Be-7- and B-8-reaction dynamics at Coulomb barrier energies

7Be- and 8B-reaction dynamics at Coulomb barrier energies M. Mazzocco1,2,A.Boiano 3,C.Boiano 4,M.LaCommara 3,5, C. Manea2, C. Parascandolo3,D.Pierroutsakou 3, C. Signorini6, E. Strano1,2,D.Torresi 1,2, H. Yamaguchi7,D.Kahl 7, L. Acosta8,9,P.DiMeo 3, J.P. Fernandez-Garcia9, T. Glodariu10, J. Grebosz11,A.Guglielmetti 4,12,N.Imai 7,13,Y.Hirayama 13, H. Ishiyama13,N.Iwasa 14,S.C.Jeong 13,15,H.M.Jia 16, N. Keeley17,Y.H.Kim 13,S.Kimura 13,S.Kubono 7,18,J.A.Lay 1,2, C.J. Lin16, G. Marquinez-Duran8,I.Martel 8,H.Miyatake 13, M. Mukai13,T.Nakao 19,M.Nicoletto 2,A.Pakou 20,K.Rusek 21, Y. Sakaguchi7,A.M.S ´ anchez-Ben´ ıtez8,T.Sava 10,O.Sgouros 20, C. Stefanini1,2,F.Soramel 1,2,V.Soukeras 20,E.Stiliaris 22, L. Stroe10,T.Teranishi 23,N.Toniolo 6, Y. Wakabayashi18, Y.X. Watanabe13,L.Yang 16 and Y.Y. Yang24 201 ,0600 (2016) EPJ Web of Conferences DOI: 10.1051/ conf/201611 0600 epj 5 1177 NN  © The Authors, published by EDP Sciences SI . 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/). F - 66 1Dipartimento di Fisica e Astronomia, Universit`a di Padova, Padova, Italy 2INFN, Sezione di Padova, Padova, Italy 3INFN, Sezione di Napoli, Napoli, Italy 4INFN, Sezione di Milano, Milano, Italy 5Dipartimento di Fisica, Universit`a di Napoli “Federico II”, Napoli, Italy 6INFN - Laboratori Nazionali di Legnaro, Legnaro (PD), Italy 7CNS - The University of Tokyo, RIKEN campus, Wako, Saitama, Japan 8Departamento de F`ısica Aplicada, Universidad de Huelva, Huelva, Spain 9INFN, Sezione di Catania, Catania, Italy 10 NIPNE, M˘agurele, Romania 11 IFJ PAN, Krak´ow, Poland 12 Dipartimento di Fisica, Universit`a di Milano, Milano, Italy 13 KEK, Tsukuba, Ibaraki, Japan 14 Department of Physics, Sendai, Miyagi, Japan 15 Institute for Basic Science, Daejeon, Korea 16 China Institute of Atomic Energy, Beijing, China 17 National Centre for Nuclear Research, Otwock, Poland 18 RIKEN Nishina Center, Wako, Saitama, Japan 19 Advanced Science Research Center, JAEA, Tokai, Ibaraki, Japan 20 Department of Physics, University of Ioannina, Ioannina, Greece 21 Heavy Ion Laboratory, University of Warsaw, Warsaw, Poland 22 Department of Physics, University of Athens, Athens, Greece 23 Department of Physics, Kyushu University, Hakozaki, Fukuoka, Japan 24 Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou, China Abstract We investigated the reaction dynamics induced by the Radioactive Ion Beams 7Be and 8Bona208Pb target at energies around the Coulomb barrier. The two measurements are strongly interconnected, being 7Be (Sα= 1.586 MeV) the loosely bound core of the even more exotic 8B(Sp= 0.1375 MeV) nucleus. Here we summarize the present status of the data analysis for the measurement of the elastic scattering process for both reactions and the preliminary results for the optical model analysis of the collected data. 201 ,0600 (2016) EPJ Web of Conferences DOI: 10.1051/ conf/201611 0600 epj 5 1177 NN 66 2 1 Introduction The reaction dynamics induced by light weakly-bound Radioactive Ion Beams (RIBs) at near-barrier energies has attracted the interest of the Nuclear Physics community for the last 20 years at least. The exotic features of several light RIBs together with their extremely weak binding energies (Sp, Snand Sαtypically lower than 1.0 MeV) make these nuclei an ideal playground for the nuclear reaction theory. From an experimental point-of-view, a large enhancement of the reaction cross section at sub-barrier energies has been observed. This enhancement