Fragmentation reactions of relativistic 238U and 208 Pb projectiles have been used to investigatethe production of heavy neutron-rich nuclei approaching the r-process waiting point at A≈195.The relativistic energies, together with the use of a high resolving-power magnetic spectrometerwere key conditions for the unambiguous identification of nuclei in the region of interest. Usingthis technique we were able to identify 73 new heavy neutron-rich nuclei expanding considerablythe north-west frontier of the chart of nuclide. Moreover, we were able to determine the halflives of 13 of those nuclide. The measured values are signific antly shorter than the predictionsused for r-process model calculations. The confirmation of these results for r-process nuclei atA≈195 would indicate that the r-process at this point is faster than expected, leading to a largerproduction of the heaviest nuclei

ALVAREZ-POL, H.

AUDOUIN, L.

BECKER, F.

BENLLIURE, J.

BLANK, Bertram

BORZOV, I.

CASAREJOS, E.

CORTINA-GIL, D.

ENQVIST, T.

FERNANDEZ, B.

GIOVINAZZO, J.

HENZLOVA, D.

JURADO, B.

KURTUKIAN NIETO, T.

LANGANKE, K.

MARTÍNEZ-PINEDO, G.

MORALES, A.I.

NAPOLITANI, P.

PEREIRA, J.

REJMUND, F.

SCHMIDT, K.H.

YORDANOV, O.

GIOVINAZZO, Jerome

JURADO APRUZZESE, Beatriz

Oskar Bordeaux

Production and $\beta$ half-lives of heavy neutron-rich nuclei approaching the r-process path at N=126

International audienceFragmentation reactions of relativistic 238U and 208 Pb projectiles have been used to investigatethe production of heavy neutron-rich nuclei approaching the r-process waiting point at A≈195.The relativistic energies, together with the use of a high resolving-power magnetic spectrometerwere key conditions for the unambiguous identification of nuclei in the region of interest. Usingthis technique we were able to identify 73 new heavy neutron-rich nuclei expanding considerablythe north-west frontier of the chart of nuclide. Moreover, we were able to determine the halflives of 13 of those nuclide. The measured values are signific antly shorter than the predictionsused for r-process model calculations. The confirmation of these results for r-process nuclei atA≈195 would indicate that the r-process at this point is faster than expected, leading to a largerproduction of the heaviest nuclei

Benlliure, J.

Alvarez-Pol, H.

Kurtukian Nieto, T.

Morales, A.I.

Casarejos, E.

Cortina-Gil, D.

Pereira, J.

Becker, F.

Borzov, I.

Enqvist, T.

Henzlova, D.

Langanke, K.

Martínez-Pinedo, G.

Schmidt, K.H.

Yordanov, O.

Blank, Bertram

Giovinazzo, J.

Jurado, B.

Audouin, L.

Napolitani, P.

Fernandez, B.

Rejmund, F.

