He+ 6,7 Li and 6 He+ 12 C reactions. Matko Milin Ruđer Bošković Institute Zagreb, Croatia

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1 6 He+ 6,7 Li and 6 He+ 12 C reactions Matko Milin Ruđer Bošković Institute Zagreb, Croatia

2 introduction experimental details elastic scattering transfer reactions sequential decay reactions quasi-free scattering fusion conclusions and outlook 6 He+ 6,7 Li and 6 He+ 12 C reactions Matko Milin Ruđer Bošković Institute Zagreb, Croatia

3 Introduction: 6 He structure Borromean structure two loosely bound neutrons orbiting an α-core large interaction radius! R RMS 2.5 fm vs. R RMS (α) 1.7 fm components: di-neutron and cigar-like 6 He J π = 0 +, S 1n =1.87 MeV, S 2n =0.97 MeV, T 1/2 =0.807 s M.V. Zhukov et al, Phys.Rep. 231 (1993) 151

4 Introduction: 6 He induced reactions elastic scattering on different targets two-neutron transfer reaction 4 He( 6 He,α) break-up reactions suppression or enhancement of 6 He-induced fusion? what happens when another particle is added to such an loose object? would nuclear molecules be formed?

5 Introduction: nuclear molecules similar to atomic case: two strongly bound inert cores (e.g. α-particles) weak attractive potential that becomes repulsive at small distances weakly bound single-particle orbits of valence particles large transfer probability of valence particles essential difference: mass and identity of valence and core particles (Pauli-principle!) discussed already by Seya et al. in the early 80's, in the mid- 90 s reintroduced by von Oertzen W.von Oertzen Z.Phys. A354 (1996) 37

6 6 He+ 6,7 Li: goals of the experiment Main: 6,7 Li( 6 He, 10 Be) reaction and molecular states of 10 Be the 10 Be states at and 11.8 MeV are essential for molecular theories their spins and parities to be looked for using single events, double and triple coincidences Additional: elastic scattering (comparison with the 6 Li scattering) other transfer reactions, like 6 He+ 6 Li α+ 8 Li quasi-free scattering of loose 6 He on different clusters in the 6 Li and 7 Li targets

7 6 He+ 6,7 Li: experimental details facility: Louvain-la-Neuve (May 1999, 80 hours in total) beam: 6 He +, pps, only detected impurity: HeH 2 + targets: 6 LiF (493 µg/cm 2 ), 7 LiF (437 µg/cm 2 ), 12 C (105 µg/cm 2 ) and Au detectors: 20 segments with 16 silicon strips each, grouped into 3 arrays: - LEDA (4-12 deg) - LAMP1 (20-65 deg) - LAMP2 ( deg) particle identification: TOF + Mylar foil

8 Detectors LEDA: Louvain-Edinburgh Detector Array developed together with Micron Semiconductor Ltd. silicon-strip charged-particle detector sectors can be arranged into two different geometries: eight-sector disc ( LEDA ) and six-sided cone ( LAMP ) T. Davinson et al, Nucl.Instrum.Meth. A454 (2000) 544

9 Experimental results elastic and inelastic scattering on 6 Li, 7 Li, 12 C and 19 F measured large cross sections for 6 He induced α-particle pick-up to some 10 Be states first results obtained for the ( 6 He, 8 Be) two-proton pick-up reactions on 12 C, 16 O and 19 F the ( 6 He,α) reactions studied on 6 Li, 7 Li, 12 C and 19 F widths, spins and parities obtained for several 10 Be states decay mode studied for 9 Be and 14 C states quasi-free scattering of 6 He on deuteron in 6 Li observed and studied production of B in 6 He+ 6 Li and 6 He+ 7 Li reactions studied

10 Elastic and inelastic scatterings fair agreement with optical model predictions, using the 6 Li potentials half of 15% enhancement for the 12 C target due to the Borromean structure of 6 He (Matsumoto et al., PRC 70, ) inelastic scattering observed only for the first excited state of 12 C M.Milin et al, Nucl.Phys. A730 (2004) 285 M.Milin et al, Nucl.Phys. A746 (2004) 183

11 Transfer reactions partial angular distributions: 6 Li( 6 He,α) 8 Li 6 Li( 6 He,t) 9 Be 6 Li( 6 He,d) 10 Be 7 Li( 6 He,α) 9 Li 7 Li( 6 He,t) 10 Be 12 C( 6 He,α) 14 C M.Milin et al, Nucl.Phys. A746 (2004) 183

12 Transfer reactions: 6 He+ 6 Li α+ 8 Li direct transfers of 2n and d allow comparison of the α+2n configuration in 6 He with the α+d configuration in 6 Li preliminary results give spectroscopic factors equal within 10% 2-mode DWBA (preliminary!) more complete angular distributions needed! M.Milin et al, to be published

