NUCLEAR CLUSTERS AND NUCLEAR MOLECULES IN LIGHT NUCLEI

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NUCLEAR CLUSTERS AND NUCLEAR MOLECULES IN LIGHT
NUCLEI

• Neven Soic
• Ruder Bokovic Institute
• Zagreb, Croatia

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• Research programme cluster structure of light
  nuclei and reactions between light deformed
  nuclei
• 8,9,10Be, 10,11,12B, 11,12,13,14C, 16,17,18O,
  20,22Ne
• Accelerator facilities RBI Zagreb, LNS Catania,
  Vivitron Strasbourg, ANU Canberra, UCL
  Louvain-la-Neuve, GANIL Caen
• Short introduction to clustering phenomena in
  light nuclei and nuclear molecules
• The first experimental results on clustering in
  10Be Zagreb experiment
• Further studies of 10Be at LNS Catania, CRC UCL
  Louvain-la-Neuve and Ganil
• Experimental evidence for the first molecular
  structure in 10Be experiments at
  Louvain-le-Neuve
• Current and future research 12Be, 10B, 10C, 14C,
  16C

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Introduction - Light nuclei

• Light nuclei 3 lt A lt 20
• Unique quantum laboratories number of particles
  between a few (exact description) and many
  (statistical approach)
• Experimental results in last 2 decades a number
  of new and interesting quantum phenomena with no
  analogues in other areas of physics

neutron halo
proton halo
neutron skin
neutron drip line
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• Strange bindings of three-body system that have
  no bound two-body subsystems Borromean nuclei
• 6He a2n, binding energy 0.97 MeV
• 9Be 2an, binding energy 1.57 MeV
• 8He 6He2n, binding energy 2.14 MeV
• 14Be 12Be2n, binding energy 1.12 MeV
• Super-Borromean nucleus 10C four-body system
  with no bound three- and two-body subsystems
• 10C 2a2p, binding energy 3.73 MeV
• Spatially extended and deformed nuclei
• Closely related to those phenomena are recently
  established nuclear molecules
• Ordinary nuclei strength and short range of
  strong force majority of ground states and
  low-lying excited states are spherical

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• Deformations are pronounced in light nuclei
  ground and excited states - clusterization
  increase binding energy
• Basic subunit a-particle (4He) very stable and
  strongly bound nucleus doubly magic (1st
  excited state at 20.21 MeV)
• Strong repulsive force (Pauli principle) between
  nucleons in 2 a-particles holds deformed
  structure
• 
  
• 6Li 7Li
  7Be 8Be
• 
  
• 
  
  
  16O
• 
  12C
• 
  
  

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Ikeda diagram for na nuclei
Cluster structures appear mainly at excitation
energies close to the thresholds for nucleus
decomposition into clusters these excitations
are labeled in Ikeda diagram
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Nuclear molecules

• Structures formed by two or more strongly bound
  clusters (e.g. a-particles) surrounded with
  valence neutrons
• 
  9Be
• Additional neutrons dont destroy cluster
  structure, they actually enhance it
• This idea was introduced already in 30s,
  discussed by Seya in early 80s, in the mid 90s
  reintroduced by von Oertzen (Z. Phys. A 354
  (1996) 37)
• Valence neutrons are transferred between two
  cores exchange force between the cores
  stronger bindings
• Exchange force quantum effect
• known in atomic physics covalent
• bindings of atomic molecules
• Analogy cores atoms,
• valence neutrons - electrons

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• Potential between the cores is very similar to
  the van der Waals potential repulsive at small
  distance, attractive at larger distances
• Molecular local potential
• between two a-particles (8Be)
• Large transfer probability of valence particles
  between the cores
• Molecular structure may appear only if the core
  system is intrinsically very deformed
• Essential difference between atomic and nuclear
  molecules neutron mass comparable to the core
  mass, valence neutrons identical to the core
  neutrons (Pauli principle)

