Direct processes for the systems 7Be, 8B + 208Pb at Coulomb barrier energies

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Published under licence by IOP Publishing Ltd 27th International Nuclear Physics Conference (INPC2019) Journal of Physics: Conference Series 1643 (2020) 012096 IOP Publishing doi:10.1088/1742-6596/1643/1/012096 1 Direct processes for the systems 7Be,8B + 208Pb at Coulomb barrier energies M Mazzocco1,2, N Keeley3, A Boiano4, C Boiano5, M La Commara6,4, A Lagni1, C Manea2, C Parascandolo4, D Pierroutsakou4, C Signorini1,2, E Strano1,2, D Torresi1,2, H Yamaguchi7, D Kahl7, L Acosta8,9, P Di Meo4, J P Fernandez-Garcia9, T Glodariu10, J Grebosz11, A Guglielmetti12,5, Y Hirayama13, N Imai7,13, H Ishiyama13, N Iwasa14, S C Jeong13,15, H M Jia16, Y H Kim13, S Kimura14, S Kubono7,17, G La Rana18,4, C J Lin16, P Lotti2, G Marqu´ınez-Dur´an8, I Martel8,19, H Miyatake13, M Mukai13, T Nakao7, M Nicoletto2, A Pakou20, K Rusek21, Y Sakaguchi7, A M S´anchez-Ben´ıtez22,23, T Sava10, O Sgouros20, V Soukeras20, F Soramel1,2, E Stiliaris24, L Stroe10, T Teranishi25, N Toniolo26, Y Wakabayashi17, Y X Watanabe13, L Yang16,7, Y Y Yang27 and H Q Zhang16 1Dipartimento di Fisica e Astronomia, Universit`a di Padova, via F. Marzolo 8, I-35131 Padova, Italy 2INFN-Sezione di Padova, via F. Marzolo 8, I-35131 Padova, Italy 3National Centre for Nuclear Research, ul. Andrzeja So ltana 7, 05-400 Otwock, Poland 4INFN-Sezione di Napoli, via Cintia, I-80126, Napoli, Italy 5INFN-Sezione di Milano, via Celoria 16, I-20133, Milano, Italy 6Dipartimento di Farmacia, Universit`a di Napoli “Federico II”, via D. Montesano, I-80131, Napoli, Italy 7Center for Nuclear Study (CNS) - The University of Tokyo, Wako Branch, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan 8Departamento de F´ısica Aplicada, Universidad de Huelva, Campus de El Carmen, E-21071 Huelva, Spain 9INFN-Sezione di Catania, via Santa Sofia 64, I-95123, Catania, Italy 10 NIPNE, 30 Reactorului Street, 077125 Magurele, Romania 11 Institute of Nuclear Physics Polish Academy of Science, ul. Radzikowskiego 152, 31-342 Krak´ow, Poland 12 Universit`a degli Studi di Milano, Dipartimento di Fisica, Via Celoria 16, I-20133 Milano, Italy 13 KEK, Oho 1-1, Tsukuba, Ibaraki 305-0801, Japan 14 Department of Physics, Tohoku University, Sendai, Miyagi 980-8578, Japan 15 Institute for Basic Science (IBS), 55, Expo-ro, Yuseong-gu, 34126 Daejeon, Korea 16 China Institute of Atomic Energy, P. O. Box 275 , Beijing 102413, China 17 RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan 18 Dipartimento di Fisica, Universit`a di Napoli “Federico II”, via Cintia, I-80126, Napoli, Italy 19 Department of Physics, University of Liverpool, Liverpool L69 7ZE, United Kingdom 20 Department of Physics and HINP, University of Ioannina, 45110 Ioannina, Greece 21 Heavy Ion Laboratory, University of Warsaw, ul. Pasteura 5a, 02-093 Warsaw, Poland 22 Nuclear Physics Center, University of Lisbon, P-1649-003 Lisbon, Portugal 23 Centro de Estudios Avanzados en F´ısica, Matem´aticas y Computaci´on (CEAFMC), Department of Integrated Sciences, University of Huelva, E-21071 Huelva, Spain 27th International Nuclear Physics Conference (INPC2019) Journal of Physics: Conference Series 1643 (2020) 012096 IOP Publishing doi:10.1088/1742-6596/1643/1/012096 2 24 Institute of Accelerating Systems and Applications and Department of Physics, University of Athens, Athens, Greece 25 Department of Physics, Kyushu University, Motooka 744, Fukuoka 819-0395, Japan 26 INFN-LNL, viale dell’Universit`a 2, I-35020, Legnaro (PD), Italy 27 Institute of Modern Physics, Chinese Academy of Sciences, 509 Nanchang Rd., Lanzhou 730000, China E-mail: [email protected] Abstract. The elastic scattering process for the nuclear reactions induced by the Radioactive Ion Beams 7Be and 8B on a 208Pb target was measured for the first time in the energy range around the Coulomb barrier. Extensive theoretical calculations within the framework of the optical model were performed. An excellent agreement between experimental data and theoretical predictions was achieved for the reaction 7Be + 208Pb, while a comprehensive understanding of the reaction dynamics induced by the more exotic projectile 8B is still far to be reached. Predictions of the cross section for the breakup for both systems will also be given. 