βn measurements In the next few years, new measurements of β-delayed neutron emitting isotopes are planned at the RIB facilities of TRIUMF, Vancouver BC (Canada) and RIKEN, Wako-shi, Saitama (Japan). At TRIUMF, the DEUterated Scintillator array for Neutron Tagging (DESCANT) detector [29,30] is ready to measure βn events. The system allows one to measure βn events in correlation with the γ-rays, from the excited states populated in the daughter nuclei, with the GRIFFIN germanium array [31,32](see Fig. 4-left). At RIKEN, the BRIKEN campaign [33]is expected to measure the most exotic neutron emitters ever produced, including dozens of β1n, several β2n and, for the first time, β3n emitters in the heavy mass region A>50. Currently there are three experiments approved in the regions around A =130 [34], the doubly-magic 78Ni [35] and in the region A=100 ∼125 [36]. The BRIKEN array is presently under commission and will take first data in the fall of 2016. A picture of the setup is shown in Fig. 4-right. This work is supported by the Spanish Ministerio de Econom´ ıa y Competitividad under grants CPAN CSD-2007- 00042 (Ingenio2010), FPA2008-04972-C03-03, FPA2008-06419, FPA2010-17142, FPA2011-28770-C03-03, FPA2011-24553, FPA2014-52823-C2-1-P, FPA2014-52823-C2-2-P and the program Severo Ochoa (SEV-2014-0398). It is also supported by the Academy of Finland under the Finnish Centre of Excellence Programme 2012–2017 (Project No. 213503, Nuclear and Accelerator-Based Physics Research at JYFL), and by the European Commission under the FP7/EURATOM contract 605203. I.D. and M.M. acknowledge the support of the German Helmholtz Association via the Young Investigators Grant No. VH-NG 627. W.G. acknowledges the support of the UK Science Technology Faculties Council (STFC) under grant No. ST/F012012/1 and the University of Valencia. 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