is mostly due to direct processes, such as the 2n-transfer for the 2n-halo nucleus 6He [1–3] and the neutron skin nucleus 8He [4,5] and, apparently, the breakup process for 8B- [6] and 11Li- induced [7, 8] reactions. Contrary to initial expectations, only a marginal enhancement, when recorded, of the fusion cross section at sub-barrier energies has been reported [4, 5]. Several review papers have been published on this topic over the last decade [9–15] In this contribution we describe the present status of the data analysis for our measurements of the elastic scattering process for the systems 7Be +208Pb and 8B+208Pb at near-barrier energies. 2 The system 7Be + 208Pb This reaction was studied at the Laboratori Nazionali di Legnaro (LNL, Italy) of the Istituto Nazionale di Fisica Nucleare (INFN), where a 7Be RIB is routinely delivered by the facility EXOTIC [16, 17]. The 7Be beam for this experiment was obtained starting from a 48.8 MeV 7Li3+ primary beam impinging on a 5-cm long gas target filled with 1 bar of H2gas cooled down to liquid nitrogen temperature (∼90 K). In these conditions, a 40.5-MeV 7Be secondary beam with an average intensity 2-3×105pps was produced. A second 7Be energy of 37.6 MeV was obtained by means of an approximately 12.5 μm thick Al degrader inserted along the beam line right before the main scattering chamber. Finally, a third energy of 42.4 MeV was achieved at the end of the experiment after warming up the target station at room temperature. In all cases the secondary beam purity was as good as 99%. Charged particles originated by the interaction with a 1 mg/cm2 208Pb target were detected in the angular ranges θlab =[55 ◦,84◦], [96◦,125◦]and [138◦,165◦]bymeansof6ΔE-Eres telescopes of the newly developed detector array EXPADES [18]. The telescopes were displaced in a cylindrical configuration around the target at an average distance of about 111.5 mm. 201 ,0600 (2016) EPJ Web of Conferences DOI: 10.1051/ conf/201611 0600 epj 5 1177 NN 66 3 40 60 80 100 120 140 160 180 Tc.m. (deg) 101 100 Ratio to Rutherford 37.6 MeV 40.5 MeV 42.4 MeV 7Be+208Pb Figure 1: Elastic scattering angular distributions for the system 7Be + 208Pb at three bombarding energies. Continuous lines represent the results of an optical model best-fit analysis of the experimental data. Three modules were located in the left hemisphere and the other three in symmetrical locations of the right hemisphere. Each telescope consisted of a 43-57 and 300 μm thick Double Sided Single Strip Detector (DSSDS) as inner and outer layer, respectively. Each DSSSD had an active area of 62.3 mm ×62.3 mm and each side was segmented into 32 strips, determining a position resolution of about 2 mm ×2 mm. Fig. 1 shows a preliminary evaluation of the elastic scattering angular distributions for the system 7Be + 208Pb. We can directly appreciate the drop of the differential cross section at backward angles due to the nuclear absorption. To contain statistical fluctuations, at this stage of the analysis, the data were grouped into bins of 8 vertical strips. A better angular resolution should be achieved after completing the sorting procedure of the collected data. Continuous lines in Fig. 1 are the result of an optical model best fit analysis of the experimental data within the framework of the optical model. The analysis was performed with the SFresco subroutine of the main code Fresco [19] and allowed the extraction of the reaction cross section. The comparison of the reaction cross sections for the system 6,7Li, 7Be and 8B +208Pb will be presented in the last section of this contribution. 