Reactions induced by 11 Be beam at Rex-Isolde

The collision induced by the three Beryllium isotopes, Be, on a Zn target were investigated at Ec.m. ≈ 1.4 the Coulomb barrier. The experiments with the radioactive Be beams were performed at the Rex-Isolde facility at CERN. In the case of Be, elastic scattering angular distributions were measured whereas, in the Be case, the quasielastic scattering angular distribution was obtained. A strong damping of the quasielastic cross-section was observed in the Be case, in the angular range around the Coulomb-nuclear interference peak. In this latter case a large totalreaction cross-section is found. Such a cross-section is more than a factor of two larger than the ones extracted in the reactions induced by the non-halo Beryllium isotopes. A large contribution to the total-reaction cross-section in the Be case could be attributed to transfer and/or break-up events.


Introduction
Elastic scattering, being a peripheral process, allows one to investigate the surface properties of the halo nuclei.In this contribution we investigate to what extent information can be obtained from elastic scattering measurements of high quality, at low bombarding energies.Low energy elastic scattering and reaction experiments, involving halo nuclei, have been performed mostly with 6 He, which is a 2n-halo nucleus, on several targets over a wide range of energies and masses.The studies performed at low bombarding energy have shown that coupling to the continuum strongly affects the elastic cross-section especially on heavy targets and that direct processes are favored a e-mail: dipietro@lns.infn.itb present address: LPC-ENSICAEN, IN2P3-CNRS and Université de Caen, France c present address: Departamento de Fysica Atomica, Molecular y Nuclear, Universidad Complutense,Madrid, Spain d present address: Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, USA over, for example, fusion.On heavy targets, coupling with the Coulomb dipole excitation of the low-lying E1 strength, produces a suppression of the elastic crosssection at forward angles, in the Coulomb-nuclear interference region [1].This suppression has not been observed in reactions induced on low charge targets as for example 64 Zn [2].On this target, however, the elastic cross-section is overall suppressed when compared to the elastic cross-section of the well bound 4 He nucleus at the same E c.m. on the same 64 Zn target [2].In the 6 He induced reactions the total-reaction crosssection is large (see e.g.[2,3]) and at low energies, as above mentioned, direct processes such as transfer and break-up are the ones that mostly contribute to this cross-section.In the reaction induced by the 1n-halo 11 Be nucleus on a 209 Bi target, at energies that exceed 10% the Coulomb barrier, the extracted total-reaction crosssection was found to be similar to the one of 9 Be+ 209 Bi [4,5], measured by the same group [6].In this contribution results of experiments performed with the three Beryllium isotopes 9,110,11 Be on a medium mass 64 Zn target will be discussed.

EPJ Web of Conferences 2 Experiments
Two separate experiments were performed in order to study the three reactions 9,10,11 Be+ 64 Zn.The experiment with the stable 9 Be beam was done at Laboratori Nazionali del Sud (LNS) in Catania.The 9 Be beam was delivered by the 14MV Tandem accelerator of LNS and it was impinging on a 550 µg/cm 2 99% enriched 64 Zn target at a center of mass energy of 24.9 MeV.The elastic-scattering angular distribution was measured using five collimated surface barrier Si detector telescopes (10 µm ∆E and 200 µm E detectors), placed on a rotating platform, in the angular range 10 0 ≤ θ c.m. ≤ 110 0 .The experiment with the radioactive beams was performed at Rex-ISOLDE (CERN).The radioactive 10 Be and 11 Be beams are produced using spallation reactions induced by 1GeV p beams impinging on a thick Ta target.The 1 + ionized Beryllium atoms are extracted from the Ta target, purified in the High Resolution Separator and after charge breading, performed with the Electron Beam Ion Source (EBIS), are postaccelerated in the Rex-Isolde linear accelerator and sent to the target for the experiment.The average beam intensity was 10 6 pps for 10 Be and 10 4 pps for 11 Be with a purity of better than 99%.In the 10,11 Be+ 64 Zn experiment the beam energy was E c.m. ≈ 24.5 MeV and the target was a 550 µg/cm 2 and a 1000 µg/cm 2 for the 10 Be and 11 Be beams respectively.The detector system used was an array of six Si-detector-telescopes each formed by a 40 µm, 50 × 50 mm 2 , ∆E DSSSD detector (16+16 strips) and a 1500 µm single pad E detector.This array was placed very close to the target; this allowed a large solid angle coverage.The angular range covered by the detector was 10 0 ≤ θ lab ≤ 150 0 .However due to the limited statistics, only data up to θ c.m. ≈100 0 were analised.More details on the experiment can be found in [7].In the 11 Be case, due to the energy resolution of the radioactive beam and the energy straggling in the target, it was not possible to separate the ground state from the bound 1 2 − (E x =0.32MeV) state in 11 Be.As a consequence, in the 11 Be case, the quasielastic scattering rather than the elastic scattering was measured.