HAL-IN2P3

Production and β half-lives of heavy neutron-rich nucleiapproaching the r-process path at N=126J. Benlliure, H. Alvarez-Pol, T. Kurtukian-Nieto, A.I. Morales, E. Casarejos,D. Cortina-Gil, J. Pereira, F. Becker, I. Borzov, T. Enqvist, et al.To cite this version:J. Benlliure, H. Alvarez-Pol, T. Kurtukian-Nieto, A.I. Morales, E. Casarejos, et al.. Produc-tion and β half-lives of heavy neutron-rich nuclei approaching the r-process path at N=126.Proceedings of the 11th Symposium on Nuclei in the Cosmos, Jul 2010, Heidelberg, Germany.NIC XI, pp.084, 2010. <in2p3-00672525>HAL Id: in2p3-00672525http://hal.in2p3.fr/in2p3-00672525Submitted on 21 Feb 2012HAL is a multi-disciplinary open accessarchive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come fromteaching and research institutions in France orabroad, or from public or private research centers.L’archive ouverte pluridisciplinaire HAL, estdestine´e au de´poˆt et a` la diffusion de documentsscientifiques de niveau recherche, publie´s ou non,e´manant des e´tablissements d’enseignement et derecherche franc¸ais ou e´trangers, des laboratoirespublics ou prive´s.PoS(NIC XI)084Production and β half-lives of heavy neutron-richnuclei approaching the r-process path at N=126J. Benlliure∗†, H. Alvarez-Pol, T. Kurtukian-Nieto‡, A.I. Morales, E. Casarejos, D.Cortina-Gil, J. PereiraUniversidad de Santiago de Compostela, 15782 Santiago de Compostela, SpainF. Becker, I. Borzov, T. Enqvist, D. Henzlova, K. Langanke, G. Martínez-Pinedo,K.-H. Schmidt, O. YordanovGSI Helmholtzzentrum für Schwerionenforschung GmbH, Planckstr. 1, D-64291 Darmstadt,GermanyB. Blank, J. Giovinazzo, B. JuradoUniversité Bordeaux I, CNRS/IN2P3, CENBG, BP 120, F-33175 Gradignan, FranceL. Audouin, P. NapolitaniIPN, IN2P3-CNRS, Université Paris-Sud 11, UMR 8608, F-91406 Orsay, FranceB. Fernández, F. RejmundGANIL CEA/DSM-CNRS/IN2P3, BP 55027, F-14076 Caen, Franceand the RISING collaborationFragmentation reactions of relativistic 238U and 208Pb projectiles have been used to investigatethe production of heavy neutron-rich nuclei approaching the r-process waiting point at A≈195.The relativistic energies, together with the use of a high resolving-power magnetic spectrometerwere key conditions for the unambiguous identification of nuclei in the region of interest. Usingthis technique we were able to identify 73 new heavy neutron-rich nuclei expanding considerablythe north-west frontier of the chart of nuclide. Moreover, we were able to determine the halflives of 13 of those nuclide. The measured values are significantly shorter than the predictionsused for r-process model calculations. The confirmation of these results for r-process nuclei atA≈195 would indicate that the r-process at this point is faster than expected, leading to a largerproduction of the heaviest nuclei.11th Symposium on Nuclei in the Cosmos19-23 July 2010Heidelberg, Germany.c© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/PoS(NIC XI)084Production and beta half-lives of heavy neutron-rich nuclei approaching the r-process path at N=126J. Benlliure1. IntroductionOne of the most challenging contributions of nuclear physics to the understanding of the stel-lar nucleosynthesis r-process is probably the production and determination of ground properties ofr-process nuclei at the N=126 shell closure. During the last years, existing radioactive beam facil-ities helped us to progress a lot in the production and characterization of medium-mass r-processnuclei [1, 2, 3]. However, the region of heavy neutron-rich nuclei remained almost unexploreddespite its interest. This region plays an important role in fundamental nuclear structure investiga-tions, because of the presence of the neutron shell N=126, but it is also extremely relevant for thecharacterization of the r-process at the waiting point around A≈195. Indeed, the r-process matterflow towards the heaviest elements is to a large extend determined by the half lives of the r-processnuclei at the N=126 waiting point. Unfortunately, present model descriptions of the beta half livesof heavy neutron-rich nuclei are rather discordant [5, 6]. It was then the purpose of this work toinvestigate the production of heavy neutron-rich nuclei but also to measure the β half lives of someof them in order to benchmark the existing theoretical predictions.2. Production of heavy neutron-rich nucleiIn order to produce heavy neutron-rich nuclei we proposed to use fragmentation reactions ofrelativistic projectiles. Although fragmentation reactions produce mostly neutron-deficient nuclei,the large fluctuations in the nature of the abraded nucleons (protons or neutrons) and in the ex-citation energy