Fusion of 8He with 206Pb around Coulomb barrier energies

Fusion of 8He with 206Pb around Coulomb barrier energies V. V. Parkar1,a, G. Marquinez1, I. Martel1, A. M. S´ anchez-Ben´ ıtez1, L. Acosta1, R. Berjillos1, J. Due˜ nas1, J. L. Flores1, J.P. Bol´ ıvar1, A. Padilla1, M.A.G. Alvarez2, D. Beaumel3, M.J.G. Borge4, A. Chbihi5, C. Cruz4, M. Cubero4, J. P. Fernandez Garcia2, B. Fern´ andez Mart´ ınez2, J. Gomez Camacho2, N. Keeley6, J. A. Labrador2, M. Marquis7, M. Mazzocco8, A. Pakou9, N. Patronis9, V. Pesudo4, D. Pierroutsakou10, R. Raabe11, K. Rusek12, R. Silvestri10, Ł. Standylo6, I. Strojek12, N. Soic13, O. Tengblad4, R. Wolski14, and A. H. Ziad2 1Departamento de F´ ısica Aplicada, Universidad de Huelva, E-21071 Huelva, Spain 2Centro Nacional de Aceleradores, Universidad de Sevilla, 41092 Sevilla, Spain 3Institut de Physique Nucl´ eaire, IN2P3-CNRS, 91406 Orsay, France 4Instituto de Estructura de la Materia, CSIC, Madrid, Spain 5GANIL, CEA/DSM-CNRS/IN2P3, Bd Henri Becquerel, BP 55027, F-14076 Caen Cedex 5, France 6Department of Nuclear Reactions, The Andrzej Soltan Institute for Nuclear Studies, ul. Ho˙ za 69, PL-00-681 Warsaw, Poland 7LPC, CNRS/IN2P3, Ensicaen, Universit´ e de Caen Basse Normandie, F-14050 Caen cedex, France 8Dipartimento di Fisica, INFN, I-35131 Padova, Italy 9Department of Physics and HINP, The University of Ioannina, GR-45110 Ioannina, Greece 10 INFN - Sezione di Napoli, via Cinthia, I-80126 Napoli, Italy 11 Instituut voor Kern-en Stralingsfysica, K.U. Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium 12 Heavy Ion Laboratory, University of Warsaw, Pasteura 5A, 02-093 Warsaw, Poland 13 Rudjer Boskovic Institute, Bijenicka 54, HR-10000 Zagreb, Croatia 14 Institute of Nuclear Physics PL-31342, Krakow, Poland Abstract. The experimental study of the fusion of light neutron-rich nucleus 8He with 206Pb is reported in this work. A fusion stack of 206Pb targets has been used for this study. The most prominent evaporation residue (210Po), which has half-life of 138 days and decays by alpha emission, is populated in the reaction. Radiochemical analysis technique is used to extract the yield of this evaporation residue. 1 Introduction From the first experiments where evidences for a halo structure in 11Li were reported [1], similar effects were also found in other exotic nuclei far away from the valley of stability. The availability of these exotic nuclei has triggered new interest in the field of nuclear structure and reaction dynamics [2,3]. There have been also many improvements in the theoretical side which provide a deeper understanding of experimental findings by using different approaches. The coupled channels approach is nowadays extensively being used to understand simultaneously different reaction channels by means of codes like FRESCO [4]. The strength and coupling between the different reaction channels can be properly described by the use of coupled channels calculations [5], including coupling to the continuum. Even for four body correlations [6], the theoretical framework was reasonably well established along last few decades from the study of nuclear reactions with stable heavy ions. At coulomb barrier energies, the data can also be described using semi-classical models [7], comae-mail: [email protected] plementary to the sophisticated coupled channels calculations, providing an elegant insight on the relevant mechanisms participating in the dynamics of the collision process. Sub-barrier fusion of neutron rich nuclei depends strongly on the reaction channels still present at energies below the barrier. In neutron-rich nuclei the fusion cross section could be increased due to a reduction in the effective Coulomb barrier [8]. However the characteristic low binding energy should favour the break-up channel and reduce the fusion probability [9]. Several experiments were performed to study the effect of neutron halo on the fusion process, and extraordinary experimental efforts have been carried out in the past for measuring sub-barrier fusion of 6He [9–11]. The case of 8He is different, as we are dealing with a skin nucleus, a system with a thick layer of neutron matter surrounding the inner nuclear core. Whether the halo or the skin will produce strong effects on fusion cross sections is still a matter of investigation. Very recently, a first measurement to understand the fusion mechanism was performed at GANIL using 197Au targets [12]. The motivation of the present work is the understanding of the interplay between the various reaction channels EPJ Web of Conferences 17, 16009 (2011) DOI: 10.1051/epjconf/20111716009 © Owned by the authors, published by EDP Sciences, 2011 This is an Open Access article distributed under the terms of the Creative Commons Attribution-Noncommercial License 3.0, which permits unrestricted use, distribution, and reproduction in any noncommercial medium, provided the original work is properly cited. Article available at http://www.epj-conferences.org or http://dx.doi.org/10.1051/epjconf/20111716009 EPJ Web of Conferences [A] [B] Fig. 1. [A] Picture of experimental setup used for the detection of the charged particles produced in the 8He+208Pb collisions (see text for details) [B] Picture of the fusion stack which was placed downstream of the scattering chamber in a separate chamber. in the fusion process. A new experiment was performed using the SPIRAL RIB facility at GANIL (Caen, France) in October 2010, where two targets along the beam line were used. In this experiment, the 8He beam was first scattered on a 208Pb target, where direct reaction processes were investigated by using the Huelva University charged particle array (GLORIA). A few meters downstream the beam line, a fusion stack of four 206Pb targets (separated by mylar in front and aluminium in back) was placed to measure subbarrier fusion cross sections. Here the evaporation residues (ER) produced after the fusion of 8He with 206Pb were implanted on 206Pb and successive Al assembly, where the yield of 210Po (an active alpha emmiter having a half life of 138 days) was accumulated during several hours of irradiation. The details of experimental setup, radiochemical procedure and the results are given in the following sections. 