Systematical Study of Optical Potential Strengths in Reactions Involving Strongly, Weakly Bound and Exotic Nuclei on 120Sn
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Systematical study of optical potential strengths in reactions involving strongly, weakly bound and exotic nuclei on 120Sn M. A. G. Alvarez, J. P. Fern´andez-Garc´ıa, J. L. Le´on-Garc´ıa Departamento FAMN, Universidad de Sevilla, Apartado 1065, 41080 Sevilla, Spain M. Rodr´ıguez-Gallardo Departamento FAMN, Universidad de Sevilla, Apartado 1065, 41080 Sevilla, Spain and Instituto Carlos I de F´ısica Te´orica y Computacional, Universidad de Sevilla, Spain L. R. Gasques, L. C. Chamon, V. A. B. Zagatto, A. L´epine-Szily, J. R. B. Oliveira Universidade de Sao Paulo, Instituto de Fisica, Rua do Matao, 1371, 05508-090, Sao Paulo, SP, Brazil V. Scarduelli Instituto de F´ısica, Universidade Federal Fluminense, 24210-340 Niter´oi,Rio de Janeiro, Brazil and Instituto de F´ısica da Universidade de S~aoPaulo, 05508-090, S~aoPaulo, SP, Brazil B. V. Carlson Departamento de F´ısica, Instituto Tecnol´ogico de Aeron´autica, S~aoJos´edos Campos, SP, Brazil J. Casal Dipartimento di Fisica e Astronomia "G. Galilei" and INFN-Sezione di Padova, Via Marzolo, 8, I-35131, Padova, Italy A. Arazi Laboratorio TANDAR, Comisi´onNacional de Energ´ıa At´omica, Av. Gral. Paz 1499, BKNA1650 San Mart´ın, Argentina D. A. Torres, F. Ramirez Departamento de F´ısica, Universidad Nacional de Colombia, Bogot´a,Colombia (Dated: December 20, 2019) We present new experimental angular distributions for the elastic scattering of 6Li + 120Sn at three bombarding energies. We include these data in a wide systematic involving the elastic scattering of 4;6He, 7Li, 9Be, 10B and 16;18O projectiles on the same target at energies around the respective Coulomb barriers. Considering this data set, we report on optical model analyses based on the double-folding S~aoPaulo Potential. Within this approach, we study the sensitivity of the data fit to different models for the nuclear matter densities and to variations in the optical potential strengths. PACS numbers: 25.70.Bc,24.10.Eq,25.70.Hi I. INTRODUCTION picture, which is associated to the weak binding of the halo neutrons (S2n = 0.98 MeV - Table I) [6], that favours arXiv:1912.09130v1 [nucl-th] 19 Dec 2019 6 Nuclei present cluster structures [1]. Light, strongly or the dissociation of the He projectile. weakly bound, stable or exotic, nuclei such as 6He, 6;7Li, 7Li is one of the heaviest nuclides formed with very 7;8;9Be, 12;13;14C, 16;18O, among others (isotopes and nu- small yields during the primordial Big-Bang nucleosyn- clei), can be considered as results of n, 1;2;3H and 3;4He thesis. Stable nuclei heavier than 7Li were formed much combinations. It has been evidenced by experimental ob- later through light nuclei reacting during stellar evolu- servations on break-up or transfer reactions (e.g. [2{5]). tion or explosions. Despite small amounts of 6Li and 7Li The 4He possesses a significantly higher binding energy being produced in stars, they are expected to be burned per nucleon than its light neighbors (see Table I), and a very fast. Additional small amounts of both, 6Li and 7Li, first excited state with very high excitation energy (20.6 may be generated from cosmic ray spallation on heavier MeV) that makes it a rather robust and inert nucleus. atoms in the interstellar medium, from solar wind and Unlike 4He, 6He is an exotic nucleus that decays, by from early solar system 7Be and 10Be radioactive decays beta minus emission, in 6Li, with a half-life of 806.7(15) [7]. ms [6]. It is a Borromean nucleus, i.e., the two sub- Both 6Li and 7Li have an anomalous low nuclear bind- systems, 4He-n and n-n, are not bound. Reactions in- ing energy per nucleon compared to their stable neigh- duced by 6He on different targets, at energies around the bors (see Table I). In fact, these lithium isotopes have Coulomb barrier, exhibit a remarkable large cross section lower binding energy per nucleon than any other stable for α particles production [3, 4]. It confirms a break-up nuclide with Z > 3. As a consequence, even being light, 2 6Li (see Table I). Even so, in reactions of 7Li, the break- TABLE I: Binding energy per nucleon, proton and neutron up channel of the α + t cluster is relevant [2]. Notwith- separation energies, possible mode of break-up and corre- 8 sponding Q value for some nuclei. All energies are provided standing, Be formation (with subsequent α + α decay) in MeV. through a proton pick-up transfer process (Q = 6:658 MeV) is more probable. nucleus BE=A S S cluster Q 1p 1n The 9Be nucleus presents a Borromean