1 Reaction Mechanisms with low energy RIBs: limits and perspectives Alessia Di Pietro INFN-Laboratori Nazionali del Sud

2 Radioactive Ion Beams: many new problems can be studied… Using the radioactive beams available today one can study reactions induced by proton or neutron rich nuclei. Some of such nuclei have low break-up thresholds. In some cases like 11 Li, 11 Be, 6 He, the last weakly bound nucleon(s) form a large diffuse HALO around a well bound core.

3 r VpVpVpVp Nuclear Halo  Nuclear Halo can show-up if a s o p bound state close to the emission particle threshold.  Low binding energy ( < 1 MeV) of outer nucleons make possible quantum tunneling of such nucleons outside nuclear core. Halo states: bound states whose wave function extends to classical forbidden region

4 The study of reactions and in particular fusion at low bombarding energies in collision induced by halo but also weakly bound nuclei is an important issue since it gives a great incentive to better understand the continuum.  Which are the theoretical expectations?  Which the experimental methods adopted?  Limits of the results obtained with the present facilities.  Have we learned something? Reaction mechanisms around the barrier

5 Effect of halo behaviour on reaction mechanism:  Static effects due to long tail in density distribution: longer tail in ion-ion potential, lowering of Coulomb barrier, larger sub-barrier fusion probabilities, etc longer tail in ion-ion potential, lowering of Coulomb barrier, larger sub-barrier fusion probabilities, etc  Dynamical effects due to coupling to states in the continuum: polarization term in optical potential, effect on sub-barrier fusion, etc.. polarization term in optical potential, effect on sub-barrier fusion, etc.. Well established that coupling of colliding nuclei relative motion to intrinsic excitations or other open reaction channels causes large enhancement of fusion cross-section at sub-barrier energies over prediction of simple penetration models.

6 E cm (MeV)  CF (mb) Fusion excitation function for: 58 Ni + 58,64 Ni and 64 Ni + 64 Ni M. Beckerman et al. Phys. Rev. Lett 45 (1980) 1472, M. Beckerman et al. Phys. Rev. C23 (1981) 1581 M. Beckerman et al. Phys. Rev. C25 (1982)

7 Some examples of different theoretical predictions K.Hagino, et al. PR C 61, (2000)A.Diaz-Torres, et al. PR C 65, (2002)M.Ito et al. PL B637,53,(2006) CDCC calculations CDCC calculations+continuum- continuum coupling Time evolution of a three body system: core, halo, target. 11 Be+ 208 Pb 11 Be+ 209 Bi 10 Be+ 209 Bi a)CDCC calculations  coupling between ground state and continuum up to 2 MeV. b)CDCC calculations  11 Be 1 st excited state and continuum-continuum coupling. Continuum considered up to 8 MeV. c) Time dependent wave packet approach  Interaction: halo-core, core-target, halo-target Results depend upon phase space in the continuum (considered range of energies and relative angular momentum). Other approximations considered in the calculations. a)b) c)c)

8 Present facilities where low energy beams of halo nuclei have been used. Present facilities where low energy beams of halo nuclei have been used. ISOL beams:  Louvain la Neuve 6 He (no more RIBs available from next summer)  REX-ISOLDE 11 Be  SPIRAL 6 He  Dubna 6 He Fragmentation beams:  Riken 11 Be (after energy degradation) In flight separated beams:  Notre Dame ( 6 He) Available beam intensities 10 5 ÷10 7 pps The required intensities..... comparable with stable beam intensities!

9 Experimental methods Different techniques have been used for the detection of the reaction products: Silicon strip arrays,  detectors, X-ray detectors, n detectors… Problems:low beam intensity and small cross-sections  low rate  high background Problems: low beam intensity and small cross-sections  low rate  high background Fusion channel identification : Heavy targets  Fission Fragments Lighter targets  Evaporation Residues but… Direct ER detection difficult  Activation techniques: Detection of  particles, X- rays or  following the ER radioactive decay. Alternative technique: characteristic  rays (but very efficient detection systems needed)

10 6 He+ 238 U: fission cross section ISOL beam ~ 10 6 pps Experimental set-up 6 He The strong enhancement of the fission cross-section comes from transfer reactions. R.Raabe et al. Nature 431(2004)823