2 Experimental Details In the experimental setup, a new simple but highly efficient charged particle detector array developed at the University of Huelva (Spain) named GLORIA (GLObal ReactIon Array) was used to measure the elastic scattering and direct reaction channels. In addition, a stack of 206Pb foils was used to extract the fusion residues using the activation technique. The schematics of the experimental setup, the scattering chamber and the GLORIA array, is shown in Fig. 1[A]. The fusion stack was placed downstream of the scattering chamber for irradiation, as it is shown in Fig. 1[B]. The GLORIA silicon ball is a compact and high granularity detection system, covering a wide angular range from θlab =10◦to 170◦. The system is made of 12 DSSSD (16 x 16 strips each) particle telescopes. For each telescope: ∆E- 40 µm, E-500 µm. The detector system is fully equipped by its own analog chain, logic and DAQ system on a VME platform. The typical two dimensional spectrum of ∆E vs. Etot for 8He+208Pb reaction measured at Elab =22 MeV and θlab =26.7◦is shown in Fig. 2. In the case of the fusion stack used for irradiation, it consists of stack of four 206Pb targets separated by mylar (∼2.5 µm) in the front (to stop the backscattered ERs) and aluminium (∼14.7 µm) Fig. 2. A typical two dimensional spectrum of ∆E vs. Etot for 8He+208Pb reaction measured at Elab =22 MeV and θlab = 26.7◦. Different isotopes of ‘He’ can be easily identified in this telescope. in the back (to degrade the beam energy). The whole stack is irradiated for 22 MeV. The incident beam energy on the successive target foils was determined based on the energy loss for the mylar, Pb and Al foils using SRIM [13]. After irradiation, the activated 206Pb foils went through a radiochemical treatment in which the 210Po was isolated and deposited on a silver coin. The coins were introduced in the alpha station system at University of Huelva, where the yield of 210Po was obtained. 2.1 Preparation of samples and radioactivity measurement The fusion stack irradiated during the experiment has been subjected to chemical processing at the Radiochemical laboratory at the University of Huelva. The picture of the measurement cell is shown in Fig. 3. Each target was dissolved and a known amount of 209Po was added as a tracer in order to fix an efficiency of the chemical process. The element of interest was extracted and put into dedicated alpha activity measurement cells. As the half-life of alpha decay from 210Po is 138 days, the activity is measured for 30 days to accumulate sufficient statistics. We also took a background run to confirm that no other peaks were present in the region around 5300 keV (emitted alphas from 210Po). A typical spectrum for one of the samples after one month of 16009-p.2 FUSION11 Fig. 3. Picture of measurement system at the Radiochemistry laboratory at the University of Huelva. In the inset, the detector and the sample holder can be seen. statistics is shown in Fig. 4[A]. The distinct peak of alpha particles from 210Po at 5300 keV can be seen along with the room background (228Th peak) and the radiotracer 209Po peaks. The peak of interest (210Po) has been fitted (shown in Fig. 4[B]) and the area under that peak was extracted. 3 Results and Discussion In order to determine the cross-section of interest (production of 210Po), the total flux of the projectiles during the irradiation is necessary. It will be obtained by normalizing the elastic scattered particles detected in the forward angle (θ < 30◦), where Rutherford scattering is expected. The analysis of the elastic scattering channel is in progress to get the final (production of 210Po) cross-sections. 4 Summary The production of 210Po populated in the fusion of 8He with 206Pb has been measured for energies around the Coulomb barrier. The method of activation in a stack of foils has been utilized for this study. The elastically scattering projectiles from 208Pb target at forward angles will be utilised for absolute normalization to determine the production crosssections. The full elastic scattering distribution data acquired in the same experiment will also be reported shortly. 210Po 228Th Counts Channel No. [A] [B] Fig. 4. [A] Typical energy spectrum from the sample after radiochemical and self deposition process on the silver disk. The peak corresponding to the alpha decay of 210Po at 5300 keV is clearly seen to be separated from other radioactive contaminants. The bigger peak corresponding to the alpha decay energy of the radiotracer 209Po at 4880 keV can also be seen. [B] Fitting procedure used to extract the yield of 210Po. 5 Acknowledgments The authors would like to thank the GANIL accelerator crew for a steady delivery of the beam throughout the experiment, and the staffof the Radiochemistry Laboratory at the University of Huelva. This work was partially supported by the Spanish Ministry of Science and Innovation (MICINN) under projects FPA2007-63074 (FINURA) and 16009-p.3 EPJ Web of Conferences INGENIO-2010 (CPAN). It has also been supported by the European Union ”SPIRAL2 preparatory phase” Grant agreement 212692, through the FP7 European program. References 1. I. Tanihata et al., Phys. Rev. Lett. 55, (1985) 2676. 2. N. Keeley, R. Raabe, N. Alamanos, and J. L. Sida, Prog. Part. Nucl. 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