Scattering of 8He on 208Pb at Energies Around the Coulomb Barrier

Vol. 43 (2012) ACTA PHYSICA POLONICA B No 2 SCATTERING OF 8He ON 208Pb AT ENERGIES AROUND THE COULOMB BARRIER∗ G. Marquínez-Durána, A.M. Sánchez Beníteza, I. Martela R. Berjillosa, J. Dueñasa, V.V. Parkara, L. Acostaa,b K. Rusekc, M.A.G. Álvarezd,e, J. Gómez-Camachod,e M.J.G. Borgef, C. Cruzf, M. Cuberof, V. Pesudof, O. Tengbladf A. Chbihig, J.P. Fernández-Garcíae, B. Fernández-Martínezd J.A. Labradord, A.H. Ziadd, J.L. Floresh, N. Keeleyi L. Standyloi, I. Strojeki, M. Marquesj, M. Mazzoccok A. Pakoul, N. Patronisl, D. Pierroutsakoum, R. Silvestrim R. Raaben, N. Soico, R. Wolskip aDepartamento de Física Aplicada, Universidad de Huelva, 21071 Huelva, Spain bLaboratori Nazionali del Sud, INFN, Via Santa Sofia 62, 95123, Catania, Italy cHeavy Ion Laboratory, University of Warsaw Pasteura 5a, 02-093 Warszawa, Poland dCentro Nacional de Aceleradores, 41092 Sevilla, Spain eDepartamento de Física Atómica, Molecular y Nuclear, Universidad de Sevilla 41080 Sevilla, Spain fInstituto de Estructura de la Materia, CSIC, 28006 Madrid, Spain gGANIL, CEA and IN2P3-CNRS, B.P. 5027, 14076 Caen cedex, France hDepartamento de Ingeniería Eléctrica, Universidad de Huelva 21071 Huelva, Spain iNational Centre for Nuclear Research, A. Soltana 7, 05-400 Otwock, Poland jLaboratoire de Physique Corpusculaire, 14050 Caen cedex, France kDip. di Fisica and INFN, Università di Padova via F. Marzolo 8, 35131 Padova, Italy lDepartment of Physics, University of Ioannina, 45110 Ioannina, Greece mINFN — Sezione di Napoli, via Cintia, 80126 Napoli, Italy nInstituut voor Kern-en Stralingsfysica, K.U. Leuven, Belgium oRudjer Boskovic Institute, Bijenicka 54, 10000 Zagreb, Croatia pInstitute of Nuclear Physics PAN, Kraków, Poland (Received November 28, 2011) We have measured the angular distributions of elastic scattering and 6,4He fragments produced in the collisions of an exotic beam of 8He on a208Pb target at laboratory energies of 18 and 22 MeV, just around the Coulomb barrier (19 MeV). The measurements were performed at the SPIRAL/GANIL facility in Caen, France. In this paper, we present preliminary data on elastic cross sections and discuss the results using optical model and coupled reaction channel calculations. DOI:10.5506/APhysPolB.43.239 PACS numbers: 01.30.Cc, 13.85.Dz, 24.10.Ht, 25.10.+s ∗Presented at the XXXII Mazurian Lakes Conference on Physics, Piaski, Poland, September 11–18, 2011. (239) 240 G. Marquínez-Durán et al. 1. Introduction Nuclei far from stability have small binding energies and large asymmetry between proton and neutron numbers. These structural features produce extended wave functions for the few valence nucleons producing the so-called halo, often arranged in Borromean configurations, where any of its two-body subsystems are unbound [1]. These features largely affect the interaction with heavy targets at low collision energies, and a number of detailed experimental studies have been carried out so far for the case of light neutron halo systems like 11Li, 6He or 11Be [2,3]. Due to the extended mass distribution and the loosely bound structure, the behaviour of neutron halo nuclei is very different from stable, tightly bound nuclei. In the scattering with heavy targets at low energies, the combination of the nuclear interaction and the strong electric field will make the core and the halo neutrons move in opposite directions, thus producing strong coupling to dipole modes of the system at relatively low excitation energy [4,5]. These effects are amplified at energies near the Coulomb barrier, where the time scale of the collision process is comparable to the typical excitation time of the collective degrees of freedom and the coupling to the corresponding continuum state. This strong coupling scenario dominates the shape of the angular distribution of the reaction fragments, allowing a detailed study of reaction processes and extraction of useful structural quantities like spectroscopic factors or Coulomb dipole transition strengths [6]. Post-accelerated 6He beams are among the most popular, presently available at many low energy Radioactive Beam Facilities (RBFs). Recent experiments to investigate near-barrier halo effects in the system 6He + 208Pb have been performed in the Cyclotron Research Center/UCL in Louvain-la- Neuve (Belgium) [5,6,7,8]. The results show that the Coulomb rainbow, a typical feature of near-barrier heavy ion collisions involving tightly bound stable beams, has completely disappeared as compared with 4He scattering on similar systems at the same centre of mass collision energy. The existence of long-range absorption mechanisms, acting at large distances, can be directly deduced from the shape of the elastic angular distributions. The angular and energy distributions of the 4He fragments produced in the interaction with heavy targets revealed the main contribution of the