13 Transfer reactions: ( 6 He, 10 Be) 6 Li and 7 Li: α-cluster structure ideal targets for α-pickup large cross sections for α- transfer to some highexcited 10 Be states observed at least one of the states of the doublet at 7.5 MeV shows strong α-clustering M.Milin et al, Europhys.Lett. 48 (1999) 616 and to be published

14 Transfer reactions: ( 6 He, 8 Be) 8 Be gs = 2α+93 kev: unique signature simple extraction other advantages: 0 +, Q- values, wave-function overlaps, clearly observed on the 12 C (clearly direct!), 16 O and 19 F (populating many states in 10 Be, 14 C and 17 N) M.Milin et al, Phys.Rev. C70 (2004)

15 Sequential decay reactions triple coincidences observed: 6 He+ 6 Li 6 He+α+d 6 He+ 7 Li 6 He+α+t 6 He+ 6 Li 2α+t+n 6 He+ 12 C 10 Be+2α decay and structure of some 9 Be and 14 C states studied (double coincidences not enough!) M.Milin et al, Nucl.Phys. A753 (2005) 263 several 10 Be states seen in the 6,7 Li( 6 He, 6 He+α) 3,2 H reaction large reduced width for α-decay found for the 2 + state at E x = 7.54 MeV

16 Angular correlations analysis: 10 Be the MeV state: always the strongest peak in the 6 He+α coincidences (regardless of geometry) 10 Be, E x = 10.2 MeV, J π = 4 + Monte Carlo simulations (including DWBA calculation of transition amplitudes) compared with the exp. results strongly favour J π =4 + M.Milin et al, Nucl.Phys. A753 (2005) 263

17 Results for 10 Be known 0 + intruder state at E x = 6.18 MeV 2 + state at E x = 7.54 MeV very strong in α-transfer observed α-decay large α-reduced width state at E x = MeV the strongest peak in 6 He+α coincidences J π =4 + assignment favoured rotational band moment of inertia >2.5 times larger than for already deformed 10 Be ground state band extreme deformation! NUCLEAR MOLECULE! M.Milin et al, Nucl.Phys. A753 (2005) 263

18 Results for 10 Be axis-ratio 2.5:1! very small density overlap of 2 alpha-particles M.Milin et al, Nucl.Phys. A753 (2005) He+α resonant elastic scattering the same conclusion! M.Freer et al, Phys.Rev.Lett. 96 (2006) Y. Kanada-En'yo et al, Phys. Rev. C 60 (1999)

19 Quasi-free scattering 6 He quasi-free scattering off d- and α-clusters in 6 Li observed, and not observed for the case of 7 Li (L=0 for α-d in 6 Li and L=1 for α-t in 7 Li) d 3 σ ~ k.f. φ(q) dedω 1 dω 2 d2 σ 12 2 dω Ð.Miljanić et al, to be published

20 Fusion strong contribution from the x Li+α coincidences observed (in both 6 He+ 6 Li and 6 He+ 7 Li) peaks corresponding to the B states identified

21 Summary different reactions measured with 6 He radioactive beam new results obtained from both nuclear structure and reaction mechanism aspects: 6 He elastic scattering on 6 Li, 7 Li and 12 C measured deuteron, triton, two-proton and α-particle pick-up by 6 He beam observed for the 1 st time evidence found for existence of extremely deformed (axis ratio 2.5:1) rotational band in 10 Be nuclear molecules! α-cluster states in 14 C observed 6 He quasi-free scattering off clusters in other nuclei observed for the 1 st time

22 Collaborators M.Milin, Ð.Miljanić, N.Soić, M.Zadro, Ruđer Bošković Institute, Zagreb, Croatia S.Cherubini, A.Musumarra, A.Ninane, Institut de Physique Nucléaire, Université Catholique de Louvain, Louvain-la-Neuve, Belgium T.Davinson, A.Di Pietro, A.N.Ostrowski, A.C.Shotter, Department of Physics and Astronomy, University of Edinburgh, Edinburgh, Great Britain P.Figuera, M.G.Pellegriti, C.Spitaleri INFN-LNS and Università di Catania, Catania, Italy

23 Outlook 6 He+ 14 C experiment in Louvain-la la-neuve advantages: a large number of excess neutrons both particles in entrance channel have J π = 0 + disadvantages: low intensity of radioactive beam radioactivity of the 14 C target ( 6 He, 10 C) on different targets? very favourable four-proton pick-up reaction e.g. on 40 Ca target Q-value is MeV! 6 He+ 14 C Q (MeV) α+ 16 C Be+ 10 Be Be+ 12 Be He+ 12 C can also be used to determine nuclear masses far from stability