Kiss, G. G.

Vitéz-Sveiczer, A.

English

Repository of the Academy's Library

Vol. 51 (2020) Acta Physica Polonica B No 3STUDYING THE EXOTIC DECAY 70Kr → 70Br∗A. Vitéz-Sveiczera,b,c,†, A. Algoraa,b,‡, A.I. Moralesa, B. RubioaG.G. Kissb,d,§, G. de Angelise, F. Recchiaf , S. NishimuradJ. Agramunta, V. Guadillaa, A. Montaner-Pizáa, S.E.A. OrrigoaA. Horváthc, D. Napolie, S. Lenzif , A. Bosof , V.H. Phongd, J. WudP.-A. Söderströmd, T. Sumikamad, H. Suzukid, H. Takedad, D.S. AhndH. Babad, P. Doornenbald, N. Fukudad, N. Inabed, T. Isobed, T. KubodS. Kubonod, H. Sakuraid, Y. Shimizud, S. Chend, B. BlankgP. Ascherg, M. Gerbauxg, T. Goigouxg, J. Giovinazzog, S. GrévygT. Kurtukián Nietog, C. Magrong, W. Gelletlya,h, Zs. DombrádibY. Fujitai,j, M. Tanakai, P. Aguilerak, F. Molinak, J. Eberthl, F. DiellD. Lubosm, C. Borcean, E. Ganiogluo, D. Nishimurap, H. OikawapY. Takeip, S. Yagip, W. Kortenq, G. de Francer, P. Daviess, J. LiutJ. Leet, T. Lokotkot, I. Kojouharovu, N. Kurzu, H. ShaffneruaIFIC (CSIC-University of Valencia), SpainbInstitute for Nuclear Research (Atomki), HungarycELTE Budapest, HungarydRIKEN Nishina Center, JapaneINFN Legnaro, ItalyfINFN Padova, ItalygCEN Bordeaux-Gradignan, FrancehDepartment of Physics, University of Surrey, UKiDepartment of Physics, Osaka University, JapanjResearch Center for Nuclear Physics (RCNP), Osaka University, JapankComisión Chilena de Energía Nuclear, Santiago, ChilelInstitute of Nuclear Physics, Universität zu Köln, GermanymPhysik Department, Technische Universität München, GermanynIFIN-HH, Bucarest, RomaniaoDepartment of Physics, University of Istanbul, TurkeypTokyo University of Science, JapanqCEA Saclay, FrancerGANIL, Caen, FrancesDepartment of Physics, York University, UKtDepartment of Physics, University of Hong Kong, ChinauGSI, Germany(Received January 14, 2020)∗ Presented at the XXXVI Mazurian Lakes Conference on Physics, Piaski, Poland,September 1–7, 2019.† Present address: University of Debrecen, Hungary.‡ Corresponding author: algora@ific.uv.es§ Corresponding author: ggkiss@atomki.mta.hu(587)588 A. Vitéz-Sveiczer et al.Beta-decay of the very neutron-deficient Kr isotope, 70Kr, was studiedat RIKEN-RIBF using the EURICA cluster array. The experiment signif-icantly increased our knowledge of the beta-decay of this isotope. Namely,16 new γ-ray transitions were identified and the half-life was derived fromtime correlations of the beta particles (tiβ1/2 = (44.99± 0.16) ms) and fromthe decay curves of the observed γ-ray transitions (tiβγ1/2 = (45.16±0.71) ms),respectively.DOI:10.5506/APhysPolB.51.5871. IntroductionType I X-ray bursts, characterized by Tpeak = 1–3 GK and ρ = 106–107 g cm−3, are suggested as possible sites for the astrophysical rp-process[1–3]. In these binary systems, the hydrogen-rich material accretes from alow-mass star, typically from a Main Sequence or a Red-Giant star, towarda neutron star. When the temperature and density in the accreted envelopebecome high enough to allow breakout from the hot CNO cycle, thermonu-clear ignition takes place and via a series of rapid proton capture reactions,isotopes up to 105Te are formed [4]. As the nucleosynthesis path approachesthe drip-line, further proton capture is inhibited by strong reverse photodis-integration reactions or by direct proton emission, and the reaction flow hasto wait for the relatively slow β-decay process. According to this picture,the modeling of the rp-process requires precise knowledge of the β-decayhalf-lives and the structure of the involved isotopes.Presently, very little is known about the β-decay of 70Kr: its lifetimehas been determined at CERN ISOLDE by measuring the time distributionof the emitted β particles [5]. Recently the β-decay of 70,71Kr was investi-gated at RIKEN Nishina Center. In the present contribution, details on thedetermination of the half-life of the 70Kr isotope are presented.2. Experimental setupThe 70Kr nuclei were produced at RIKEN Nishina Center by fragmenta-tion of a 78Kr primary beam, with 345 MeV/nucleon energy and ∼ 40 pnAaverage intensity, impinging on a 5 mm thick 9Be target. The nuclei ofinterest were separated and identified by measuring event-by-event the en-ergy loss (∆E), magnetic rigidity (Bρ) and time-of-flight (TOF) of the ions,in the BigRIPS spectrometer [6]. Figure 1 shows a particle identificationmatrix, which was used to correlate implantation and decay events.The fragments were transmitted to the exit of the ZeroDegree spectrome-ter where a setup for decay studies was