Elastic scattering angular distributions
In Fig. 1 the elastic scattering angular distribution for 9,10 Be+ 64 Zn and the quasielastic scattering angular distribution for 11 Be+ 64 Zn are shown.As one can see from the figure, the angular distributions for 9,10 Be+ 64 Zn appear to be very similar.This result is not surprising.It is known in fact [8], that although 9 Be is a weakly bound nucleus, at energies above the Coulomb barrier the break-up channel does not give an important contribution to the total reaction crosssection.In the 9 Be+ 64 Zn collision measured in [8] the reaction channel that mostly contribute to the total reaction cross-section is the total-fusion.This result is similar to what expected in reactions induced by well bound nuclei such as 10 Be.On the other hand, the 11 Be+ 64 Zn quasielastic angular distribution shows a very different behaviour; the peak due to the interference between the Coulomb and nuclear amplitude is missing and the quasielastic cross-section appears to be suppressed at the angles at which the nuclear interaction is felt.The observed feature, which cannot be attributed to the strong coupling to the bound 1 st excited state of 11 Be since this channel is included in the quasielastic cross-section, is typical of absorption occurring at large distances.Similar features have been observed when a strong coupling with the Coulomb excitation of quadrupole states in heavy deformed nuclei is present due to a long-range interaction such as the Coulomb interaction [9].
In the 11 Be case, absorption at large distances may occur due to the large radial extension of the halo nucleus.As shown in [10], the suppression of the elastic cross-section in the Coulomb-nuclear interference region cannot be attributed to Coulomb dipole coupling even though there is the presence of a low-lying E1 strength near the threshold but this suppression is mainly due to the nuclear contribution.In [10] one can see that the Coulomb-nuclear interference contribute further to the suppression of the elastic cross-section in the so called "rainbow" region.A strong coupling to the Coulomb dipole strength is instead expected to be important in the scattering with heavy targets having high Z [11], as indeed observed in collisions induced by the 2n-halo 6 He nucleus see e.g.[1].

Optical Model analysis.
The 9,10,11 Be+ 64 Zn scattering angular distributions were analyzed within the Optical Model (OM) framework using the code PTOLEMY [12].For the  and 10 Be+ 64 Zn elastic scattering, a volume potential having a Woods-Saxon (W-S) shape for both the real and imaginary parts was used; the radius and diffuseness (real and imaginary) were fixed and the best χ 2 was obtained by varying the potential depths.Before fixing radius and diffuseness, a series of calculations were performed where they were varied at steps of 0.05 fm.In the case of the 11 Be+ 64 Zn quasielastic angular distribution, the coupling to the break-up was taken into consideration using a Dynamic Polarization Potential (DPP).The DPP used was an imaginary surface potential having the shape of a W-S derivative.No real part for the DPP potential was considered.As volume potential we used the one extracted from the 10 Be+ 64 Zn elastic scattering fit.This potential was responsible for the core-target interaction.The OM fit was performed using as free parameter the depth of the DPP potential and varying the diffuseness at steps of 0.05 fm.The best χ 2 were obtained for a DPP diffuseness parameter of a si ≈ 3.5 fm.A similar diffuseness (a si =3.2 fm) of the surface DPP potential was obtained by [13,14].The results are shown in Fig. 1.
The values of the obtained potential parameters can be found in [7].As above mentioned, in the 11 Be+ 64 Zn case, the inelastic excitation of the bound 11 Be state at E x =0.32 MeV could not be separated from the elastic peak and therefore the quasielastic scattering angular distribution was obtained.We have verified, by means of DWBA calculations, that at each angle the inelastic contribution to the quasi elastic scattering angular distribution is small and hence the quasielastic angular distribution can be considered as elastic scattering angular distribution.This can be seen in Fig. 2. Details of the calculations can be found in [7].The total-reaction cross-sections deduced from OM analysis for 9,10,11 Be+ 64 Zn are σ R =1090 mb, σ R =1260 mb and σ R =2730 mb respectively.