1. Introduction The reaction dynamics induced by light weakly-bound nuclei in the energy range around the Coulomb barrier is a quite interesting research topic in present-day nuclear physics. These studies was originally motivated by the expectation that the peculiar properties of many light exotic nuclei, such as halo structure, neutron skin structure or very weak binding energy, could magnify the strong enhancement of near-barrier fusion cross section observed in collisions between stable nuclei. Despite earlier measurements, it was soon realized that the unusual features of exotic nuclei tend to increase the total reaction probability rather than the fusion cross section. The investigation then moved to try to shed some light on which reaction channels were mainly responsible for the enhancement of the total reaction probability, whether this effect was mainly related to transfer channels or to the breakup process. Several review articles have been written over the past 15 years on this topic [1, 2, 3, 4, 5, 6, 7, 8, 9]. These studies are usually very complicated and typically suffer of low statistical accuracy. In fact, these exotic projectiles are generally radioactive and short-lived, therefore we need first a nuclear reaction between stable nuclei to occur, in order to produce the radioisotopes, and then a suitable device to select and purify the secondary beam. As a consequence, Radioactive Ion Beams nowadays available still have rather low intensities. Thus, the first series of experiments performed with exotic beams in the energy range around the Coulomb barrier are typically addressed to the investigation of the most probable reaction mechanism, i.e. the elastic scattering process. The analysis of the elastic scattering angular distribution within the framework of the optical model can provide a first indication about the overall reactivity of an exotic projectile and (possible) modification on the shape the experimental data with respect to the theoretical predictions for the scattering process can give some hints on the magnitude of cross sections for other nuclear processes and their influence on the scattering channel. 2. Experiments 2.1. Production of the secondary beams Both Radioactive Ion Beams used in these studies were produced by means of the in-flight technique. The 8B experiment was performed with the CRIB facility [10, 11], located inside the RIKEN campus in Japan. We started with a 11.2 MeV/u 6Li3+ primary beam with an average intensity of about 1 pµA impinging on a 8-cm long gas target, filled with 3He gas at a pressure of about 1 bar and kept at cryogenic temperature. The 8B was then selected and purified by means of 27th International Nuclear Physics Conference (INPC2019) Journal of Physics: Conference Series 1643 (2020) 012096 IOP Publishing doi:10.1088/1742-6596/1643/1/012096 3 the ion-optical elements of CRIB and impinged on a 2.2 mg/cm2 208Pb target with an energy of 50 ±1 MeV, an average intensity of 104pps and a purity around 20%. The 7Be experiment was carried out at the Laboratori Nazionali di Legnaro (LNL, Italy), where the radioactive ion beam was produced with the facility EXOTIC [12]. In this case, we used a 7Li3+ beam with an energy of 48.8 MeV and an intensity of 50-60 pnA. The primary beam was hitting a 5-cm long gas target, filled with H2at a pressure of about 1 bar. The outgoing 7Be secondary beam was selected with the facility EXOTIC and delivered with at intensity of 2-3 ×105pps and a nearly 100-% purity on a 1 mg/cm2 208Pb target. Three different secondary beam energies were obtained by operating the target at room and cryogenic temperatures and by inserting a 12.5-µm thick aluminum degrader at a suitable position along the beam line. The 7Be energies on target were: 42.2±0.4 MeV, 40.5±0.4 MeV and 37.4±0.5 MeV. 2.2. Detector set-up In both experiments, six modules of the detector array EXPADES [13] were employed for the detection of charged particles. Each module was composed by two Double-Sided-Silicon-Strip- Detectors (DSSSDs), the inner one with a thickness of 43-57 µm, while the outer one had a thickness of 300 µm. Each DSSSD had an area of 64.0 mm ×64.0 mm and the front and back sides were segmented into 32 vertical