The Scientific program with RIBRAS (Radioactive Ion Beams in Brasil)

AIP Conference Proceedings 1139, 76 (2009); https://doi.org/10.1063/1.3157833 1139, 76 © 2009 American Institute of Physics. The Scientific program with RIBRAS (Radioactive Ion Beams in Brasil) Cite as: AIP Conference Proceedings 1139, 76 (2009); https:// doi.org/10.1063/1.3157833 Published Online: 08 June 2009 R. Lichtenthäler, A. Lépine-Szily, V. Guimarães, P. N. de Faria, D. R. Mendes, K. C. C. Pires, V. Morcelle, E. A. Benjamim, A. M. Moro, M. S. Hussein, A. Arazi, A. Barioni, R. Pampa Condori, M. C. Morais, M. Assunção, J. Alcantara Nunẽz, I. Padron, P. R. S. Gomes, O. Camargo, Y. Otani, E. Leistenschneider, and V. Scarduelli ARTICLES YOU MAY BE INTERESTED IN The Research Program at RIBRAS (Radioactive Ion Beams in Brasil)-III AIP Conference Proceedings 1224, 461 (2010); https:// doi.org/10.1063/1.3431452 Near-barrier scattering of and AIP Conference Proceedings 1012, 333 (2008); https:// doi.org/10.1063/1.2939322 The Scientific program with RIBRAS (Radioactive Ion Beams in Brasil) R. Lichtenthäler*, A. Lépine-Szily*, V. Guimarães*, P.N. de Faria*, D. R. Mendes Jr*, K. C. C. Pires*, V. Morcelle*, E.A. Benjamim†, A. M. Moro,**, M.S. Hussein*, A. Arazi‡, A. Barioni *, R. Pampa Condori*, M.C. Morais*, M. Assunção§, J. Alcantara Nunez*, I. Padron¶, P.R.S. Gomes", O. Camargo Jr.*, Y. Otani*, E. Leistenschneider* and V. Scarduelli* *Instituto de Física da Universidade de São Paulo, C.P. 66318,05389-970 São Paulo, Brazil †Universidad de Santiago de Compostela, Depto. Fisica Particulas, Facultad Fisica, Campus Sur s/n 15786 Santiago de Compostela (Spain) **Departamento de FAMN, Universidad de Sevilla, Apdo 1065, E-41080, Sevilla, Spain ‡Laboratorio Tandar, Departamento de Física, Comisíon Nacional de Energía Atómica, Av. del Libertador 8250,(1429), Buenos Aires, Argentina § UNIFESP - Campus de Diadema, SP ¶CEADEN,P.O.Box, 6122,Havana,Cuba "Instituto de Física, Universidade Federal Fluminense, Niteroi, R.J. 24210-340, Brazil Abstract. The Radioactive Ion Beams Facility (RIBRAS) is in operation since 2004 at the Pelletron Accelerator Laboratory of the University of São Paulo and consists of two superconducting solenoids capable of producing low energy secondary beams of light exotic nuclei. Measurements of the elastic scattering, breakup and transfer reactions with radioactive projectiles such as 6He, 8Li, 7Be on several targets have been performed. A review of the research program carried on along the last four years using the RIBRAS facility is presented. Keywords: radioactive beams, elastic scattering PACS: 25.60.Bx, 29.38.-c INTRODUCTION The possibility of producing secondary beams of unstable nuclei opened a huge field of research in Nuclear Physics. There are 283 stable nuclei and thousands of nuclides out of the line of stability with half lifes long enough to allow the production of secondary beams. Nuclei very far from the line of the stability and close to the drip lines, the so called exotic nuclei, present very interesting new features as the “neutron halo” observed in neutron rich nuclei such as 11Li [1], 11Be, 6He and others. The very low binding energy and low angular momentum of the valence neutrons in these nuclei favours the formation of a halo of neutrons which extends over large distances from the stable core [2] and has densities much lower than the normal nuclear density. The 11Li and 6He nuclei are examples of Borromean nuclei consisting of two neutrons loosely bound to a core (n + n + c) forming a bound three body system. The study of the nuclear structure of the exotic nuclei is very motivating since it represents a challenge for the nuclear theorists who developed their models for the nuclei around the line of the stability. The modification or even the disappearance of the magic numbers observed for nuclei CPU 39, Nuclear Physics 2008, XXXI Workshop on Nuclear Physics in Brazil, edited by V. Guimaraes, J. R. B. Oliveira, K. C. D. Macario, and F. A. Genezini © 2009 American Institute of Physics 978-0-7354-0676-6/09/$25.00 76 near to the neutron drip line is an example of this challenge. The presence of a very difuse nuclear surface due to the halo could have an effect on the spin-orbit interaction of the surface nucleons causing modifications in the shell structure. The existence of the exotic nuclei, on the other side, has certainly consequences in other fields as the astrophysics. The nucleosynthesis and the energy production in massive stars, where the temperatures and densities are sufficiently high to produce exotic species, can be affected by its presence in the stellar enviroment. For instance, the presence in massive stars of light neutron rich nuclei as the 6He and 8Li can play a role in the nucleosynthesis of heavy elements helping to bridge the mass A = 5 and A = 