6He+6,7Li and 6He+12C

Matko MilinRuđer Bošković Institute

Zagreb, Croatia

reactions

introduction experimental details elastic scattering transfer reactions sequential decay reactions quasi-free scattering fusion conclusions and outlook

6He+6,7Li and 6He+12C

Matko MilinRuđer Bošković Institute

Zagreb, Croatia

reactions

Introduction: 6He structure

Borromean structure two loosely bound neutrons orbiting an αααα-core large interaction radius! RRMS≈ 2.5 fm vs. RRMS(αααα)≈ 1.7 fm components: di-neutron and cigar-like

M.V. Zhukov et al, Phys.Rep. 231 (1993) 151

6He Jππππ= 0+,

S1n=1.87 MeV, S2n=0.97 MeV, T1/2=0.807 s

Introduction: 6He induced reactions

elastic scattering on different targets two-neutron transfer reaction 4He(6He,αααα) break-up reactions suppression or enhancement of 6He-induced fusion? …

what happens when another particle is added to such an loose object? would “nuclear molecules” be formed?

Introduction: nuclear molecules similar to atomic case:

two strongly bound inert cores (e.g. αααα-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 OertzenZ.Phys. A354 (1996) 37

6He+6,7Li: goals of the experiment

Main:• 6,7Li(6He,10Be) reaction and “molecular” states of 10Be

• the 10Be states at 10.15 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 6Li scattering)

• other transfer reactions, like 6He+6Li αααα+8Li

• quasi-free scattering of loose 6He on different clusters in the 6Li and 7Li targets

• …

facility: Louvain-la-Neuve (May 1999, 80 hours in total)

beam: 6He+, 4×106 pps, only detected impurity: HeH2+

targets: 6LiF (493 µµµµg/cm2), 7LiF (437 µµµµg/cm2), 12C (105 µµµµg/cm2)and Au

detectors: 20 segments with 16 silicon strips each, grouped into 3 arrays: - LEDA (4-12 deg)

- LAMP1 (20-65 deg)- LAMP2 (115-160 deg)

particle identification: TOF + Mylar foil

6He+6,7Li: experimental details

Detectors

T. Davinson et al, Nucl.Instrum.Meth. A454 (2000) 544

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”)

Experimental results elastic and inelastic scattering on 6Li, 7Li, 12C and 19F

measured large cross sections for 6He induced αααα-particle pick-up to some 10Be states

first results obtained for the (6He,8Be) two-proton pick-up reactions on 12C, 16O and 19F

the (6He,αααα) reactions studied on 6Li, 7Li, 12C and 19F

widths, spins and parities obtained for several 10Be states

decay mode studied for 9Be and 14C states

quasi-free scattering of 6He on deuteron in 6Li observed and studied

production of 10-12B in 6He+6Li and 6He+7Li reactions studied

…

Elastic and inelastic scatterings

M.Milin et al, Nucl.Phys. A746 (2004) 183

fair agreement with optical model predictions, using the 6Li –potentials

half of 15% enhancement for the 12C target due to the Borromean structure of 6He (Matsumoto et al., PRC 70, 061601)

inelastic scattering observed only for the first excited state of 12C

M.Milin et al, Nucl.Phys. A730 (2004) 285

Transfer reactions

M.Milin et al, Nucl.Phys. A746 (2004) 183

partial angular distributions:

6Li(6He,αααα)8Li

6Li(6He,t)9Be

6Li(6He,d)10Be

7Li(6He,αααα)9Li

7Li(6He,t)10Be

12C(6He,αααα)14C

…

Transfer reactions: 6He+6Li αααα+8Li

M.Milin et al, to be published

direct transfers of 2nand d allow comparison of the αααα+2n configuration in 6He with the αααα+dconfiguration in 6Li

preliminary results give spectroscopic factors equal within 10%

more complete angular distributions needed!

2-mode DWBA(preliminary!)