gained in the collision make it possible to populate cold fragmentation reactionchannels. These reaction channels are characterized by the abrasion of mostly protons while theexcitation energy gained in the process is rather low, limiting then the evaporation of neutrons. Itwas shown some years ago [7] that these reaction channels produce the most neutron-rich nucleiin fragmentation reactions, in particular proton-removal channels where only protons are abradedand no neutrons are evaporated.In this particular case we used beams of 238U and 208Pb projectiles accelerated at 1 A GeV atthe SIS18 synchrotron in GSI. Pulsed beams with a typical bunch length of 3 seconds a repetitionrate of 10 seconds and a typical intensity of ≈108 ions/s where extracted from the accelerator anddirected to a 2 g/cm2 thickness beryllium target. Projectile fragments, flying forward because of thekinematical conditions, were analyzed with the magnetic spectrometer FRagment Separator (FRS)[8]. This is a zero-degree magnetic spectrometer with two symmetric sections in order to preservethe achromatism of the system. A profiled energy degrader placed at the intermediate image planewas fundamental for the separation of the transmitted nuclei [9] and the identification of atomiccharge states. The nuclei traversing the spectrometer were identified by determining their magneticrigidity, velocity and atomic number. A description of the identification method for these nucleican be found in [10, 11].In figure 1 we represent on top of chart of nuclide all nuclei produced in this work. As canbe seen we were able to produce 73 new neutron-rich isotopes of elements between Ytterbium and∗Speaker.†E-mail: j.benlliure@usc.es‡Present address: Centre d’Etudes Nuclèaires Bordeaux, Gradignan, France2PoS(NIC XI)084Production and beta half-lives of heavy neutron-rich nuclei approaching the r-process path at N=126J. BenlliureNZ82126 233Fr232Rn230At227Po224Bi220Pb217Tl215Hg210Au205Pt203Ir201Os198Re195W193Ta190Hf187Lu184Yb> 10 µb>  1 µb>  100 nb> 10 nb> 1 nb> 100 pb> 10 pbFigure 1: Nuclei produced in this work on top of a chart of nuclide, the color scale represents intervals ofproduction cross section. Black squares correspond to stable nuclei. The dotted line represents the limit ofknown nuclei as in 1958 and the thick-solid line the limit of known nuclei before these experiments. Thehatched area corresponds to an estimation of the path followed by the stellar nucleosynthesis r-process.Francium. Some of these results were previously reported in [12, 13]. The present results constitutea major step towards the production of nuclei at the A≈195 r-process waiting point. Indeed in Fig.1 the dotted line indicates the limits of the chart of nuclide as in 1958. Therefore, the presentexperiment allowed us to produce in few days of beam time as many new nuclei in this region ofthe chart of nuclide as in the previous fifty years.In this work we could also determine the production cross sections of these nuclei normalizingthe production yields to the integrated beam current and the number of target nuclei. The beam cur-rent was continuously measured during the experiment using a secondary-electron monitor placedbefore the reaction target. This monitor was carefully calibrated during the experiment using aplastic scintillator [10]. The measured yields were also corrected by the losses inherent to the ex-perimental technique used in this work. The main corrections were due to the limited momentumacceptance of the spectrometer, in particular, for those nuclei with magnetic rigidities far from thecentral value defined by the FRS tune, the fraction of non fully stripped nuclei, and secondary re-actions in all layers of matter placed along the spectrometer. Smaller corrections were due to theacquisition dead time and detectors efficiency. These cross sections were used to benchmark modelcalculations predicting the production of heavy neutron-rich nuclei in future radioactive beam fa-cilities. These calculations clearly show that new radioactive beam facilities presently under designor construction will allow us to produce nuclei at the A≈195 r-process waiting point.3PoS(NIC XI)084Production and beta half-lives of heavy neutron-rich nuclei approaching the r-process path at N=126J. BenlliureFigure 2: Half lives of heavy neutron-rich nuclei measured in this work normalized to different theoreticalpredictions, gross theory (green symbols), finite-range droplet model [5] (blue symbols) and energy-densityfunctionals (red symbols) [6]. The measured half-lives for 198Ir(8±2 s), 199Ir(6±4 s) and 204Au(37±1 s) arenot shown for the clarity of the figure.3. Half-lives of heavy