installed. It consisted of the EURICAgermanium cluster detector array [7] surrounding the WAS3ABi double-sided silicon strip detector (DSSSD) system [8]. The three DSSSD detectorsStudying the Exotic Decay 70Kr → 70Br 589Fig. 1. Identification plot of ions implanted in WAS3ABi, showing the atomicnumber Z versus the mass-to-charge ratio A/Q of the ions.used in the present experiment were segmented into 60 vertical (X) and 40horizontal (Y ) strips, corresponding to 2400 pixels with an active area of1 × 1 mm2 each. The full range and the resolution of the X and Y stripswere 5 MeV/25 keV and 10 MeV/30 keV, respectively.EURICA is an array of twelve EUROBALL cluster germanium detectors,each containing seven crystals. The average distance between the center ofthe DSSSDs and the front face of the HPGe detectors is 22 cm, correspondingto a total absolute detection efficiency of about 8% at the energy of 1332 keV.3. Event selectionThe event separation criteria are discussed in detail in Ref. [9], here onlya brief summary is given. Namely, we identified events as an implantationof a 70Kr nucleus when a high-energy signal in the plastic scintillator, lo-cated after the F11 focal plane of Zero Degree spectrometer, is detectedtogether with an overflow energy signal in a DSSSD, but without an energysignal from the plastic veto detector placed behind WAS3ABi. According tothese requirements, about 1.6× 106 70Kr nuclei were implanted into the 2ndDSSSD. Each implantation was followed, on average, by saturated signals of3.4 X strips and 4.3 Y strips, respectively. The strip, where the implanta-tion took place, was identified using the time information of the strips thatfired in an implantation event. For that, the strip with the fastest signalwas used. Then the selected implantation events in WAS3ABi were corre-590 A. Vitéz-Sveiczer et al.lated in time with decay events taking place in the same strips where theimplantation event was observed. The β-delayed γ-ray energy spectra weremeasured by the EURICA array and the events were selected using the fol-lowing conditions: the maximum time difference between an implantationand the corresponding detection of a β particle was fixed to 20 s and γ rayswere recorded up to 800 ns after the detected β events.4. Determination of the half-lifeThe half-life of 70Kr was derived first from the time distribution ofimplantation-β (i-β) correlations (without any conditions on γ rays). Thei-β histogram was fitted with a function including the exponential β-decayand a fixed background extracted from a linear fit to the backward-timedistribution of i-β correlations. Figure 2 shows the time distribution of i-βcorrelations, the lines correspond to the activity of 70Kr (dashed/green),70Br (dotted/blue) and to the background (long-dashed/red), respectively.The resulting half-life from i-β correlations ist1/2 =(44.99± stat0.14 ± sys0.06)ms = (44.99± 0.16) ms .Fig. 2. (Color online) Time distribution of implantation-β correlations. The fittingfunction includes the Bateman formula and a linear background, determined froma fit to the negative-time distribution.The background-subtracted β-delayed γ-ray energy spectrum, measuredwith the EURICA array, is shown in Fig. 3. The Eγ = 933 keV transitionwas already assigned to the de-excitation of the 1st excited state of 70Br [10].Several other γ-ray transitions are attributed to the β-decay of 70Kr. Theβ(γ–γ) coincidence spectrum gated on the Eγ = 933 keV γ-ray is shown inFig. 4.Studying the Exotic Decay 70Kr → 70Br 591Fig. 3. (Color online) Compton-suppressed γ-ray spectrum. The nine strongestγ rays used to derive the half-life are labeled, and the corresponding energies aregiven in keV. They were assigned to the β-decay of 70Kr as stated in the text. Aspectrum tagged with β particles with negative time correlations with respect toimplantation is shown in dark gray/red. These events are interpreted as a possiblecontamination since they are uncorrelated to implantations.Fig. 4. The energy spectrum of γ-ray transitions in coincidence with the Eγ =933 keV transition. The transitions in coincidence with the Compton backgroundsampled around the Eγ = 933 keV peak were subtracted from the spectrum afternormalization in order to evaluate the effect of coincidences uncorrelated with thepeak, on which the gate was taken. The labeled peaks were all verified via gatingback on them as well, and looking for significant Eγ = 933 keV coincidences in thespectrum.The half-life of 70Kr was also extracted from the decay