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Elastic Scattering and Reaction Mechanisms of the Halo Nucleus 11Be Around the Coulomb Barrier

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Elastic Scattering and Reaction Mechanisms of the Halo Nucleus 11Be Around the Coulomb Barrier week ending PRL 105, 022701 (2010) PHYSICAL REVIEW LETTERS 9 JULY 2010 Elastic Scattering and Reaction Mechanisms of the Halo Nucleus 11Be around the Coulomb Barrier A. Di Pietro,1 G. Randisi,1,2,* V. Scuderi,1,2 L. Acosta,3 F. Amorini,1,2 M. J. G. Borge,4 P. Figuera,1 M. Fisichella,1,2 L. M. Fraile,5,† J. Gomez-Camacho,6 H. Jeppesen,5,‡ M. Lattuada,1,2 I. Martel,3 M. Milin,7 A. Musumarra,1,8 M. Papa,1 M. G. Pellegriti,1,2 F. Perez-Bernal,3 R. Raabe,9 F. Rizzo,1,2 D. Santonocito,1 G. Scalia,1,2 O. Tengblad,4 D. Torresi,1,2 A. Maira Vidal,4 D. Voulot,5 F. Wenander,5 and M. Zadro10 1INFN–Laboratori Nazionali del Sud and Sezione di Catania, Catania, Italy 2Dipartimento di Fisica ed Astronomia, Universita´ di Catania, Catania, Italy 3Departamento de Fisica Aplicada, Universidad de Huelva, Huelva, Spain 4Instituto de Estructura de la Materia CSIC, Madrid, Spain 5ISOLDE, CERN, CH-1211 Geneva 23, Switzerland 6Departamento de Fisica Atomica Molecular Nuclear, Universidad de Sevilla and Centro Nacional de Aceleradores, Sevilla, Spain 7Department of Physics, Faculty of Science, University of Zagreb, Zagreb, Croatia 8Dipartimento di Metodologie Fisiche e Chimiche per l’Ingegneria, Universita´ di Catania, Catania, Italy 9Instituut voor Kern-en Stralingsfysica, Katholieke Universiteit, Leuven, Belgium 10Division of Experimental Physics, Ru‘er Bosˇkovic´ Institute, Zagreb, Croatia (Received 7 March 2010; published 6 July 2010) Collisions induced by 9;10;11Be on a 64Zn target at the same c.m. energy were studied. For the first time, strong effects of the 11Be halo structure on elastic-scattering and reaction mechanisms at energies near the Coulomb barrier are evidenced experimentally. The elastic-scattering cross section of the 11Be halo nucleus shows unusual behavior in the Coulomb-nuclear interference peak angular region. The extracted total-reaction cross section for the 11Be collision is more than double the ones measured in the collisions induced by 9;10Be. It is shown that such a strong enhancement of the total-reaction cross section with 11Be is due to transfer and breakup processes. DOI: 10.1103/PhysRevLett.105.022701 PACS numbers: 25.60.Bx, 25.70.Bc A hundred years after Rutherford’s scattering experi- cally forbidden region, giving rise to a diffuse tail sur- ment [1], heavy-ion-elastic-scattering angular distributions rounding a well-bound core. The behavior of the system in (AD) are usually plotted as a ratio of the Rutherford cross nuclear reactions is mostly determined by the tail of the section (i.e., pure Coulomb scattering). Such representa- wave function [3]. The reaction mechanisms may also be tion usually shows a decrease of the elastic cross section affected by the weak binding: at energies around the with the angle due to absorption at small impact parame- Coulomb barrier, couplings between the entrance channel ters by nonelastic processes, and an oscillatory behavior. and the continuum [4–8], as well as to the various reaction The latter, using the language of optics, is described in channels [9–12], are expected to be very important. Direct terms of refraction by nonabsorbing lenses (Coulomb rain- reactions, such as breakup or transfer, may be favored bow model) or diffraction by sharp-edged, nonrefracting owing to the low binding energy, the extended tail of apertures (Fraunhofer or Fresnel diffraction model). halo nuclei, and the large Q values for selected transfer However, the refraction or diffraction descriptions are channels. oversimplifications of the realistic process; rather, the nu- Almost all elastic-scattering and reaction mechanism cleus behaves as a ‘‘cloudy crystal ball.’’ The elastic- studies around the barrier with halo nuclei have been scattering AD may show a peak resulting from the inter- performed with the 2n halo nucleus 6He. All authors agree ference between the Coulomb and nuclear amplitudes that, due to the 6He structure, one has an enhancement of (Coulomb-nuclear interference peak) [2], which, in anal- the total-reaction (TR) cross section due to strong transfer ogy with the Coulomb rainbow model, is sometimes called and breakup channels which may saturate almost com- the rainbow peak. Since elastic scattering is a peripheral pletely the TR cross section at and below the barrier process, it does not give information on the interior region (e.g., [10,13–15]). To our knowledge, only two attempts of nuclei. It probes the tail of the wave function, and hence to measure elastic-scattering and reaction mechanisms one can learn about surface properties, such as size of around