Elastic scattering of 17 O ions from 58 Ni at near-barrier energies

Elastic scattering has been studied for the collisions induced by 17O on 58Ni target at energies around and above the Coulomb barrier. The elastic scattering angular distributions were measured for several energies and were analyzed within the framework of the optical model to obtain total reaction cross sections. The reaction cross-sections of the tightly bound 17O were compared with those of weakly bound 17F on the same targets in order to investigate the effects of the low binding energy in the reaction dynamics.


Introduction
Over the past years many efforts were devoted to the study of the reaction dynamics in collision induced by light stable and unstable nuclei at energies around the Coulomb barrier.In particular reactions induced by radioactive nuclei nuclei has been one of the most studied topics.The effects of the breakup channel on the fusion and on the elastic scattering processes of weakly bound projectiles have been studied extensively, both experimentally and theoretically (see, e.g., Ref. [1] and references therein).The effects of the high breakup probability that characterizes such exotic projectiles nuclei on the fusion cross section it has been rather controversial.On one hand one may expect the fusion cross section for these weakly bound nuclei to be reduced due to the loss of flux in the entrance channel.On the other hand the strong coupling to the breakup channel should enhance the fusion cross section.This two opposite views were somehow reconciled by Hagino et al. [2] who predicted that couplings a e-mail: domenico.torresi@pd.infn.it to breakup states lead to the enhancement of the complete fusion cross section at sub-barrier energies and to a reduction of the complete fusion cross section above the barrier.Information on the structure of such nuclei and on its effect on the reaction dynamics can be obtained from high-quality elastic scattering angular distributions data such as shown for the systems 9,10,11 Be on 64 Zn [4] and 6 He on 208 Pb [5].Within this framework we undertook the study of the 17 F reaction dynamics on a 208 Pb [8] and, more recently, on a 58 Ni target [3].The results obtained in the latest experiments were compared with those of system 16 O+ 58 Ni [7].With the aim to add a further system to the entire scenario, we measured the elastic scattering angular distributions for the systems 17 O on 58 Ni and 208 Pb targets at several energies around the Coulomb barrier. 17O is indeed the mirror nucleus of the radioactive weakly-bound 17 F.The two isobars have similar shell model configurations with d 5/2 ground states and low-lying s 1/2 first excited states, but while the the first is very well bound S n = 4.143 MeV for 17 O, the second is weakly bound S p = 0.600 MeV for 17 F.

Experimetal set-up
The measurements of the elastic scattering for the system 17 O + 58 Ni was included in the framework of the commissioning of the new detector system EXPADES [9].This array consists of eight identical modules, each module is a telescope made of a 64×64 mm 2 large Double Sided Silicon Strip Detector (DSSSD), 300 µm thick as second stage.As first stage alternatively a 40 µm thick DSSSD or a gas chamber can be used.DSSSD has both electrodes segmented into 32 strips.For the thick DSSSDs the readout electronics is based on an 32-channel ASIC chip-set, manufactured by IDEAS-GM (Norway), while for the thin detectors a home-made traditional electronics was developed.
The measurement was performed using the 17 O beam delivered by the TANDEM-XTU accelerator at the Laboratori Nazionali di Legnaro.The targets were 150 µg/cm 2 thick 58 Ni foil with a backing of 208 Pb (50 µg/cm 2 thick) for normalization purpose.The targets were tilted by 45˚with respect to the beam axis.For the first target energy ranged from 42.5 to 55 MeV in steps of 2.5 MeV, while for the second target five energies in the interval 78-87 MeV were studied.In this experiment two of the total eight modules of the EXPADES detector array were used.The first was placed at forward angles covering the angular range 35˚-70˚, while the second was placed at backward angle covering the angular range 80˚-110˚.They both consist of a single stage DSSSD 300 µm thick detector.Elastically scattered 17 O particles were selected according to their energy.The angular resolution allowed by the EXPADES pixel geometry, as can be seen from figure 1 was better than 1˚.For normalization purposes two additional monitor detectors were placed at 20˚.

Preliminary results
The elastic scattering angular distributions were properly normalized by assuming that the scattering at small angles (i.e.where the monitor detectors were located) was purely of the Rutherford type.Preliminary angular distributions for the system 17 O+ 58 Ni at energies: 42.5, 45, 47.5, 50, 52.5, 55 MeV are shown in figure 1.A very preliminary optical model analysis of the experimental data has been also performed.The interaction potential between the 17 O and 58 Ni nuclei was described according to a standard Akyüz-Winther [12] parameterization.The fits were performed with the SFRESCO subroutine of the main coupled-channel code FRESCO [10] and are displayed in figure 1 with continuous lines.For the moment we assumed all events as originating from a pure elastic scattering process, even if the experimental energy resolution did not allow to separate events leading to the excitation of the first excited state at Ex = 0.871 MeV from elastic events.The preliminary results for the reaction cross sections for the system 17 O + 58 Ni are the following: 253 mb at 42.5 MeV, 452 mb at 45 MeV, 590 mb at 47.5 MeV, 694 mb at 50 MeV, 779 mb at 52.5 MeV and 869 mb at 55 MeV.In figure 2 are shown the total reaction cross section of three systems 17 O + 58 Ni, 16 O + 58 Ni [7] and 17 F + 58 Ni [3].In order to account for the different geometrical sizes of the colliding nuclei and the different Coulomb barrier heights, the data were reduced according the procedure described in Ref. [11].

Concluding remarks
The elastic scattering angular distribution for the system 17 O on a 58 Ni has been measured at energy close and above the Coulomb barrier.The data have been analyzed within the framework of the optical model in order to extract the total reaction cross section.The differences in the reaction dynamics between the two mirror nuclei: the weakly bound 17 F and the well bound 17 O can be ascribed, at least in first approximation, to the different binding energy of the less bound nucleon.Considering that the data reduction should account for all static effects and for dynamical effects related to bound states, the tiny difference between the 17 F data point and the 17 O curve in figure 2 leaves very small room to the breakup channel.

Figure 1 .
Figure 1.Elastic scattering angular distributions for the system 17 O + 58 Ni at different energies.Lines represent the results of optical model best-fit analysis.