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• Weakly bound single-particle orbitals of valence
  particles (neutrons in p-orbital around
  a-particle)
• Molecular wave functions in two-centre system for
  2 p levels (harmonic oscillator with (nx, ny, nz)
  (1, 0, 0) i (0, 0, 1) ), cores on z-axis
  vertical to projection plane

s orbital s.p. w.f. parallel with z axis
p orbital s.p. w.f. vertical to z axis
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• 9Be i 10Be nuclei (valence neutrons in p3/2 orbit
  around aparticle) are crucial for understanding
  of nuclear molecular phenomena
• 3-D view of p1 molecular orbit
• for m1 (p orbit)
• and m0 (s orbit)
• Experimental signatures of cluster (molecular)
  structure
• selective (strong) population of levels
  in (cluster) transfer reactions
• large reduced widths for specific
  cluster structure
• rotational bands of states corresponding
  to very deformed structure

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• Expanded Ikeda diagram (von Oertzen diagram)
  neutron-rich nuclei with valence neutrons in
  covalent molecular orbitals around 4He and 16O

core valence neutrons structures appear at
excitations close to the thresholds for
cluster decays

• Recently large interest for studies of
  neutron-rich and exotic weakly bound nuclei
  exotic cluster structure
• review article by W. von Oertzen, M. Freer, Y.
  Kanada-Enyo, Physics Reports 432 (2006) 43

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Experimental studies of 10Be nucleus
1988
2004
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RBI accelerator facility
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CRC Louvain-la-Neuve radioactive ion beam facility
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• Our first experiment (1994) on 10Be cluster
  structure
• Measurement of the 7Li7Li ? aa6He (Q 7.37
  MeV) reaction at the RBI EN tandem Van de Graaff
  accelerator, beam energy 8 MeV
• Idea of the experiment
• use of the well developed
• cluster structure of 7Li to
• excite possible deformed
• structure in 10Be
• Only 8Be contribute to
• the excitation spectra

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Q7.37 MeV
Relative energy spectra for various pairs of
reaction products
Q-value spectra (reaction total energy) for
7Li(7Li,aa)6He and 7Li(7Li,a6He)4He reactions
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Results interpreted in terms of extremely
deformed structure
rotational band 0 at 6.18 MeV, 2
at 7.54 MeV, (4) at 10.2 MeV
New excited state at 10.2 MeV which decays
exclusively by a-particle emission
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• Further experiments
• 7Li7Li ? aa6He E030 MeV at tandem Van de
  Graaf accelerator of Laboratori Nazionali del
  Sud, Catania
• 9Be7Li ? a6Li6He E052 MeV at tandem Van
  de Graaf accelerator of Laboratori Nazionali del
  Sud, Catania
• 7Li7Li ? aa6He E08 MeV at RBI tandem
  Van de Graaf
• Fizika B 10 235 (2001)

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• Inclusive 10Be excitation energy spectra for two
  measured reactions

10Be excitation spectra for coincidence events
for three measured reactions
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• Idea for following experiments use of
  radioactive 6He ion beam to pickup a-particle and
  excite 10Be states with deformed structure
• 6He ß-decays with half-life of 800 ms
• structure compact a-core with two
• weakly bound valence neutrons
• Measurements of the 6He6Li ? 6Head and
  6He7Li ? 6Heat reactions in two experiments in
  1998. and 1999. at Louvain-la-Neuve
• Beam energies 17 and 18 MeV, beam intensity 3 x
  106 p/s
• Particle identification time of flight, reaction
  kinematics

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Bed coincidence events for 6Li(6He,d)10Be reactio
n
Experimental angular distributions for
6Li(6He,10Be)2H reaction Theory disturbed wave
Born approximation (FRDWBA) Large cross-section
for a-particle transfer for doublet of states at
7.5 MeV ? well developed a6He structure
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• results confirmed in measurements at higher beam
  energy
• a-spectroscopic factor for 7.5 MeV doublet 3-5
  times larger than s. f. for the ground and first
  excited state
• one of the doublet states has cluster structure,
  more likely it is 7.54 MeV state