Transfer reactions: (6He,10Be)

6Li and 7Li: αααα-cluster structure ideal targets for αααα-pickup

large cross sections for αααα-transfer to some high-excited 10Be states observed

at least one of the states of the doublet at ≈7.5 MeV shows strong αααα-clustering

M.Milin et al, Europhys.Lett. 48 (1999) 616

and to be published

Transfer reactions: (6He,8Be)

8Begs= 2αααα+93 keV: unique signature simple extraction

other advantages: 0+, Q-values, wave-function overlaps,…

clearly observed on the 12C (clearly direct!), 16O and 19F (populating many states in 10Be, 14C and 17N)

M.Milin et al, Phys.Rev. C70 (2004) 044603

Sequential decay reactions

M.Milin et al, Nucl.Phys. A753 (2005) 263

triple coincidences observed:6He+6Li 6He+αααα+d6He+7Li 6He+αααα+t 6He+6Li 2αααα+t+n 6He+12C 10Be+2αααα

decay and structure of some 9Be and 14C states studied

several 10Be states seen in the 6,7Li(6He,6He+αααα)3,2H reaction

large reduced width for αααα-decay found for the 2+

state at Ex= 7.54 MeV

(double coincidencesnot enough!)

the 10.15 MeV state: always the strongest peak in the 6He+αααα coincidences (regardless of geometry)

Monte Carlo simulations (including DWBA calculation of transition amplitudes) compared with the exp. results strongly favour Jππππ=4+

10Be, Ex= 10.2 MeV, Jππππ= 4+

Angular correlations analysis: 10Be

M.Milin et al, Nucl.Phys. A753 (2005) 263

Results for 10Be

22++ state at Estate at Exx= 7.54 MeV= 7.54 MeV very strong in αααα-transfer

observed αααα-decay large αααα-reduced width

state at Estate at Exx= 10.15 MeV= 10.15 MeV the strongest peak in 6He+ααααcoincidences

Jππππ=4+ assignment favoured

known 0known 0++ intruder state at intruder state at EExx= 6.18 MeV= 6.18 MeV

NUCLEAR MOLECULE!NUCLEAR MOLECULE!

M.Milin et al, Nucl.Phys. A753 (2005) 263

rotational bandrotational band

moment of inertia >2.5 times larger than for already deformed 10Be ground state band

extreme deformation!

Results for 10Be axis-ratio≈ 2.5:1!

very small density overlap of 2 alpha-particles

M.Freer et al, Phys.Rev.Lett. 96 (2006) 042501

M.Milin et al, Nucl.Phys. A753 (2005) 263

6He+αααα resonant elastic

scattering the same conclusion!

Y. Kan

ada-

En'yo

et al,

Phys

. Rev

. C 6

0 (1999) 064304

Quasi-free scattering

6He quasi-free scattering off d- and αααα-clusters in 6Li observed, and not observed for the case of 7Li (L=0 for αααα-d in 6Li and L=1 for αααα-t in 7Li)

Ð.M

iljan

ićet al,

to be pu

blished

d3σσσσ

dEdΩΩΩΩ1dΩΩΩΩ2~ k.f. |φφφφ(q)|2

d2σσσσ12dΩΩΩΩ

Fusion

strong contribution from the xLi+αααα coincidences observed (in both 6He+6Li and 6He+7Li)

peaks corresponding to the 10-12B states identified…

Summary different reactions measured with 6He radioactive beam

new results obtained from both nuclear structure and reaction mechanism aspects:

6He elastic scattering on 6Li, 7Li and 12C measured

deuteron, triton, two-proton and αααα-particle pick-up by 6He beam observed for the 1st time

evidence found for existence of extremely deformed (axis ratio – 2.5:1) rotational band in 10Be nuclear molecules!

αααα-cluster states in 14C observed

6He quasi-free scattering off clusters in other nuclei

observed for the 1st time

…

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.SpitaleriINFN-LNS and Università di Catania, Catania, Italy

Outlook66He+He+1414C experiment in C experiment in LouvainLouvain--lala--NeuveNeuve advantages:

a large number of excess neutrons

both particles in entrance channel have Jππππ= 0+

disadvantages:

low intensity of radioactive beam

radioactivity of the 14C target

6He+14C Q (MeV) αααα+

16C 4.49510Be+10Be -4.5998Be+12Be -9.404 8He+12C -10.894

((66He,He,1010C)C) on different targets?on different targets?

very favourable four-proton pick-up reaction

e.g. on 40Ca target Q-value is -2.29 MeV!

can also be used to determine nuclear masses far from stability