Cross-section for direct reactions
The analysis of the ∆E − E spectra in the 11 Be+ 64 Zn case showed the presence of 10 Be events next to the 11 Be quasi-elastic peak as can be seen in Fig. 3.Those events could be produced in both 1n transfer or breakup processes.By selecting these events on the ∆E − E spectra the associated angular distribution was extracted and it is shown in Fig. 4. The integrated crosssection, obtained by assuming that dσ dθ =0 at θ lab =0 and 60 • is σ=1100 ± 150 mb, and it corresponds to ≈40% of the exstracted total-reacion cross-section.Therefore, contrary to what found in the weakly bound 9 Be case, where at a similar E c.m. energy the totalreaction cross-section is mainly due to fusion [8], in the 11 Be case direct processes are giving a large contribution to the extracted total reaction cross-section.These results are in agrement with what previously found in 6 He induced collision.

Conclusions
In this contribution have been discussed results of elastic and quasielastic scattering angular distribution for the reactions 9,10 Be+ 64 Zn and 11 Be+ 64 Zn respectively at E c.m. ≈1.4 the Coulomb barrier energy.In the case of 9,10 Be+ 64 Zn similar elastic-scattering angular distributions are observed.These angular distributions show the typical Coulomb-nuclear interference peak.Instead, in the 11 Be quasielastic scattering this 13001-p.3EPJ Web of Conferences peak is strongly suppressed, showing that absorption occurs at much longer distances than for the other two non-halo Be isotopes [7].An OM analysis was performed on the scattering data and the total reaction cross-section was extracted.In the 9,10 Be+ 64 Zn a W-S potential was used and only a volume potential was considered.However, in the 11 Be case, in order to take the coupling with the break-up into consideration, a surface DPP potential was considered in addition to the volume potential.The DPP used had the shape of a W-S derivative.The volume potential was extracted from the 10 Be+ 64 Zn elastic scattering OM fit.The best fit was obtained with the surface term having a very large diffuseness a si ≈3.5 fm in agreement with what found by [13,14].In the case of 11 Be induced collision, a much larger total-reaction cross-section is obtained with respect to the 9,10 Be induced reactions.Moreover, in the 11 Be+ 64 Zn experiment, transfer and/or break-up events were identified.By measuring the angular distribution for such events it is found that they correspond to about 40% of the total-reaction cross-section.At the measured energy, above the Coulomb barrier, break-up processes do not play an important role in the weakly-bound 9 Be induced collision, where the total-reaction cross-section is found to be similar to the well bound 10 Be case.A behaviour of the elastic angular distribution similar to the one observed in the 11 Be case is found in collisions induced by the 2n-halo 6 He nucleus on heavy targets where coupling with the Coulomb dipole breakup is important.However, the large suppression of the elastic cross-section in the Coulomb-nuclear interference region has not been observed in the 6 He+ 64 Zn [2].The larger radial distribution of the 11 Be compared to the 6 He [14] could be responsible for such a difference.

Fig. 1 .
Fig. 1.Elastic scattering angular distribution for 9 Be+ 64 Zn open symbols and 10 Be+ 64 Zn, blue symbols.Quasielastic angular distribution for 11 Be+ 64 Zn, red symbols.Lines represent the result of the Optical Model fit to the data (see text for details).

FUSION11Fig. 2 .
Fig. 2. Quasielastic angular distribution for 11 Be+ 64 Zn, red symbols.Full red line represent the result of the Optical Model fit to the data.Dashed black line represent DWBA calculations of inelastic scattering.

Fig. 4 .
Fig. 4. Angular distribution of transfer/break-up events in 11 Be+ 64 Zn obtained by selecting 10 Be events in the ∆E − E spectrum.