Elastic and break-up of the 1n-halo 11Be nucleus

Elastic and break-up of the 1n-halo 11Be nucleus A. Di Pietro1,a, A.M. Moro2, L. Acosta3, F. Amorini1,4, M.J.G. Borge5, P. Figuera1, M. Fisichella1,6, L.M. Fraile7, J. Gomez-Camacho2,8, H. Jeppesen9b, M. Lattuada1,4, I. Martel4, M. Milin10, A.Musumarra1,2, M. Papa1M.G. Pellegriti1,4, F.Perez-Bernal4, R. Raabe11, G. Randisi1,4c, F. Rizzo1,2, V. Scuderi1,4, O. Tengblad5, D. Torresi1,4, A. Maira Vidal5, D. Voulot9, F. Wenander9, and M. Zadro13 1INFN-Laboratori Nazionali del Sud and Sezione di Catania, Italy 2Departamento de FAMN, Universidad de Sevilla, Spain 3Departamento de Física Aplicada, Universidad de Huelva, Huelva, Spain 4Dipartimento di Fisica ed Astronomia Universitá di Catania, Catania, Italy 5Instituto de Estructura de la Materia CSIC, Madrid, Spain 6Dipartimento di Fisica ed Astronomia Universitá di Messina, Messina, Italy 7Departamento de Física Atómica, Molecular y Nuclear, Universidad Complutense, Madrid, Spain 8Centro Nacional de Aceleradores, Sevilla, Spain 9ISOLDE, CERN, CH-1211 Geneva 23, Switzerland 10Department of Physics, Faculty of Science University of Zagreb, Zagreb, Croatia 11Instituut voor Kern-en Stralingsfysica KU Leuven, Belgium 12Centro Siciliano di Fisica Nucleare e Struttura della Materia, Catania, Italy 13Division of Experimental Physics Ruder Boškovi´ c Institute, Zagreb, Croatia Abstract. The elastic and break-up angular distributions of the 10,11Be+64Zn reactions measured at Ec.m.≈1.4 VChave been analysed within the CCDC and O.M. frameworks. The suppression of the Coulomb-nuclear interference, observed in the 11Be scattering case with respect to the 10Be, has been interpreted as due to a long range absorption owing to the coupling with the break-up (Coulomb and nuclear) channels. The presence of 10Be events on the 11Be experiment data have been explained as due mainly to break-up processes. 1 Introduction Nuclear halo arises from the very weak binding of the last valence nucleon(s) which decouples from an inert core [1]. The single-particle wave function of the valence nucleons in halo nuclei has a long tail, which extends mostly outside the potential well. Over the years, a lot of experiments have been performed in order to investigate this peculiar structure also in the attempt to understand how it affects the reaction processes (see e.g. [2–5]). Elastic scattering, being a peripheral process, can be used to probe the tail of the wave function ae-mail: [email protected] bpresent address: Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, USA cpresent address: Instituut voor Kern-en Stralingsfysica KU Leuven, Belgium DOI: 10.1051/ C Owned by the authors, published by EDP Sciences, 2014 , / 03023 (2014) 201 66 epjconf EPJ Web of Conferences 46603023 This is an Open Access article distributed under the terms of the Creative Commons Attribution License 2.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Article available at http://www.epj-conferences.org or http://dx.doi.org/10.1051/epjconf/20146603023 thus allowing to learn about the surface properties of these nuclei. The observed striking shape of the elastic scattering angular distribution for halo nuclei, required to introduce in the calculations the explicit inclusion of coupling effects between relative motion and internal degrees of freedom. In particular, the coupling not only to bound and unbound excited states but also to the continuum (break-up) is needed (see e.g. [6–9]). A common feature observed in the low-energy halo-induced elastic scattering, on medium mass and heavy targets, is a reduction of the Coulomb-nuclear interference peak (e.g. [4, 10]). This suppression seems to be attributed to long range absorption which smooths the surface-jump of the scattering matrix |S(l)|. A further consequence of this suppression is a much larger total-reaction cross-section with respect to the one measured in reactions induced by the corresponding non-halo isotope (e.g. [10, 11]). Moreover, weakly bound halo systems are characterized by a low-lying large dipole strength just above the break-up threshold, it has been found that coupling with this B(E1) strength is important in collisions with heavy targets where, in turn, this strength is also responsible for the presence of a very large break-up cross-section which is partially responsible of the large total reaction cross-section that has been found in these collisions. In this paper, will be summarised the results concerning elastic scattering and break-up in the collision 11Be+64Zn performed at REX-Isolde at energies around the Coulomb barrier. 2 Elastic scattering The 11Be+64Zn experiment was performed at the REX-ISOLDE facility of CERN at Ec.m.