and horizontal strips, respectively. Details on the electronic chains used for both detector stages can be found in [13]. The six modules of EXPADES were arranged in a cylindrical configuration around the target position ensuring a solid angle coverage of about 14 % of 4πsr. A symmetrical displacement, covering the angular range θlab = [52◦, 167◦], around the beam axis was chosen for the 7Be experiment. On the other hand, for the 8B experiment a slight asymmetrical arrangement of the EXPADES modules was preferred in order to enlarge the polar angle coverage θlab = [8◦, 166◦]. More details on the detector geometry are given in the recent publication [14]. 3. Results 3.1. Elastic scattering and total reaction cross section The first step of the data analysis was the evaluation of the elastic scattering angular distributions for both systems and the extraction, via an optical model fit of the experimental data, of the total reaction cross sections. All the steps of the data evaluation and of the fitting procedure are described in [14] and here we discuss the results and the comparison with similar mass systems. When comparing different systems, both bombarding energies and cross sections need to the normalize in order to account for the different Coulomb barrier and the different geometrical size of the colliding nuclei. In doing that, we used the formalism suggested by L.F. Canto and collaborators [15]. This method was originally developed and works pretty nicely for the comparison of fusion cross section data, but can also be employed, paying some attention [16], also to reaction cross section data. In our case, we are going to compare reactions induced by projectiles with similar masses (in the range A = 6-8) and all interacting with the same target nucleus (208Pb). It has been discussed for a long time whether, from the point of view of the reaction dynamics, the 7Be behavior would have resembled more that of 6Li, which has a separation energy very similar (within 100 keV) to that of 7Be, or that of its mirror nucleus 7Li, with an analogue nuclear structure but a separation energy 60% larger. Our study provides a first answer to this question, at least for the interaction with a very massive target such as 208Pb, since the three data points we measured for the system 7Be + 208Pb follow essentially the trend of the reaction 6Li + 208Pb [17]. This result suggests that nuclear binding energy plays a major role than nuclear structure in collisions involving weakly-bound nuclei, at least in the energy range around the Coulomb barrier. 27th International Nuclear Physics Conference (INPC2019) Journal of Physics: Conference Series 1643 (2020) 012096 IOP Publishing doi:10.1088/1742-6596/1643/1/012096 4 If we now add to the comparison also the mirror nuclei of mass A = 8, i.e. 8Li [18] and 8B, again interacting with a 208Pb target, we observed in [14] that, in both cases, the ”‘normalized”’ total reaction cross section data are enhanced with respect to region of the graph where projectiles with mass A = 6-7 are located. A simple argument related only to the projectile binding energy would not stand in this occasion, since the 8Li separation energy is exactly in between those of 6Li/7Be and 7Li. The actual reason for the 8Li enhancement has therefore to be searched elsewhere and, most likely, relies on the large probability, i.e. cross section, for the n-stripping process (leading to 7Li in the reaction exit channel) to occur. Specific experimental measurements for this process are therefore needed in order to confirm this scenario. Even more striking is the comparison of 8B with all other light projectiles. In fact, its reaction cross section is enhanced by a factor 3-4 with respect to that measured for the reaction induced by its core 7Be. Following the analogies with the system 8+58Ni [19], the enhancement should be due to the extremely weak binding energy of 8B (Sp= 137.5 keV) and consequently the very high probability for this nucleus to break up while approaching a target nucleus. To this extend, we performed extensive Coupled-Channel calculations in order to provide an estimate of the cross section for the breakup process 8B→7Be + p. 3.2. Breakup process Theoretical calculations were performed for both systems. Being both