Inclusive breakup measurements of the 7 Li+ 119 Sn system. M. A. G. Alvarez 1, A. Di Pietro 2, B. Fernández 3, J. P. Fernández-García 2,4, P. Figuera 2, L. R. Gasques 1, J. Gómez-Camacho 3, M. Lattuada 2,3, Jin Lei 5, A.M. Moro 6, D. Santonocito 2, A. C. Shotter 7, D. Torresi 8, M. Zadro 9, V. A. B. Zagatto 1 1 Instituto de Física, Universidade de São Paulo, São Paulo, Brazil. 2 INFN, Laboratori Nazionali del Sud, Catania, Italy 3 Centro Nacional de Aceleradores, Seville, Spain 4 Dipartimento di Fisica e Astronomia, Catania, Italy 5 Department of Physics and Astronomy, Ohio University. OH, EEUU 6 Departamento FAMN, Universidad de Sevilla, Sevilla, Spain. 7 School of Physics and Astronomy, University of Edinburgh, UK 8 School of Physics and Astronomy, University of Birmingham, UK 9 Ruder Bošković Institute, Bijenička, Zagreb, Croatia 1 Instituto de Fisica da Universidade Federal Fluminense, Niteroi, Brazil Abstract We propose to measure the 7 Li+ 119 Sn reaction at energies below, around and above the Coulomb barrier in order to study the inclusive breakup cross-sections of the fragments coming from the dissociation of the 7 Li projectile. These experimental data will be compared with the predictions of calculations based on a recent implementation of the inclusive breakup model proposed by Ichimura, Austern and Vincent, recently revisited by several groups. Introduction During the last decades, the study of collisions induced by halo nuclei has revealed that their diffuse structure associated with a very low breakup threshold have an effect in their reaction dynamics (see e.g. [1] and references therein). Due to its low binding energy, the dissociation of the projectile into two or more fragments is an important mechanism occurring in collisions induced by these radioactive nuclei and coupling effects to the continuum deeply affects the other reaction channels around the barrier. Stable weakly bound nuclei, such us 6,7 Li, have similar characteristics as halo nuclei, such as a low breakup threshold associated with a marked cluster structure of their ground state. Therefore, the study of reactions induced by stable weakly bound nuclei (where good quality data are easier to obtain due to the higher beam intensity) enables to check the validity of new models which will also help in understanding the reaction dynamics with halo nuclei. The analysis of breakup experiments has provided important structure information on these weakly bound nuclei, such as spectroscopic factors, separation energies, positions and widths of resonances [2]. Due to the low beam intensities and the limitations of the experimental setups, fully exclusive breakup measurements involving exotic weakly-bound nuclei are difficult to obtain. More typically, only the charged fragments are detected and the measured cross sections are therefore inclusive with respect to the unobserved fragment(s). The inclusive cross sections can be split into two parts: (i) the elastic breakup (EBU), in which the projectile fragments survive after the collision and the target remains in its ground state, and (ii) the non-elastic breakup (NEB), which involves the target excitation or the absorption or transfer 1
of one or more projectile constituents. The EBU observables can be accurately obtained with standard reaction models, such as the continuum-discretized coupled-channels (CDCC) method. In contrast, NEB cross sections are difficult to compute owing to the large number of final states involved. For that reason, closed-form inclusive breakup models were proposed in the 198s, although the computational limitations at that time prevented their full implementation and assessment. Recently, these models have been reexamined and implemented by several groups [3, 4]. In particular, the comparison of the model proposed by Ichimura, Austern and Vincent [5] with inclusive breakup data for d and 6 Li induced reactions has shown very encouraging results. Further comparisons against experimental data are nevertheless necessary to establish the validity and limitations of these models. Motivation In Ref. [3], the NEB method has been tested with success for the case of d and 6 Li, in which the projectile ground state is in a dominant s-wave. To test the validity of this new method, it is important to extend the study to more general cases, in which the internal angular momentum is l. This is the case of 7 Li, with l=1, having a marked α-t cluster structure with a breakup