Study of fusion in 6 , 7 Li+ 197 Au at near barrier energies

. Excitation functions are measured for complete fusion and transfer reactions of 6 Li and 7 Li with 197 Au at energies around the Coulomb barrier. Coupled channel calculations including the couplings to both target and projectile excited states have been performed and are found to explain the data at energies below the barrier. At above barrier energies the complete fusion cross sections are found to be suppressed compared to the coupled channel calculations for both the systems. A systematic comparison of fusion cross-section for halo nuclei 6 , 8 He and weakly bound stable nuclei 6 , 7 Li on 197 Au target is also presented. Large neutron transfer cross-sections are observed for 6 , 7 Li as compared to tightly bound projectiles 12 C, 16 O.


Introduction
The dynamics of complete fusion and breakup reactions induced by weakly bound stable nuclei have been extensively studied in recent years [1]. The weakly bound stable nuclei like 6,7 Li, 9 Be, with a well def ned cluster structure and small separation energies have a large breakup probability. It is now well established that couplings to internal degrees of freedom of target/projectile nuclei have a strong inf uence on fusion process at near and sub-barrier energies. Theoretical models involving dynamic interactions such as coupling to low lying rotational states (arising due to nuclear deformation) and/or vibrational states (due to surface modes) of colliding nuclei, predict the fusion cross section enhancement at sub-barrier energies over the 1D barrier penetration model [2]. The experimental study of reactions with weakly bound nuclei and heavy mass targets have shown an enhancement in the cross section at subbarrier energies and suppression at above barrier energies compared to 1D barrier penetration model. Also, in case of reactions with radioactive ion beams it has been observed that coupling to transfer channel results in large enhancement of fusion probability [3]. Recent results with 8 He on 197 Au target, indicate that the sub-barrier total reaction cross section is completely dominated by direct reactions, in the form of 1n and 2n stripping [4]. Further, the σ 2n /σ 1n ratio can provide information on structural correlations [5]. A simultaneous measurement of direct and compound nuclear processes with weakly bound nuclei can therefore be used to study the inf uence of direct reactions on fusion process. In this paper, we present the excitation function a e-mail: nanal@tifr.res.in b Present address: Departamento de Fisica Aplicada, Universidad de Huelva, E-21071 Huelva, Spain measurement for complete fusion and transfer reaction in 6,7 Li + 197 Au over an energy range of 0.6 ≤ E/V b ≤ 1.5. A systematic comparison of fusion and transfer processes of weakly bound stable nuclei with halo and tightly bound projectiles is also presented.

Experimental Details
Self supporting, rolled target foils of 197 Au (∼1.5-1.65 mg/cm 2 thick) were irradiated with 6,7 Li beams of energies 23-44 MeV (I∼ 10-20 pnA) from the Pelletron Linac facility, Mumbai. Aluminum catcher foils of thickness ∼3 mg/cm 2 were mounted behind the target to capture all reaction products. For optimal utilization of the beam time, cascaded targets with aluminum degrader foils were used in some cases. The beam current was recorded at regular preset intervals (10 to 30 s) during irradiation. Products originating from complete fusion and transfer reactions are predominantly beta active with long half-lives (12 min to 3 days) and were measured by off-line gamma counting. The irradiated target along with the catcher foil was mounted at a distance of ∼ 10 cm from an efficiency calibrated HPGe detector. For data collection at sub-barrier energies, the detector was housed inside a low background setup consisting of graded shielding of thin Ni+Cd foils and 5cm thick lead and the target was mounted in a close geometry. Data was recorded using a CAMAC based acquisition system LAMPS. Table 1 lists half-lives of reaction products together with their characteristic γ-rays and branching ratios. Figure 1 shows a typical off-line γ-ray spectrum for 6 Li + 197 Au at E lab = 40MeV. Half-lives of various γrays were followed for unambiguous identif cation of the residues. The cross sections of various channels were extracted from observed γ−ray yields. For many of the decay Table 1. Reaction products with half-lives (T 1/2 ), characteristic γ-rays and relative intensities I γ (%) [6]. products, more than one γ-ray were observed and extracted cross sections from different γ-rays were consistent within a few %. . The compound nuclear angular momentum distribution obtained from coupled channel code CCFULL [8] was used as input for these calculations. The total fusion cross sections were obtained from the sum of partial residue cross sections. The coupled channel calculations were performed using code CCFULL [8], including the coupling to the f rst excited state in the projectile (i.e. to the unbound 3 + , 2.186 MeV state of 6 Li, and to the bound 1/2 − , 0.478 MeV of 7 Li). In addition, a coupling to the target inelastic state with deformation parameter β 2 = 0.1 has also been included. The results thus obtained are shown in f gure 4.The 1D barrier penetration model calculations for 6 Li are also shown for comparison. It is clearly seen that the CCFULL calculations are in excellent agreement with experimental data at energies below the barrier for both 6,7 Li, but at above barrier energies measured cross-sections are lower than the CCFULL prediction. It should be pointed out that the suppression of measured cross-sections at higher energies is much more signif cant in case of 6 Li than in 7 Li. Scaled cross sections are obtained using procedure described in ref. [9] taking into account experimental values 16017-p.2  Comparison of reduced fusion cross sections (adopted from [9]) for 6 Li + 198 Pt [11], 6 He + 197 Au [10], 8 He + 197 Au [4] together with present data 6,7 Li + 197 Au. of the fusion cross section and height, radius and curvature of the barrier. Figure 5 shows a systematic comparison of the present data with 6,8 He on 197 Au from ref. [4,10]. The 6 Li + 198 Pt [11] is also shown in the same f gure. The scaled cross-sections for 6 Li on 197 Au and 198 Pt are very similar, indicating no signif cant target dependence. Similar results have been reported in [12] and [13] showing no target dependence. A large enhancement of the scaled fusion cross section for halo nucleus 8 He as compared to the weakly bound nuclei is clearly seen at sub-barrier energies.