the barrier with n-halo nuclei different from 6He nuclei and surface diffuseness. Therefore, elastic scattering are reported in literature. In [16,17] 11Be þ 209Bi, the is an ideal tool to study peculiar nuclear structures as, for quasielastic cross section, which included inelastic excita- example, the nuclear halo. Such structure originates when tions up to 2.6 MeV, was measured using a fragmentation very weakly bound nucleon(s) can tunnel into the classi- beam degraded in energy. At energies exceeding 10% of 0031-9007=10=105(2)=022701(5) 022701-1 Ó 2010 The American Physical Society week ending PRL 105, 022701 (2010) PHYSICAL REVIEW LETTERS 9 JULY 2010 the barrier, the extracted TR cross section was found to be stable 9Be beam was performed at Laboratori Nazionali similar to the one of 9Be þ 209Bi.In[18] 11Be þ 120Sn, del Sud (LNS) in Catania. The 9Be beam was delivered by 11 ð1ÀÞ quasi-elastic-scattering [the Be 2 inelastic excitation the 14 MV SMP Tandem of LNS and was impinging on a was included] AD was measured, but in a very limited 550 g=cm264Zn target. Five Si-detector telescopes angular range. (10 mE and 200 m E detectors), placed on a rotating This Letter reports, for the first time, clear experimental arm, allowed the measurement of the elastic-scattering AD evidence of strong effects of the 11Be halo structure on up to 110 . ; ; elastic-scattering and reaction mechanisms in collisions In Fig. 1 the AD for the scattering of 9 10 11Be þ 64Zn are close to the Coulomb barrier. We measured high quality shown in linear scale. As one can see, in spite of the very elastic-scattering AD of 9;10;11Be on a 64Zn target, in a wide different binding energies of 9Be and 10Be, their elastic- E 11 angular range and with small angular step, at c:m: scattering AD are similar. Be scattering shows a very 24:5 MeV, corresponding to about 1.4 the Coulomb bar- different pattern; the main feature that one can observe in rier. Moreover, in the case of the 11Be halo nucleus, the Fig. 1 is a dramatic reduction of the elastic cross section at breakup or transfer AD was extracted. The three beryllium forward angles. A similar reduction of the elastic cross isotopes have different structures, namely, 9Be is a section is observed in collisions involving deformed nuclei Borromean weakly bound nucleus (Sn ¼ 1:67 MeV), [21] where it arises from coupling with the strong Coulomb þ with a well-developed - -n cluster structure (see, e.g., excitation of the 2 state in the target. In [4] the effect of [19]). With an additional nucleon and due to pairing, 10Be coupling with a large Coulomb dipole excitation due to the in its ground state is equally deformed but much more presence of the continuum low-lying E1 strength is inves- 9 11 6 bound (Sn ¼ 6:81 MeV) than Be. Finally, Be is a one tigated for He projectile on different target charges and neutron halo nucleus whose core is 10Be and its binding beam energies. It is concluded that close to the Coulomb energy is only Sn ¼ 503 keV [20]. By comparing the barrier, coupling to Coulomb dipole breakup should be elastic-scattering AD for these three systems, the separate evident only in scattering with targets having high charge effect of the weak binding and halo structure can be (ZT 80) and that measurements with lighter targets 11 investigated. (ZT 28) are not sensitive to this coupling. Be has The data with the radioactive beams were obtained, in strong low-lying continuum dipole strength, as 6He. The the same experiment, using the new postaccelerated 10;11Be observation of a strong reduction of the Coulomb-nuclear beams of REX-ISOLDE at CERN. The detection system interference peak (CNIP) in the scattering of 11Be with a used consisted of an array of Si-detector telescopes each light charge target must be due to other mechanisms be- formed by a 40 m, 50 Â 50 mm2, ÁE DSSSD detector sides coupling to Coulomb breakup. These mechanisms (16 Â 16 pixels) and a 1500 m single pad E detector. The could be associated with the halo structure. In 6He-induced detectors were placed very close to the target in order to collisions, a clear reduction of the elastic scattering in the have a large angular (10 150) and solid angle CNIP region is observed, although not as large as in the coverage. Because of the high granularity, the AD could be present case only for heavy targets [22]. The 6He elastic- obtained with a 1 step. The beam energy resolution was insufficient to separate 11Be elastic from inelastic scatter- 11 ing of the Be 1st excited state at Ex ¼ 320 keV, but as we will see in the following, the inelastic channel contributes very little to the measured AD. A 550 and 1000 g=cm2 64Zn target was used with 10Be and 11Be beams, respec- tively. The target was tilted at 45 to facilitate the mea- surement in the angular region around 90.
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