≈24.5 MeV. The detection system used consisted of an array of Si-detector telescopes which were placed very close to the target allowing for a large angular and solid angle coverage [9, 10]. In the same experiment the elastic-scattering angular distribution for 10Be+64Zn was also measured. The beam energy resolution was insufficient to separate 11Be elastic from the inelastic scattering of the 11Be 1st excited state at Ex=320 keV therefore, in this case, the quasi-elastic angular distribution was measured and it is shown in Fig. 1 in linear scale. However, the contribution of inelastic scattering on the quasi-elastic angular distribution was found to be small [9, 10]. As shown in Fig. 1, a strong reduction of the cross-section in the Coulomb-nuclear interference region is observed for the 11Be+64Zn collision, much stronger than in the 6He case [11, 12]. In the same figure, the 10Be+64Zn elastic scattering angular distribution is also shown; this last angular distribution presents the typical Fresnel diffraction pattern. The total-reaction cross-sections have been extracted via O.M. analysis of the elastic angular distribution. It was found that in the 11Be case (σ=2.73 b) this is ≈2.2 times larger than for the 10Be one (σ=1.26 b). The optical potential deduced from the 10Be+64Zn analysis was used as bare potential for the 11Be+64Zn calculations. In the Optical Model (O.M.) analysis of 11Be+64Zn, in addition to the bare potential, two types of Dynamic Polarization potentials (DPP) were used. 1) A phenomenological DPP having the shape of a Woods-Saxon derivative; 2) a DPP potential derived from the semiclassical theory of Alder and Winter for Coulomb excitation which takes into account the effect of the coupling to the inelastic and break-up channels due to the dipole Coulomb interaction. In the first case a good fit of the data was obtained with a DPP having a very large diffuseness of 3.5 fm, in agreement with what found by [13]. In the second case, adding the DPP to the bare potential produced a strong reduction of the elastic cross-section in the Coulomb-nuclear interference region; however, this reduction is not sufficient to reproduce the measured data showing that other processes, not included in the calculations must be contributing. EPJ Web of Conferences 03023-p.2 Figure 1. (color online). Elastic scattering angular distribution of 10Be+64Zn (red squares) and 11Be+64Zn (blue circle) at c.m. energy of ≈24.5 MeV. The full line represent the CDCC calculations for the elastic-scattering 11Be+64Zn while the dot-dashed line represent O.M. calculations for the scattering 10Be+64Zn. See text for details. The scattering angular distribution for the 11Be case was also compared with CDCC calculations. In these calculations the 11Be+64Zn reaction is treated within an effective three-body model, 10Be+n+64Zn, and coupling to break-up channels is explicitly taken into account. The calculations show that the suppression of the Coulomb-nuclear interference peak observed in the angular distribution is due to a combined effect of Coulomb and nuclear couplings to break-up channels, although they act in very different way. Coulomb coupling gives a large contribution to the break-up cross section and therefore produces an increase of the reaction cross section; nuclear coupling has a large effect on the elastic phase-shift which becomes almost real, thus affecting the shape of the elastic angular distribution but not so much the break-up cross section [9]. The results of all these calculations are also shown in Fig. 1. 3 Break-up In the 11Be+64Zn data, it was possible to identify, along with the scattered 11Be events also 10Be events having an energy close to the quasi-elastic peak. These events correspond to transfer or breakup processes. However, DWBA calculations predict very low cross-section for the transfer process, therefore mainly break-up contributes to the 10Be production. The break-up angular distribution was extracted and it is shown in Fig 2. For the largest measured angles the thickness of the ∆E detector was too large to allow 10Be particles to punch through and being identified. Therefore the break-up angular distribution was extracted only in a limited angular range. The integrated cross-section is σ=1.10±0.15 b. The measured angular distribution was compared with the CDCC calculations described in the previous section. In the calculations it was approximated that the angle of the outgoing 10Be fragment is the same as that of 11Be∗in the laboratory frame as for the data. The CDCC calculations underestimate the measured cross-section. The CDCC calculations provide only the elastic break-up cross-section i.e. neither the core nor the target are excited. Moreover, core excitation effects are expected to be non-negligible [14] and by introducing them in the calculations the agreement with the data could INPC 2013 03023-p.3 improve. The 1n-trasfer contribution is also not included in the CDCC calculations. Figure 2. (color online). Beak-up angular distribution of 11Be+64Zn. The full line represent the CDCC calculations for the break-up process. See text for details. 4 Summary The collision induced by the 11Be halo nucleus on a 64Zn target has been investigated. The quasielastic scattering angular distribution shows that the peak due to Coulomb-nuclear interference disappears, being the indication that long-range absorption occurs. The long-range absorption is due to the combined effects of Coulomb and nuclear couplings to the continuum. The Coulomb break-up is also responsible for the large break-up cross-section observed. References [1] A. Jensen, K. Riisager and D. V. Fedorov, Rev. Mod. Phys. 76 215 (2004). [2] R. 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