11 4,6 He+ 64 Zn fusion excitation function ISOL beam 10 6 pps Experimental technique: Off-Line X-ray detection. 4 He + 64 Zn 6 He + 64 Zn Beam 64 Zn targets Nb catcher Si-Strip Si-strip Experimental set-up A. Di Pietro Europhys. Jour. Special Topics 150, 15 (2007) A. Di Pietro et al. Phys.Rev.C 69(2004)044613

12 6 He Exp. data from: J.J. Kolata et al: Phys.Rev.Lett.81(1998)4580 Comparison from: N.Alamanos et al: Phys.Rev.C65,054606,(2002) Enhancement of fusion cross- section below the Coulomb barrier is observed when compared with 4 He+ 209 Bi cross-section or calculations. 6 He+ 209 Bi fusion cross sections In-flight separated beam 10 6 pps Experimental technique: off-line  detection 6 He+ 209 Bi 4 He+ 209 Bi 2n+3n+4n 4 He+ 209 Bi 1n Fusion excitation function

13 6 He+ 206 Pb collision: fusion cross sections 6 He ISOL beam degraded in energy. Experimental technique: off-line  particle detection. A large enhancement of the fusion cross section for the 6 He+ 206 Pb is claimed Yu. E. Penionzhkevic et al. Phys. Rev. Lett. 96 (2006) n 1n (measured) (calculated)

14 11 Be+ 209 Bi Energy degraded fragmented beam i ~ 10 5 pps Experimental technique: off-line  detection+ FF detection 9,10,11 Be+ 209 Bi C.Signorini et al. Nucl.Phys.A 735(2004)329 No differences of fusion cross-section are observed among the different Be isotope induced fusion reactions. Only statistical errors considered. Beam profile  spectrum from radioactive decay

15 Other reaction mechanisms E.F. Aguilera Phys.Rev.C 63(2001) A strong  particle yield due to transfer+B.U. events is observed. The associated cross section saturates 80% of total reaction cross section at the barrier and almost all the total reaction cross section below the barrier.  -n angular correlation measurements suggest that about 20% of the  particle yield is due to 1n transfer events whereas the remaining 80% is shared between 2n transfer and break-up J.P.Bychowski et al. Phys. Lett. B 596,26,(2004)  fus  reac E cm =12.4 MeV  T+bu =1200  150 mb  T+bu /  R  80% 6 He+ 64 Zn  particle a.d. A. Di Pietro et al. Phys.Rev.C 69(2004) He+ 209 Bi

16 11 Be+ 209 Bi 17 F+ 208 Pb In these two cases  R ~  Fus.  R similar to reaction induced by well bound systems. No strong direct reaction process contribution. For other systems studied…. M.Mazzocco et al. EPJ A28,295(2006) M.Romoli et al. PRC 69,064614(2004)

17 From the data so far collected a completely clear picture of structure effects of halo bound nuclei on reaction mechanisms is still not available and the role of break-up has still to be understood The experiments so far performed have reached their limit, only little improvements can be done. New data (possibly taken in exclusive experiments and/or experiments looking at once at elastic + all open reaction channels) are needed for a complete understanding of the reaction dynamics in collision around the Coulomb barrier. More precise experiments extending to lower energies below the barrier should be performed. This is not possible with the existing facilities higher intensities mandatory (EURISOL). Summarising

18 What could be done with stable beam intensities? M.Dasgupta et al. Phys.Rev. C70,024606,(2004) 9 Be+ 208 Pb E c.m. (MeV) E cm (MeV)  CF (mb) M. Beckerman et al. Phys. Rev. Lett 45 (1980) 1472, M. Beckerman et al. Phys. Rev. C23 (1981) 1581 M. Beckerman et al. Phys. Rev. C25 (1982)

19 Will this be possible at EURISOL? Beam currents much higher then the ones currently available (in some cases comparable with stable beam currents). Possibility to detect different reaction products (e.g. neutrons,  ) with the new, more efficient detection systems which will be available at EURISOL to discriminate the different reaction mechanisms. More species available.