8 gaps [3]. In order to explore some of these new interesting subjects a group of physicists decided to install the Radioactive Ion Beams Facility (RIBRAS) in the São Paulo Pelletron Accelerator Laboratory. The 8MV São Paulo Pelletron is a low energy Tandem accelerator, capable of producing primary beams of stable elements in the energy range of 3-5 MeV/A and intensities of 1mAe maximum. At these low energies the transfer reactions available for producing unstable nuclei have cross sections of about 10 -20 mb to be compared to the larger cross sections of fragmentation reactions of 100 - 200mb at higher energies. For this reason a system of large acceptance is required to obtain useful secondary beam intensities. Solenoids are selectors with acceptance of about 30 msr in comparison to 5 msr of dipoles. For this reason a system of solenoids was considered to be the best option for our accelerator. The decision of using superconductors solenoids was taken in the perspective of the installation of a 10 MeV/A superconducting LINAC post accelerator in the future. Since 2004 we are operating with RIBRAS in one of the Pelletron beam lines. We are performing measurements of elastic scattering angular distributions of 6He, 8Li and 7Be on several targets. These measurements allow a systematic study of the interacting potential of these exotic projetiles and the determination of the total reaction cross sections at low energies. Reaction channels other than the elastic, as the projectile breakup, neutron transfer reactions, the p(8Li,a) and 12C(8Li,9 Be)11B transfer reactions are also being investigated. In this paper we present a description of the RIBRAS system and a review of the most important measurements performed until now. THE RIBRAS SYSTEM The system consists of two superconducting solenoids [4, 5] of maximum field of 6.5T each and maximum Br 2T.m. The criostats have 1 meter long per 1 meter diameter and are mounted on an aluminum bench of 7.5m length total. The bore of the criostats has 30cm diameter The primary target is mounted on a ISO chamber just before the first solenoid and consists of a gas cell with two windows. To produce the 6He and 8Li beams we use a 9Be exit window which is the primary target itself. As the entrance window we use a havar foil of 2mm. Helium gas can be pressurized into the cell to cool down the Berilum foil which is heated by the high intensity primary beam. At primary beam currents above 500nAe the cooling is necessary to prevent the primary target breakdown, however, below this intensities, the cooling is not necessary and one can operate without gas in the cell. High pressure gas targets of a few bars are possible by using two havar 77 windows in the cell. The usual production reaction at RIBRAS are 9Be(7Li,8 Li6He), and 3He(7,6Li,7 Be) [6] and the intensities of secondary beams are of the order of 105pps. After crossing the primary target the primary beam is supressed in a tungsten Faraday cup which measures its intensity. A current integrator measures the total charge incident on the primary target during a run. The secondary particles produced in the forward direction are collected by the first solenoid within an angular acceptance of 2 ≤ Q ≤ 6deg. The magnetic field of the first solenoid is adjusted to focus the secondary beam of interest in an ISO-250 scattering chamber located in the middle between the solenoids. Due to the low energy of the machine we are presently using only the first solenoid and performing the experiments in the secondary scattering chamber in the midway between the two solenoids. The secondary targets are mounted in the center of the mid scattering chamber in a mounting which permits the change of up to 4 targets from the outside of the chamber without breaking the vacuum. The detectors are mounted in the scattering chamber on a rotating plate which allows the measurement of angular distributions. A systems of 4 Silicon telescopes E(500-1000mm)-DE(20mm) are normally used as the detector system and are mounted separated by 30deg. A fixed detector can be mounted on the top of the scattering chamber to be used as a monitor. The area of the silicon detectores are between 150 -300mm2 and the typical solid angles of the telescopes are in the range from 10 msr at the forward angles up to 50 msr at the backward angles. The secondary targets have typical thickness in the range of a few mg/cm2 and its thickness are measured off-line by the energy loss of alpha particles from a 241Am source. Usually a heavy element target as gold is mounted and runs are performed during the experiment. In many cases at our energies the scattering in gold is pure Rutherford and it can be used for normalization of the cross sections and monitoring the secondary beam production rate. The energy resolution of the secondary beams is tipically in the range 0.5-1 MeV and is determined mainly by the straggling on the primary target and the kinematics of the production reactions. The straggling in the primary target is calculated by the difference in energy of the reaction products coming from a reaction taking place at the beginning or at the end of the primary target. THE 8Li, 6He+51V SYSTEMS The first experiment at RIBRAS has been performed in 2004 during the XIII J.A. Swieca Summer School in São Paulo. 