neutron-rich nucleiSome of the nuclide produced in this experiment were slowed down and implanted into an ar-ray of three double-sided silicon strip detectors with a thickness of 1 mm [14]. This array providedthe position and arriving time of the nuclide and the position and emission time of the subsequentβ particles. From the position and time correlations we were able to determine the β half lives ofthe nuclide of interest. Because of the pulsed nature of the beam and the relatively long half livesof the nuclide we were investigating, we could not use standard techniques for the determination ofthe half lives. In this case, we proposed to use backward-time correlations for evaluating our back-ground and numerical fitting functions based on Monte Carlo simulations. A detailed descriptionof this method can be found in Ref. [15].In figure 2 we reported the measured half lives normalized to the predictions obtained withthree model calculations, the gross theory (green symbols), calculations based in the finite-rangedrop model by Moller et al.[5] (blue symbols) and calculations based in the energy-density func-tionals by Borzov [6] (red symbols). As can be seen, the Gross theory and Borzov’s calculationsprovide a rather good description of the data. However, the calculations based on the finite-rangedroplet model of Moller et al. overestimates the half lives by almost two orders of magnitude formost of the nuclide investigated in this work [16].Although we did not measured nuclides directly at the r-process, calculations for r-process Os-mium isotopes indicate a similar difference between the model predictions [16]. The confirmationof the tendency we observed in this work as a general trend for r-process nuclei, would indicate4PoS(NIC XI)084Production and beta half-lives of heavy neutron-rich nuclei approaching the r-process path at N=126J. Benlliurethat finite-range droplet model calculations would underestimate the speed of the r-process at thewaiting point A≈195. Since these calculations are widely used in r-process simulations, these re-sults would indicate that the r-process matter flow towards the heaviest elements would be fasterthan presently thought.4. Summary and conclusionsIn this work we have demonstrated that it is possible to produce heavy neutron-rich nucleiat the r-process waiting point A≈195 using fragmentation reactions of heavy stable projectilessuch us 238U and 208Pb at relativistic energies. Indeed, in and experiment performed at GSI withmodest primary beam intensities we were able to produce 73 new isotopes of elements betweenYtterbium and Francium. Moreover, we could determine the β half lives of thirteen of these nuclei.The measured half lives are rather well described with calculations based on the energy-densityfunctionals while those calculations based on the finite-range droplet model largely overestimatethe measured values. The confirmation of this overestimation of the β half lives for nuclei at theA≈195 r-process waiting point would indicate that the matter flow at this waiting point is fasterthan predicted by present model calculations based on the finite-range droplet model. Therefore,the production of the heaviest r-process nuclei would also be larger than presently expected.References[1] J. Benlliure et al., Phys. Rev. C 78, 054605 (2008)[2] K. Helariutta et al., Eur. Phys. J. A 17,181 (2003)[3] T. Ohnishi et al., J. Phys. Soc. Jpn. 79, 073201 (2010)[4] A. Jungclaus et al., Phys. Rev. Lett. 99, 132501 (2007)[5] P. Moller et al., Phys. Rev. C 67, 055802 (2003)[6] I. Borzov, Phys. Rev. C 67, 025802 (2003)[7] J. Benlliure et al., Nucl. Phys. A 660, 87 (1999)[8] H. Geissel et al., Nucl. Instr. and Methods B 70, 286 (1992)[9] K.-H. Schmidt et al., Nucl. Instr. and Methods A 260, 287 (1987)[10] E. Casarejos et al., Phys. Rev. C 74, 044612 (2006)[11] J. Taïeb et al., Nucl. Phys. A 724, 413 (2003)[12] H. Alvarez-Pol et al., Eur. Phys. J. A 42, 485 (2009)[13] H. Alvarez-Pol et al., Phys. Rev. C 82, 041602 (2010)[14] R. Kumar et al., Nucl. Instrum. and Methods A 598, 754 (2009)[15] T. Kurtukian-Nieto et al., Nucl. Instrum. and Meth. A 589, 472 (2008)[16] T. Kurtukian-Nieto et al., Phys. Lett. B submitted (arXiv:0711.0101v1)5

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Production and $\beta$ half-lives of heavy neutron-rich nuclei approaching the r-process path at N=126

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Production and β\betaβ half-lives of heavy neutron-rich nuclei approaching the r-process path at N=126

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Production and $\beta$ half-lives of heavy neutron-rich nuclei approaching the r-process path at N=126