threshold of 2.47 MeV. In Ref. [6], the NEB calculations have been compared with one of the few inclusive breakup measurements of 7 Li available in the literature at the barrier [7]. A disagreement between the experimental data and the predictions of the NEB method has been observed. This discrepancy is not yet fully understood, thus new inclusive breakup measurements involving 7 Li will be very useful to clarify it. The aim of this proposal is to obtain inclusive breakup measurements of α and t particles produced in the collision 7 Li+ 119 Sn at energies below, around and above the Coulomb barrier ( =18, 2 and 25 MeV). This colliding system has been chosen because these measurements will complement our recent study of Ref. [8], where the fusion excitation function of the same system was obtained for a wide energy range around the Coulomb barrier. In addition, as experimentally observed in [8], for this colliding system α particles coming from fusion evaporation reactions are not expected, since in the energy range around the Coulomb barrier, the evaporation of charged particles is strongly hindered by the Coulomb barrier and the compound nuclei decay by neutron emission. The results of the experiment here proposed will be compared with the predictions of the inclusive breakup model discussed in Ref. [3, 6]. This analysis is expected to improve our understanding of several unresolved problems in fusion studies, such as the suppression of complete fusion and the relative importance of the incomplete fusion in the total fusion cross sections. Experimental setup The measurements will be performed using the CT2 scattering chamber. A self-supporting 119 Sn target, 25 µg/cm 2 thick, will be used in the experiment. Tritons and alpha particles will 2
be detected by five silicon detector telescopes, each consisting of a 1-15 µm thick E1 detector followed by 15 µm thick E2 detector and 1 µm-thick E res detector. The telescope system will be mounted on a rotating plate. Circular collimators with diameters of 6 mm will be positioned in front of each telescope, and their angular opening with respect to the target center will be 1.. Angular distributions will be measured in the angular range of θ lab 4-16 with a step of 1. This angular range can be covered in three steps. Two telescopes, each consisting of a 15 µm thick E detector followed by a 5 µm thick E detector, placed symmetrically with respect to the beam direction at angles of ±15, with an angular opening of.2, will be used to monitor the beam position and for normalization purposes. The ratio of the solid angles of the monitors and detectors will be determined by the Rutherford scattering of 7 Li on a 1 µg/cm 2 thick 197 Au target. 119 Sn( 7 Li, αx) 119 Sn( 7 Li, tx) 8 6 4 =25 MeV NEB EBU TBU 2 15 1 =25 MeV NEB EBU TBU 2 5 =2 MeV =2 MeV 3 2 1 4 2 3 6 9 12 15 18 θ lab (deg) 3 6 9 12 15 18 θ lab (deg) Figure 1: Calculated angular distribution of α (left) and t particles (right) produced in the reaction 7 Li+ 119 Sn at incident energies of 21.2 and 26.5 MeV. The dotted, dashed and solid lines represent the EBU (calculated with the CDCC method), NEB (calculated according to Ref. [3]) and their sum, respectively. Beam time request For the beam time estimate we used as reference the sum of NEB, calculated with the model of Ref. [3], and EBU, obtained with the CDCC method, of the angular distributions of α and t particles (see Fig. 1). Taking into account the 119 Sn target thickness, detector solid angles, a beam intensity of 1 1 pps (1.6 pna) and an average of 9 BTU per energy, we expect a statistical error on the α and 3 H angular distributions smaller than 6%. This will also allow to build-up energy spectra at a given angle with a reasonable statistics for most of the angular settings. For electronics set-up, calibrations and solid angle normalization runs we ask for 2 BTU. Thus, we 3
ask 29 BTU in total. References [1] L. Canto et al, Physics Reports 596, 1 (215). [2] T. Nakamura, Y. Kondo, Clusters in Nuclei, 2, C. Beck (Ed.), Springer,Berlin-Heidelberg (212) [3] Jin Lei and A. M. Moro, Phys. Rev. C 92, 44616 (215). [4] G. Potel et al, Phys. Rev. C 92, 34611 (215). [5] M. Ichimura, N. Austern, and C. M. Vincent, Phys. Rev. C 32, 431 (1985) [6] Jin Lei, Study of inclusive breakup reactions induced by weakly bound nuclei, PhD. thesis. University of Seville (216) [7] G. R. Kelly et. al, Phys. Rev. C 63, 2461 (2). [8] M. Fisichella et. al, Phys. Rev. C, accepted (216). 4