8Li and 6He secondary beams have been produced at Elab = 26 MeV and Elab = 23 MeV respectively. The difference in energy of the secondary beams is due to the Q-values of the production reactions. Quasi-elastic angular distributions of 6He and 8Li on natural 51V have been measured and are presented in Fig.1. By a coincidence of energies between the alpha contaminant and the 6He beam we were able to measure two angular distributions at comparable energies for the 6He,a+51V systems. It allows a comparision between the halo 6He and the double magic a projetiles. One can see in Fig. 1 that the halo 6He is more absorptive. 78 FIGURE 1. 8Li,6He+51V scattering angular distributions. Comparison between 6He and 4He+51V at the same energy (right down). The solid curves are optical model calculations. THE 6He+27 Al SYSTEM Previous studies [7, 8, 9] of the elastic scattering of 6He and other stable weakly bound projectiles as on several medium mass targets have shown that there is a large enhancement of the total reaction cross section of the halo projectile in comparison to the weakly bound nuclei. The reaction channels causing this enhancement are not yet well identified and the first guess was that the projectile breakup could be responsible for the extra reaction cross section. As the 6He is a weakly bound halo nucleus it is expected that it will easily break into an alpha particle and two neutrons giving rise to this extra reaction cross section observed as an absorption in the elastic scattering. The idea was to extend the measurements of medium mass targets to lighter targets as the 27Al. In lighter targets the Coulomb breakup is less important compared to the nuclear breakup which becomes dominant. The 27Al was chosen due to the existence in the literature of previous measurements [7] of elastic angular distributions using several projectiles that would allow a systematic analysis of the reaction cross sections data. Four elastic scattering angular distributions 6He +27Al have been measured at low energies and the results are presented in [5]. The data have been analysed by the Optical Model adjusting the normalization of the imaginary part and the difuseness of the density distribution to 79 FIGURE 2. E -DE spectrum of the 6He+120Sn collision fit the angular distributions. The reduced reaction cross sections have been compared to other systems and apparently no enhancement with respect to the weakly bound projetiles is observed. THE6He+120 Sn SYSTEM As the enhancement observed in the total reaction cross section for the 6He projectile apparently disappears for lighter targets, the next step is to extend the measurements to heavier targets. The 120Sn is an interesting candidate since it is not a very deformed nucleus, magic in protons, with its first excited state at 1.171 MeV which would be easily separated from the elastic peak. We measured elastic angular distributions of 6He +120Sn at four energies (Elab = 17.4,18.1,19.8,20.5 MeV) a little above the Coulomb barrier. We used an isotopically enriched (98.29%), 3.8mg/cm2, 120Sn target. Four E - DE(20mm) silicon telescopes have been mounted in the scattering chamber and angular distributions have been measured from 20deg up to 75deg in steps of 5deg in the laboratory. We performed normalization runs with gold target in the begin and at the end of each 120Sn run. In Fig. 3 we present an E -DE spectrum of the particles emerging from the 120Sn target. The elastic peak is clearly separated of the 7Li2+ contamination. In the alpha particle line there is a group of counts with a broad distribution of energies centered a little below the energy of the elastic peak. Further we will discuss in more detail the energy spectrum of the alpha particles and present some possible reaction mechanisms which could produce them. 80 1,2 1 0,8 0,6 0,4 0,2 0 1,2 1 0,8 0,6 0,4 0,2 0 10 20 30 40 50 60 70 80 90 q (deg) 10 20 30 40 50 60 70 80 90 q (deg) - - - - ^ !L cm. E =20.5 MeV lab >£ \ - V -• ^^J 10 20 30 40 50 60 70 80 90 q (deg) FIGURE 3. 