201 ,0600 (2016) EPJ Web of Conferences DOI: 10.1051/ conf/201611 0600 epj 5 1177 NN 66 4 3 The system 8B+208Pb The study of this reaction was carried out at the CRIB (CNS Radioactive Ion Beam) [20,21] facility located in the RIKEN campus at Wako (Japan). A 50 MeV 8B RIB was produced starting from a 66 MeV 6Li3+ primary beam impinging on a 8-cm long gas target inflated with 3He gas at a pressure of 1 bar. The outcoming 8B beam had an intensity of 2×104pps and a purity of about 20%, being the 6Li scattered beam, 3He ions recoiling out from the gas target and 7Be the main contaminant beams. However, all these beams had different times of flight (ToF) through the CRIB separator and, by using proper gates in the ToF spectra, it will be possible to discriminate events originating from different components of the cocktail beam. Charged reaction products arising from the interaction with a 2.2 mg/cm2 208Pb (evaporated on a 1.5 μm thick Mylar backing foil) were detected by means of the six ΔE-Eres modules of the already mentioned detector array EXPADES. In this experiment a slightly asymmetric configuration of the telescopes was employed in order to ensure the coverage of the following angular ranges: θlab =[16 ◦,41◦], [55◦,84◦], [96◦,125◦]and [139◦,164◦]. The average distance between the detectors and the target central position was about 113 mm. Fig. 2 shows a very preliminary evaluation of the angular distribution for the elastic scattering process in the system 8B+208Pb. Only four point were plotted, each one corresponding to the mean location of the four angular ranges covered by the EXPADES array. The experimental data were normalized by using a Monte-Carlo simulation, which took into account the geometry of the detector array and the kinematics of the elastic scattering process. Simulated events were generated according to a pure Rutherford cross section. The ratio between experimental and simulated data was finally set equal to unity in the angular range covered by the detector located at the forward-most angles. In the next months we expect to improve the angular resolution of the elastic scattering differential cross section by performing a detailed strip-by-strip analysis of the gathered data and, if allowed by the collected statistics, to proceed also with a pixel-by-pixel analysis. Continuous line in Fig. 2 represent the result of an optical model best fit analysis performed with the code Fresco. This procedure allowed us to extract a first (extremely preliminary) evaluation of the reaction cross section for the system 8B+208Pb at 50 MeV beam energy. 201 ,0600 (2016) EPJ Web of Conferences DOI: 10.1051/ conf/201611 0600 epj 5 1177 NN 66 5 20 40 60 80 100 120 140 160 180 Tc.m. (deg) 0 0.2 0.4 0.6 0.8 1 1.2 Ratio to Rutherford 50 MeV 8B + 208Pb Figure 2: Preliminary elastic scattering angular distribution for the system 8B+ 208Pb at 50 MeV beam energy. The continuous line is the result of the optical model analysis of the collected data. 35 40 45 50 55 7Be Energy (MeV) 0 500 1000 1500 2000 7Be Reaction Cross Section (mb) 7Be + 208Pb 7Li + 208Pb 6Li + 208Pb 8B + 208Pb Figure 3: Reaction cross sections for the systems 6,7Li, 7Be and 8B+208Pb. The data were reduced according to the procedure recommended in Ref. [23] and then normalized to the system 7Be + 208Pb. 201 ,0600 (2016) EPJ Web of Conferences DOI: 10.1051/ conf/201611 0600 epj 5 1177 NN 66 6 4 Discussion Fig. 3 illustrates the reaction cross