neutron transfer channel through continuum states. This mechanism seems to play also a dominant role in sub-barrier fusion reactions with heavy targets, and an important experimental effort has been very recently carried out at FLRN-JINR (Dubna, Russia) and UCL to disentangle the relevant reaction processes [9,10]. The scattering of 8He with heavy targets has attracted the interest of the scientific community due to its peculiar nuclear structure. It is a well-known skin nucleus, having a larger but tighter neutron layer as compared to the Scattering of 8He on 208Pb at Energies Around the Coulomb Barrier 241 weakly bound 2n-halo in 6He. On the other hand, the binding energies for 1nand 2nsystems are similar, whereas in the case of 6He the breakup of the 2nsystem from the alpha core is energetically favoured. These differences in geometry and binding energies should be reflected in the elastic scattering and reaction cross sections. The scattering of 8He on gold and copper targets was previously investigated at GANIL (Caen, France) [11,12], where a first measurement of fusion cross sections and 1n/2ntransfer channels at Coulomb barrier energies was performed. We have performed optical model (OM) and coupled reaction channel (CRC) calculations with the code FRESCO [13] for the system 8He + 208Pb at different energies around the barrier to investigate the effect of the 1n stripping process on the elastic channel, and the results are shown in Fig. 1. The calculations were similar to those for this system described in Ref. [3], and employed double-folded real potentials and interior Woods–Saxon imaginary potentials to simulate the ingoing-wave boundary condition. Thus, all surface absorption in these calculations is generated by the couplings to the 1nstripping. The solid curves denote the results of the full CRC calculations, while the dashed curves indicate the results of OM calculations with all couplings switched off. At energies well below the barrier (16 MeV and 18 MeV) this process leads to a strong absorption pattern similar to 6He scattering on 208Pb at the same collision energies, which accounts for a more than 50%reduction in the elastic cross sections at backward angles. At colli- Fig. 1. CRC calculations (solid/black line) including the 1nstripping channel for the system 8He + 208Pb at collision energies Elab = 16, 18, 22 and 27 MeV. The neutron stripping produces a strong reduction on the elastic cross sections as compared to the results of simple OM calculations (dashed/red curves). 242 G. Marquínez-Durán et al. sion energies just around and above the barrier (22 MeV and 27 MeV) there is a partial suppression of the Coulomb rainbow. Although a sizeable part of the Coulomb-nuclear interference effect is removed due to the weak binding of the neutrons, the effect of the tight neutron skin still resembles that of a traditional stable heavy ion nucleus. This dynamical scenario is very different to the case of the scattering system 6He + 208Pb, which is dominated by the 2ntransfer process and the rainbow effect is completely suppressed. In this work we report on recent measurements performed at SPIRAL/GANIL facility in Caen (France) for the system 8He + 208Pb at energies just around the Coulomb barrier. In particular, we focus on the elastic channel and the effect of the 1nstripping channel on it. 2. Experimental setup The experiment was performed at the SPIRAL/GANIL facility in Caen (France). The 8He beam is produced by means of the fragmentation of a 13C primary beam at 75 MeV/nucleon in a dedicated target for the production of helium isotopes. After being purified and re-accelerated in the CIME cyclotron, this method allows for the delivery of an intense 8He beam with a diameter around 5mm and an intensity between 104and 105pps on target. The 8He can be ionized to +1 and +2 charge state. For nuclear reactions at low energies around the Coulomb barrier the +1 charge state is normally selected because a larger intensity can be achieved. Once the beam was produced and accelerated, it was driven through various collimators and beam diagnostics into the reaction chamber. The target was a selfsupported foil of 208Pb with a thickness of 1 mg/cm2and it was placed inside the GLORIA (GLObal ReactIon Array) silicon array developed at the University of Huelva. The GLORIA detector array consists of twelve DSSSD detectors arranged in six particle detector telescopes. All the telescopes are tangent to a sphere of 60 mm radius centered in target position, resulting in a overall solid angle of the system of 26.1%of 4π. Each telescope is made of a 40 µm∆E-detector of 50 mm ×50 mm, segmented in 16 strips on each side, and by a 1 mm E-detector of the same size and segmentation; the strip pitch was of 4 mm for all detectors of the array. The relative position of these telescopes with respect to the reaction target was optimised considering the following parameters: (i) a maximum angular range coverage, (ii) a good angular resolution (less than 5◦), (iii) an angular range overlap between telescopes and (iv) a symmetric position of detectors. The array was designed in such a way that it covers a continuous angular range between 15◦and 165◦, with no gaps and with a high granularity. The angular coverage and solid angles corresponding to the final position of the telescopes are pre- Scattering of 8He on 208Pb at Energies Around the Coulomb Barrier 243 sented in Table I. The 208Pb target was introduced tilted 30◦with respect to the vertical position avoiding the shadowing of detectors and ensuring the detection of particles around 90◦. TABLE I Angular coverage and solid angle for each telescope. Telescope θcenter [deg] Angular range [deg] Solid angle [msr]/%4π 1 Forward 38 15.0–62.5 547.2/4.35 2 Backward 142 117.0–165.0 547.2/4.35 3 Forward 38 15.0–62.5 547.2/4.35 4 Backward 142 117.0–165.0 547.2/4.35 5 Top 75 82.0–128.0 547.2/4.35 6 Bottom 105 52.0–97.5 547.2/4.35 In order to fully exploit the available beam time, a fusion stack consisting of several 206Pb, aluminium and mylar foils was placed behind the reaction chamber, at the end of the S3 frame. For the energy calibration of detectors, we used four calibration points: a triple alpha source was used before and after the measurements of 8He scattering and the elastic peak, taking into account energy loss in the target and detector dead layer. A test signal was sent to the preamplifiers with a low rate pulse generator for evaluating the efficiency of the electronic chain related to each DSSSD detector. 3. Preliminary results A typical particle ∆E(E)spectrum obtained at forward angles with beam energy Elab = 22 MeV is shown in Fig. 2(a). The elastic as well as the 6,4He production channels are clearly separated, and even a small fraction of contaminants of 6He ions, estimated below 0.0001%, was observed during the experiment. The energy resolution achieved in our experiment was around 40 keV. In Fig. 2(b) we show with red dots the preliminary results obtained for the elastic channel of 8He+208Pb system at 22 MeV. The data are normalised to the corresponding Rutherford cross sections and compared with 6He + 208Pb [5] and 6He + 208Pb [14] scattering systems at similar collision energies. The 4He data is plotted with blue dots and was measured at Elab = 23.5MeV; it exhibits a strong rainbow pattern around the grazing angle (around 75◦Lab) characteristic of light stable nuclei. The angular distribution of 6He is plotted with green dots, and shows the usual strong absorption pattern down to 50◦Lab, where the Coulomb-nuclear rainbow has disappeared. A simple inspection of the data reveals that the total re- 244 G. Marquínez-Durán et al. (a) (b) Fig. 2. (a) Typical ∆E(E)spectrum obtained at forward angles. (b) Extracted angular distribution of the elastic scattering at Elab = 224 MeV. See the text for discussion. action cross section for 8He projectiles is higher than for 6He at the same bombarding energy. The angular distribution of the elastic scattering of 8He follows the trend of the 6He data up to about the grazing angle, where the absorption becomes even stronger. This effect is consistent with the observation of a strong yield of 4He and 6He fragments at the ∆E(E)spectrum shown in Fig. 2(a). As the impact parameter reduces with increasing angle, the combined effect of nuclear and Coulomb couplings produce a larger removal of elastic flux than in the case of 6He scattering system. We also plot in Fig. 2(b) the curve for the full CRC calculation at 22 MeV taken from Fig. 1. The description of the 8He + 208Pb data at backward angles is good, as expected since it is in this angular region where 1nstripping couplings have most influence on the elastic scattering. For angles in the region where the Coulomb rainbow peak would be we see that the calculations overpredict the data, indicating significant effects due to channels not included in our coupling scheme. These are most likely breakup and/or 2nstripping (3nand 4nstripping, while in principle possible for 8He, must be considered unlikely given the possible reaction mechanisms) by analogy with results for 6He + 208Pb where it is just in this region that breakup has most influence on the elastic scattering. The effect of coupling to the 1nstripping appears to be more important for 8He compared to 6He, which may be explained by the larger spectroscopic factor for the 8He/7He overlap compared to the 6He/5He one. Scattering of 8He on 208Pb at Energies Around the Coulomb Barrier 245 4. Summary and conclusions We have measured the elastic scattering and reaction channels for the system 8He + 208Pb at 18 MeV and 22 MeV using the novel charged particle detctor array GLORIA. The experiment was performed at the SPIRAL radioactive beam facility at GANIL (Caen, France). 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