Some aspects of reaction mechanism study in collisions induced by Radioactive Beams Alessia Di Pietro

Outline of the talk Effects on fusion and elastic scattering cross-sections at energies around the Coulomb barrier in reactions induced by halo and weakly bound Radioactive Beams.  Effects on fusion and elastic scattering cross-sections at energies around the Coulomb barrier in reactions induced by halo and weakly bound Radioactive Beams.  What have we learned from the existing experimental data?  The reaction 6 He+ 64 Zn.  The reaction 13 N+ 9 Be.  Summary and conclusions.  The EXCYT facility at LNS.

Effects of Halo structure on fusion below the Coulomb barrier? Different theoretical models predicts enhancement or hindering of fusion cross-section in reaction induced by Halo nuclei depending on how the break-up is treated in the calculations.  Break-up removes flux from the fusion channel.  Strong coupling with break-up enhances fusion cross- sections.

6 He+ 209 Bi J.J. Kolata Phys.Rev.Lett.81(1998)4580 Fusion excitation function E.F. Aguilera Phys.Rev.C 63(2001) Total reaction excitation function An even larger enhancement of the total reaction cross- section was measured. Enhancement of fusion cross- section below the Coulomb barrier is observed.

The experiment 6 He+ 64 Zn Si-Strip Beam 64 Zn targetsNb catcher Si-Strip Experimental set-up  Fusion excitation function on a medium mass system. Activation technique used. Activation technique used.  Elastic scattering angular distribution.  Transfer and break-up cross section measurement.  Comparison of the results with 4 He+ 64 Zn reaction.

4,6 He+ 64 Zn Heavy residue Excitation Function 6 He+ 64 Zn 4 He+ 64 Zn CASCADE predictions compared with experimental results A strong enhancement of the fusion cross-section seems to be present! The strong enhancement comes only from one residue, 65 Zn. 65 Zn can be produced also by 1n and 2n transfer reactions. A. Di Pietro et al. Phys.Rev.C 69(2004) He+ 64 Zn 4 He+ 64 Zn

The strong enhancement of the fission cross-section comes from transfer reactions. R.Raabe et al. Nature 431(2004)823 6 He+ 238 U Experimental set-up

For halo projectiles break-up responsible for a damping in the elastic angular distribution at large angles. New features such as the disappearance of threshold anomaly effect of the Optical Potential observed in reactions induced by weakly bound and halo nuclei. The Optical Potentials basic ingredient for the description of elastic scattering but also important for break-up, transfer and fusion. Calculations should be performed using models which take into account explicitly or by Polarisation Potentials the coupling with transfer and break-up channels. Optical Model analysis at low energies performed using very large imaginary diffuseness parameter in order to reproduce data. Elastic scattering

Large reaction cross-section is found in 6 He+ 64 Zn when compared with 4 He+ 64 Zn at the same Ecm Most of the reaction cross-section corresponds to transfer and break-up events rather than fusion. Transfer+break-up  ~1.2b 6 He+ 64 cm =12.4 MeV A.Di Pietro et al. Phys.Rev.C 69(2004) A.Di Pietro et al. Europhys.Lett. 64(2003)309 Elastic scattering  rea  0.65 b  rea  1.45 b 4,6 He+ 64 Zn a I =0.85fm for 6 He+ 64 Zn

Other elastic scattering results 6 He+ 209 Bi 6 He+ 208 Pb Very large imaginary diffuseness must be used to best-fit the data a I =1.22 fm a I =3.02 fm Energy dependent a I parameter: a I = xE c.m. O.R. Kakuee et al. Nucl.Phys.A 728(2003)339 E.F. Aguilera et al. Phys.Rev.C 63(2001)061603R

P.R.S.Gomes et al. Phys.Lett.B 601(2004)20 6,7 Li+ 64 Zn Results on medium mass targets 9 Be+ 64 Zn shows no effects on fusion and reaction cross-section. 6,7 Li+ 64 Zn shows no effects on fusion but large reaction cross- section 9 Be+ 64 Zn Effects on reaction and/or fusion cross-section induced by weakly bound nuclei above the Coulomb barrier?

Results on heavy targets Suppression of fusion cross-section above the barrier of about 70%. This suppression is attributed to break-up which leads to incomplete fusion. M.Dasgupta et al. PRL82(1999) Be+ 208 Pb 6,7 Li+ 209 Bi M.Dasgupta et al. Phys.Rev.C 66(2002)041602R

Experimental set-up Detection system based on Si-strip detectors and Monolithic  E-E telescopes Monolithic detection module Si-strip detector Fusion cross-section for the reaction 13 N+ 9 Be (weakly bound projectile on weakly bound target)

 E [channels] Energy [channels]  N  E thickness  1  m Identification threshold  keV/A for Z  6-20 Monolithic Si detectors G.Cardella et al. NIMA378(1996)262 A.Musumarra et al. NIMA409(1998)414

13 N+ 9 Be =45 MeV  /  fus Z 10 B+ 12 C =42 MeV The results are in agreement with CASCADE predictions. No evident suppression of fusion cross- section is present.  /  fus Z E*( 22 Na)  40 MeV CASCADEExperiment 1/E cm (1/MeV)  fus (mb) 13 N+ 9 Be 10 B+ 12 C

Summary and conclusions From the data so far collected a clear picture of structure effects of halo and weakly bound nulcei on reaction mechanisms is still not available. The role of the break-up has still to be understood. More theoretical and experimental efforts are needed. The experiments with radioactive beams are quite difficult due to the low intensity of such beams. Our results show that X-ray off- line detection seems to be a good tool to obtain fusion excitation functions in reactions induced by light Halo nuclei on intermediate mass targets.

EXCYT installation at LNS MAGNEX

Facility scheme

RIBs intensity table

Effects on reaction and/or fusion cross-section induced by weakly bound nuclei above the Coulomb barrier? According to this systematic study there is a hindrance of fusion cross-section in reaction between light weakly bound nuclei above the Coulomb barrier. Example : A. Szanto de Toledo et al. Nucl.Phys. A 679(2000)175

Fusion excitation function measured with an activation technique Evaporation Residues produced in fusion reaction are radioactive and decay by Electron Capture. Discrimination of E.R. by X-ray energies and half-lives. 67,68 Ga  67,68 Zn 65 Zn  65 Cu X-ray spectrum T 1/2 = 67.6 m T 1/2 =3.26 d Activity curve