sections for the systems 6,7Li [22], 7Be and 8B+208Pb in the energy range around the Coulomb barrier. The data for the different systems were reduced according to the procedure described in Ref. [23] and then multiplied by the normalization factor of the reaction 7Be + 208Pb. It is interesting to observed that, according to our preliminary analysis, the 7Be (Sα= 1.586 MeV) nucleus exhibits a reactivity rather similar to that of its more bound mirror nucleus 7Li (Salpha = 2.467 MeV), rather than to that of the similarly weakly bound nucleus 6Li (Salpha = 1.474 MeV). On the other side, it is particularly evident that, even at this very preliminary stage of the data analysis, the reaction induced by the 8Bp-halo nucleus (Sp= 0.1375 MeV) on the 208Pb target has a reaction cross section much larger those measured for reactions induced by the other weakly-bound projectiles on the same target. This scenario is rather consistent with that observed for reactions induced by the same projectiles on the medium-mass target 58Ni [6]. In our case, in an ultimate stage of the data analysis, it should be possible to established whether the reaction cross section enhancement for the system 8B+208Pb is mainly due to direct channels, such as breakup and/or transfer, or to the fusion process. Acknowledgments This work was partially supported by the Italian Minister for Education, University and Research (MIUR) within the project RBFR08P1W2 (FIRB 2008) and the National Science Centre of Poland under contract No. UMO- 2014 14 M ST2 00738 (COPIN-INFN Collaboration). The research leading to these results has also received funding from the European Commission, Seventh Framework Programme (FP7 2007-2013) under Grant Agreement n. 600376. J.A.L. is a Marie Curie Piscopia fellow at the University of Padova. References [1] A.DiPietroet al.,Phys.Rev.C69, 044613 (2004). [2] A. Navin et al.,Phys.Rev.C70, 044601 (2004). [3] R. Raabe et al., Nature 431, 823 (2004). 201 ,0600 (2016) EPJ Web of Conferences DOI: 10.1051/ conf/201611 0600 epj 5 1177 NN 66 7 [4] A. Lemasson et al., Phys. Rev. Lett. 103, 232701 (2009). [5] A. Lemasson et al.,Phys.Rev.C82, 044617 (2010). [6] E.F. Aguilera et al.,Phys.Rev.C.79, 021601(R) (2009). [7] M. Cubero et al., Phys. Rev. Lett. 109, 262701 (2012). [8] J.P. Fernandez-Garcia et al., Phys. Rev. Lett. 110, 142701 (2013). [9] L.F. Canto, P.R.S. Gomes, R. Donangelo and M.S. Hussein, Phys. Rep. 424, 1 (2006). [10] J.F. Liang, and C. Signorini, Int. J. Mod. Phys. E 14, 1121 (2005). [11] N. Keeley, R. Raabe, N. Alamanos, and J.L. Sida, Prog. Part. Nucl. Phys. 59, 579 (2007). [12] N. Keeley, N. Alamanos, K.W. Kemper, and K.Rusek, Prog. Part. Nucl. Phys. 63, 396 (2009). [13] M. Mazzocco, Int. J. Mod. Phys. E 19, 977 (2010). [14] N. Keeley, K.W. Kemper and K. Rusek, Eur. Phys. J. A 50, 145 (2014). [15] L.F. Canto, P.R.S. Gomes, R. Donangelo, J. Lubian and M.S. Hussein, Phys. Rep. 596, 1 (2006). [16] F. Farinon et al., Nucl. Instrum. Meth. B 266, 4097 (2008). [17] M. Mazzocco et al., Nucl. Instrum. Meth. B 317, 223 (2013). [18] E. Strano et al., Nucl. Instr. and Meth. B 317 (2013) 657. [19] I.J. Thompson, Comput. Phys. Rep. 2, 167 (1988). [20] Y. Yanagisawa et al., Nucl. Instr. and Meth. A 539 (2005) 74. [21] H. Yamaguchi et al., Nucl. Instr. and Meth. A 589 (2008) 150. [22] N. Keeley et al., Nucl. Phys. A 571 (1994) 326. [23] P.R.S. Gomes, J. Lubian, I. Padron, R.M. Anjos, Phys. Rev. C 71 (2005) 017601. 201 ,0600 (2016) EPJ Web of Conferences DOI: 10.1051/ conf/201611 0600 epj 5 1177 NN 66 8