curve of the592 A. Vitéz-Sveiczer et al.strong β-delayed γ-ray transitions observed at Eγ = 933 keV, 1120 keV,1493 keV, 1574 keV, 1630 keV, 2230 keV, 2305 keV, 2508 keV and 2563 keV.These transitions were first used one-by-one to build decay curves, usingimplantation-β–γ (i-β–γ) time correlations. Then exponential functionswith constant background were fitted using maximum likelihood methodto obtain half-lives corresponding to the different transitions. This informa-tion was used to verify that the γ rays belong to the decay of 70Kr. Finally,these decay curves were summed to increase statistics. Figure 5 shows thetime distribution of the i-β–γ correlations and the fitting functions. System-Fig. 5. (Color online) The upper curve shows the β-decay time spectra of 70Krbased on the measurement of i-β–γ correlations. The forward and backward timedistributions are indicated with gray/blue and light gray/yellow bars, black andwhite dashed lines show the fitted exponential and the linear background, respec-tively. The lower figure shows the normalized residual of the fit, which should befluctuating randomly with a Gaussian distribution around zero.atic uncertainties were investigated by varying the fit parameters. Figure 6shows the resulting 70Kr half-life as the function of the varied fitting param-eters (such as bin size, fit ranges and background). The resulting half-lifefrom i-β–γ correlations ist1/2 =(45.16± stat0.68 ± sys0.20)ms = (45.16± 0.71) ms .Studying the Exotic Decay 70Kr → 70Br 593Fig. 6. Error contributions to the half-life of 70Kr: impact of the bin size, fit andbackground ranges on the fitted half-life values.5. SummaryUsing β–γ and β–γ–γ correlations 16 new γ-ray transitions, assigned tothe β-decay of 70Kr were identified, and the half-life was derived from thetime correlations of the β particles (tiβ1/2 = (44.99± 0.16) ms) and the decaycurves of the observed γ-ray transitions (tiβγ1/2 = (45.16± 0.71) ms).We want to express our gratitude to the RIKEN accelerator staff forproviding a stable and high-intensity 78Kr beam during the experiment, theEUROBALL Owners Committee for the loan of germanium detectors, andthe PRESPEC Collaboration for the readout electronics of the cluster de-tectors. Part of the WAS3ABi was supported by the Rare Isotope ScienceProject, which is funded by the Ministry of Science, ICT and Future Plan-ning (MSIP) and National Research Foundation (NRF) of Korea. Supportfrom the Spanish Ministerio de Economía, Industria y Competitividad undergrants No. IJCI-2014-19172, No. FPA2011-24553, No. FPA2014-52823-C2-1-P and No. FPA2017-83946-C2-1-P, Centro de Excelencia Severo Ochoa delIFIC SEV-2014-0398, and Junta para la Ampliación de Estudios Programme(CSIC JAE-Doc contract) co-financed by FSE is acknowledged. This workwas also financed by Programmi di Ricerca Scientifica di Rilevante Inter-esse Nazionale (PRIN) No. 2001024324 01302, by JSPS KAKENHI grant594 A. Vitéz-Sveiczer et al.No. 25247045, 2604808, by the Chilean FONDECYT projects INICIACIONNo. 11130049 and REGULAR No. 1171467, by the Istanbul University Scien-tific Project Unit under Project No. BYP-53195, by the Max-Planck Partnergroup, by the NSF grant No. PHY-1404442, by the UK Science and Technol-ogy Facilities Council (STFC) grant No. ST/F012012/1, by MEXT Japanunder grant No. 15K05104, and by the French–Japanese “Laboratoire inter-national associé” FJ-NSP. This work was supported by NKFIH (NN128072),by the New National Excellence Program of the Ministry for Innovation andTechnology (ÚNKP-19-4-DE-65). G.G. Kiss acknowledges support form theJános Bolyai research fellowship of the Hungarian Academy of Sciences.REFERENCES[1] R.K. Wallace, S.E. Woosley, Astrophys. J. Suppl. Ser. 45, 389 (1981).[2] H. Schatz et al., Phys. Rep. 294, 167 (1998).[3] A. Parikh et al., Prog. Part. Nucl. Phys. 69, 225 (2013).[4] H. Schatz et al., Phys. Rev. Lett. 86, 3471 (2001).[5] M. Oinonen et al., Phys. Rev. C 61, 035801 (2000).[6] N. Fukuda et al., Nucl. Instrum. Methods Phys. Res. B 317, 323 (2013).[7] P.-A. Söderström et al., Nucl. Instrum. Methods Phys. Res. B 317, 649(2013).[8] S. Nishimura, Prog. Theor. Exp. Phys. 2012, 03C006 (2012).[9] A.I. Morales et al., Phys. Rev. C 95, 064327 (2017).[10] G. de Angelis et al., Eur. Phys. J. A 12, 51 (2001).

STUDYING THE EXOTIC DECAY 70Kr -> 70Br

http://real.mtak.hu/112987/1/70Kr_VSA_acta_phys_pol.pdf

STUDYING THE EXOTIC DECAY 70Kr -> 70Br

Abstract

Similar works

Full text

Available Versions

Repository of the Academy's Library