structure com- 4He 7.07 19.81 20.58 posed of two α particles and one weakly bound neutron 6He 4.88 22.59 1.71 α + n + n −0:98 [10]. It has a binding energy for the α + α + n break- 6 6Li 5.33 4.43 5.66 α + d −1:47 up comparable to that for Li (see Table I). The 1n- 9 7Li 5.61 9.97 7.25 α + t −2:47 separation energy of Be is quite small in comparison 9Be 6.46 16.89 1.66 α + α + n −1:57 with those for the other nuclei of Table I. Thus, when col- 10B 6.47 6.59 8.44 6Li + α −4:46 liding with a target nucleus, 9Be tends (with high proba- 11B 6.93 11.23 11.54 7Li + α −8:66 bility) to transfer its weakly bound neutron, with α+α or 8Be formation (the later followed by α+α decay). In [11], Arazi et al. demonstrated the importance of couplings to 6;7 unbound states to obtain theoretical agreement with the Li are less common in the solar system than 25 of the 9 120 first 32 chemical elements [8]. The 6Li and 7Li nuclei are Be + Sn data set, at energies around the Coulomb stable weakly bound isotopes for which strong break-up barrier, corroborating break-up as an important process. 7 10 effects are expected in collisions with other nuclei. These Similar to Li, B also presents a first excited state ∗ isotopes can be considered as α + d and α + t clusters, with low excitation energy (E = 0:72 MeV). However, 6;7 9 with small Q values (see Table I). compared to Li and Be (Table I), its most favorable 10 6 4 Luong et al. [2] showed that break-up of 6Li into its break-up channel, B ! Li + He, is energetically α+d constituents dominates in reactions with heavy tar- higher and, therefore, less probable. In addition, con- gets. However, break-up triggered by nucleon transfer is sidering the different values of the 1n-separation energy highly probable. As an example, in the case of a 6Li (Table I), break-up triggered by nucleon transfer is not 10 9 beam focusing on a 120Sn target these processes could as favored for B as it is for Be. In [12], we demon- be: strated that couplings to the continuum states are not 6Li + 120Sn ! 121Sn + 4He + p; important to obtain a good agreement between theoret- 10 120 6Li + 120Sn ! 121Sb + 4He + n. ical calculations and experimental data for B + Sn, These strong break-up mechanisms triggered by nucleon at energies around the Coulomb barrier, indicating that transfer help in explaining the large number of α parti- break-up is not an important process in this case. The cles observed in different 6Li reactions [2, 9]. In Table above mentioned features indicate a very different reac- 9 10 II, we present Q values of possible break-up processes tion dynamics for Be and B weakly bound projectiles 120 triggered by transfer for systems involving some weakly reacting with Sn. bound projectiles on a 120Sn target. Studying reactions involving weakly bound stable nu- clei is a crucial step towards a better understanding of their abundances. The structural models of these nu- TABLE II: Q values of some break-up processes triggered (or clei are fundamental to determine how they interact and, not) by transfer for weakly bound projectiles colliding with a therefore, to shed light on such abundances. Weakly 120 Sn target. bound nuclei, in general, have fundamental structural projectile reaction products Q(MeV) characteristics, such as the above mentioned low break- up thresholds and cluster structures. Break-up can lead 6Li 121Sn + α + p 2.472 to a complex problem of three or more bodies, and can 6Li 121Sb + α + n 2.092 occur by direct excitation of the weakly bound projectile into continuum states or by populating continuum states 7Li 122Sn + α + p 4.036 of the target [3, 13{18]. 7 122 Li Sb + α + n 1.247 Weakly bound stable nuclei can easily be produced and 9 121 accelerated, with high intensities, in conventional particle Be Sn + α + α 4.597 accelerators. Within this context, complementary exper- 9Be 120Sn + 8Be + n 4.505 imental campaigns are being developed in two laborato- 10B 121Sn + 2α + p -1.989 ries: the 8 MV tandem accelerator of the Open Labora- 10B 121Sb + 2α + n -2.368 tory of Nuclear Physics (LAFN, acronym in Portuguese) in the Institute of Physics of the University of S~aoPaulo (Brazil), and the 20 MV tandem accelerator TANDAR Unlike 6Li, 7Li presents a first excited state with rel- (Buenos Aires, Argentina). The aim of the joint collabo- atively low excitation energy (E∗ = 0:48 MeV). The 7Li ration is to study the scattering involving stable, strongly nucleus also has a small binding energy for the α+t break- and weakly bound, nuclei on the same target (120Sn), at up, which is, however, about 1 MeV higher than that for energies around the respective Coulomb barriers.