6He+120Sn elastic angular distributions compared to CDCC calculations In figure 5 we show the elastic scattering angular distributions compared with CDCC calculations considering the breakup of the projectile 6He —> a + 2n. In this calculation an improved two body model is used to describe the wave function of the 6He [10] which simulates the realistic three body wave function. The elastic angular distributions are well reproduced by the CDCC calculations at the four energies measured. The total reaction cross obtained indicate that there is an enhancement with respect to systematics of the weakly bound projectiles on medium mass targets [7]. However the breakup cross sections which comes out from the CDCC calculations are much lower than the total alpha yield measured indicating that processes other than the projectile breakup are contributing to the alpha yield. The 1n transfer followed by the 5 He breakup and the 2« transfer between the 6He projectile and the target are processes that produce alpha particles in the exit channel and could contribute to the observed alpha yield. As the transfer processes are known to be selective in Q-value, it is interesting to analyse not only the angular distributions but also the energy distribution of the alpha particles emitted. The Q-value of a transfer reaction is determined by the difference in binding energies (EB > 0 for bound states) of the transfered particle between the final and initial states Q = Ebf — Ebi where £& is refering to the binding energy of the transfered particle in the projectile (ground state) and Eur = E?5f — Eexct refers to the transfered particle to a state of the final nucleus either the ground state Eff or any excited state Eexct. It is well known that there is an optimum value for the reaction Qopt around which the transfer process is more probable to occur. A simple estimate of the Qopt value is given by the formula:Qopt = Ec^-^3j4 — 1] [11]. This formula means that for neutrons 0 81 FIGURE 4. The picture represents the di-neutron transfer process. The transfer to weakly bound states in the final nucleus is favoured by the Q-window. FIGURE 5. 120Sn(6He,a) reaction. Energy spectra of the alphas(left) and angular distributions(right). Qopt = 0 and consequently the transfer to excited states of the final target nucleus for which Eb f Ebi would be enhanced. As for the weakly bound exotic projectile 6He, the initial binding energy of the transfered particle Ebi = 0.973 MeV is small, final states of low binding energies (but still bound states) will be favoured in the transfer process. Possibly final states in the continuum would also be populated due to the Q-window width (see Fig.4 above). 82 In the Fig. 5 we present energy spectra of the alpha particles corrected for the energy losses in the target, and compare to the Q-window calculations using the formula of [12] and suposing that the alpha particles are produced by the 2n-transfer reaction 120Sn(6He,a)122Sn. We see that at 17.4 MeV most of the alpha spectrum is reproduced by the Q-window indicating that the 2n-transfer can be an important process. The breakup (CDCC) calculation on the other side predicts an energy distribution centered at lower energies. The 1n-transfer reaction 120Sn(6He,5 He)121Sn followed by the 5He breakup predicts alpha particles of energies lower than the 2n-transfer but higher than the 6He breakup in agreement with a part of the observed spectra at 18.05,19.8 and 20.5 MeV. We are presently performing DWBA calculations for 1n and 2n transfers considering the transfer to both, bound states and continuum states. The transfer to continuum (TC) resulted a much higher cross section compared to the breakup. Also the shape of the angular distributions in the TC calculations is in better agreement with the data compared to the breakup results (see Fig. 5 (right)) indicating that the neutron transfer is an important reaction channel. SUMMARY The research program at RIBRAS so far consisted basically of elastic scattering measurements of 6He, 8Li and 7Be on light, intermediate and heavy mass targets at energies around and above the Coulomb barrier. These measurements can provide a systematic study of the interacting potential of exotic systems and the determination of total reaction cross sections which can be compared to other stable systems. We plan to extend this research program to identify other reaction channels such as fusion and transfer which could be responsible by the enhancement in the reaction cross section observed with halo projectiles. ACKNOWLEDGMENTS The authors thank the FAPESP (proc.no.2003/10099-2) and CNPq for financial support. REFERENCES 1. I. Tanihata et al., Phys. Rev. Lett. 24,2676(1985) and Phys. Lett. 160B,380(1985) 2. T. Kobayashi et al., Phys. Rev. Lett. 60,2599(1988). 3. J. Görres, H. Herndl, I.J. Thompson, M. Wiescher, Phys. Rev. C52, 2231(1995) 4. R. Lichtenthäler et al., Eur. Phys. J. A 25,s01,733 (2005); Nucl. Phys. News 15, 25 (2005). 5. E.A. Benjamim et al., Physics Letters B647,30,(2007) 6. R. Lichtenthäler et al., Eur. Physical Journal Special Topics 150,27 (2007). 7. P.R.S. 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