Transfer cross sections
In case of weakly bound projectiles like 6,7 Li the contribution from transfer and breakup processes is expected to be signif cant. The d/t transfer leads to Hg isotopes which are stable and could not be measured in the present experiment. However, the isomeric state 197m Hg resulting from  (d,2n) is observed in 6 Li induced reaction and its cross section has been measured. In 6 Li induced reaction, 1n pickup ( 196 Au) and 1n stripping ( 198 Au) channels are seen while in case of 7 Li, 1n and 2n stripping ( 198,199 Au) channels are observed. It should be mentioned that care has to be taken while extracting the 2n stripping cross-sections in the present case. The daughter nucleus 199 Hg is populated via β decay of 199 Au (2n stripping, 3.139 days) and via EC of 199 Tl (7.42 hrs) which is a granddaughter of 5n evaporation channel( 199 Pb).Therefore for measurement of 2n stripping cross-section, data collection was done 60 hours after irradiation (∼8 half-lives of 199 Tl). Figure 6 shows a typical decay plot of 199 Au at E lab = 33MeV, where the measured half-life value is in excellent agreement with reference value listed in Table 1 indicating that the contribution from the 199 Tl nucleus is negligible.
The measured excitation functions for transfer products in 6,7 Li + 197 Au reactions are shown in f gures 7 and 8. It should be mentioned that at very low energy, σ 1n is larger than σ f usion . Similar energy dependence has been reported in 6 He + 197 Au reaction for production of 198 Au [10], while 16017-p.3   [4,10], weakly bound nuclei (present data) and tightly bound nuclei [14] on 197 Au target the absolute magnitude of σ 1n is considerably large (>1 barn). A systematic comparison of transfer cross sections with 6,8 He (halo nuclei), 6,7 Li (weakly bound nuclei) and 12 C, 16 O (tightly bound stable nuclei) is shown in f gure 9. In case of 6,8 He, large Q value of 2n transfer leads to population of particle un-bound states in 199 Au and therefore is followed by neutron evaporation(s) [4,10]. Thus, the observed 198 Au cross-section contains contribution both from 1n and 2n transfer. Hence, in the f gure total transfer cross sections, i.e., 1n stripping + 1n pickup for 6 Li and 1n + 2n stripping for 7 Li are plotted. It can be seen that at subbarrier energies, large n-transfer cross-sections are observed for both 6,7 Li compared to stable nuclei, while those for halo nuclei are even larger (as expected).

Summary and Conclusion
In summary, the excitation function for complete fusion in the 6,7 Li + 197 Au systems has been measured at ener-gies around the Coulomb barrier. The fusion cross sections show enhancement below the barrier as compared to 1D BPM. The Coupled channel calculations incorporating couplings to the inelastic excitation of target and projectiles are able to explain the data at sub-barrier energies. The measured fusion cross sections above the barrier are suppressed as compared to the CCFULL calculation, while suppression is more signif cant in case of 6 Li than in 7 Li. A comparison of scaled fusion cross-sections for 6,8 He and 6,7 Li has also been presented. Large neutron transfer cross-sections are observed for 6,7 Li as compared to tightly bound projectiles 12 C, 16 O.
In both 6,7 Li induced reactions the breakup channel is very important. In the present off-line experiment, d/t capture leading to stable Hg isotopes and α capture leading to Tl isotopes (which are also produced by decay of fusion residues-Pb isotopes) could not be extracted. The measurement of breakup fusion products by online gamma spectroscopy is planned and will help in understanding the effect of breakup in reactions with weakly bound nuclei.
Authors would like to thank Ms. D. Thapa for target preparation, Mr. M.S. Pose for help during the experiment and the accelerator staff for smooth operation.