projectile very weaklybound, we described the breakup projectile by means of a discretization of the (continuous) energy spectrum above the breakup threshold in bins equally spaced in the momentum space. Also in this case, the procedure adopted for the calculations is fully described in [14] and here we summarize the main details important for the subsequent discussion. The 7Be nucleus was modelled using an 3He + 4He cluster folding model. In the calculations, we included also the possibility to excite the 7Be first excited state at 0.429 MeV (Jπ= 1/2−), the two l= 3¯hresonances at 4.57 (7/2−) and 6.73 (5/2−) MeV, the non resonant continuum up to multipolarity λ= 4 and up to an excitation energy of 9.88 MeV above the breakup threshold. The calculations were performed by using the coupled channel code Fresco [20]. First of all, we remark that we were able to reproduce the experimental elastic scattering angular distribution without any free parameter, providing an additional confirmation of the validity of the adopted approach. The calculations predict a breakup cross section of 20-38 mb in the range of energies covered by our experiment. These values correspond approximately to about 10% of the overall reaction cross section for the system 7Be + 208Pb, strongly indicating that the breakup process does not dominate the reaction dynamics for this system, in a qualitative agreement with our previous observation for the reaction 7Be + 58Ni [?]. We also had an additional confirmation of this outcome from the preliminary analysis of the production cross section of the reaction products 3He and 4He. The heavier isotopes is indeed 4-5 times more abundant than the lighter counterpart, suggesting that there are many other reaction channels (such as, for instance, npickup or p-stripping) open in this energy range and leading to extra-production of 4He with respect to 3He. Moving now to 8B, we used a 7Be + pcluster folding model to describe this very exotic projectile. To reduce the computation time, 7Be was assumed to be spinless and inert and the proton to be in a pure p3/2state. The non resonant continuum up to a multipolarity λ= 5 and up to an excitation energy of 11.7 MeV above the breakup threshold was included. Due to assumed spinless nature of 7Be, no 8B resonances were considered in the theoretical calculations. The predicted differential cross section for the elastic scattering process failed to reproduce satisfactory the experimental data, already indicating that our model to describe 8B might be too simplistic. Being the 8core, i.e. 7Be, also a weakly bound nucleus, perhaps a 4-body approach, including also the possibility of core excitation, should be more adequate for this reaction. 27th International Nuclear Physics Conference (INPC2019) Journal of Physics: Conference Series 1643 (2020) 012096 IOP Publishing doi:10.1088/1742-6596/1643/1/012096 5 Nevertheless, coupled channel calculations predict an extremely large breakup cross section of around 600 mb, which corresponds to more than 50% of the total reaction cross section. We are presently working to extract the angular distribution for the 7Be reaction product. A preliminary evaluation shows a differential cross section strongly peaked at forward angles, with a large tail extending up to very backward angles, in qualitative agreement with the earlier observations by V. Guimar˜aes and collaborators for the reaction 8B + 58Ni [22, 23]. 4. Outlook The elastic scattering process for the systems 7Be, 8B + 208Pb has been measured for the first time in the energy range around the Coulomb barrier. The experimental data have been analyzed within the framework of the optical model in order to extract the total reaction cross section. The 8B + 208Pb reactivity resulted to exceed by far those reported for projectiles of masses in the range A = 6-8 and interacting with the same target nucleus. Theoretical calculations indicate that this enhancement might be due to the breakup process 8→7Be + pand we are presently working to extract the experimental differential cross section for the 7Be reaction product. Acknowledgments The 8B experiment was performed at RI Beam Factory operated by RIKEN Nishina Center and CNS, University of Tokyo. 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