10Be excitation energy spectrum for 6He6Li
?a6Hed (triple coincidence)
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Measurements of the 7Li(7Li,a6He)4He
reaction Eb58 MeV at tandem Van de Graaf
accelerator ANU Canberra Results indicate
Jp3- for 10.15 MeV state, for state at 11.8
MeV possible are 4,6
Experimental correlation function
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Experiment with radioactive 10Be ion beam
at GANIL Measurements of 12C(10Be,a6He)12C
reaction beam energy 302 MeV
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In this experiment we used neutron detector DEMON
array of 81 modules with liquid scintillator
NE213 Reactions 12C(10Be,aa) i 12C(10Be,aan) Two
neutrons removal from 10Be mainly excite 8Be 2
state Two (or more) steps complex process,
excitation of various 9Be states, core excitation
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10Be excitation energy spectra peaks at 7.54
(130 keV above threshold) and 10.15 MeV
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Angular correlation analysis for 10.15 MeV state
Angular correlations between decay products may
provide information on spin and parity of
decaying state (if both products are Jp 0)
Ex10.15 MeV, Jp4
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• Results state at 10.15 MeV has spin
• and parity Jp4 (with assumption of
• the reaction mechanism), confirmed
• its well developed cluster structure
• a6He for 7.54 MeV state (Jp2)
• confirmed its well developed
• a6He structure

• These results confirm our previous speculation of
  very deformed structure for 10Be excited states
  and rotational band 0 (6.18 MeV), 2 (7.54 MeV),
  4 (10.15 MeV)
• Band rotational parameter ?/2I 200 keV ? 2.5
  times larger than for 8Be ground state band !
• Two neutrons move along symmetry axis between two
  separated a-particles ? s-orbitals

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Final evidence and confirmation of results
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• Experiment 6He beam and 4He gas target
• Louvain-la-Neuve RIB facility
• Resonant elastic scattering
• provides direct determination of
• spin and parity, excitation energy,
• total and partial width

Results for 10.15 MeV state can be described only
with 4 (coincidence events)
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• Ga 0.10 0.13 MeV Ga /G 0.35 0.46
• Extremely large value for spectroscopic factor
  for a-cluster
• These results are final confirmation of our
  previous claims

Singles data compared with non-resonant elastic
scattering
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• Results of antisymmetrized molecular
  dynamics (AMD) calculations for structure of 10Be
  nucleus, Y. Kanada-Enyo, H. Horiuchi, A. Dote,
  Phys. Rev. C 60 064304 (1999)
• Results of molecular orbital model
  calculations for 0 levels in 10Be, N. Itagaki,
  S. Okabe, Phys. Rev. C 61 044306 (2000)

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Conclusion and outlook

• Importance of results experimentally confirmed
  extremely deformed structure in 10Be the first
  nuclear molecule
• Two neutrons move along symmetry axis between two
  separate a-particles ? s-orbitals
• Studies of light nuclei still and again provide
  unexpected new results
• Further studies 10Be complete spectroscopy,
  isospin analog states in 10B i 10C (very exotic
  nucleus)
• Neutron-rich beryllium nuclei 12Be (2a 4
  neutrons)
• Thee-centre nuclear molecules neutron-rich
  carbon nuclei 13C, 14C, 16C
• 16C three a chain state stabilized by valence
  neutrons

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Collaborators

• RBI M. Milin, Ð. Miljanic, M. Zadro, S. Blagus,
  M. Bogovac, S. Fazinic, D. Rendic, T. Tadic
• Laboratori Nazionali del Sud INFN Catania
  Universita di Catania, Italija M. Lattuada, C.
  Spitaleri, M. Aliotta, S. Cherubini, A. Di
  Pietro, P. Figuera, A. Musumarra, R. G. Pizzone,
  S. Romano, A. Tumino, E. Costanzo, M. G.
  Pellegriti
• University of Edinburgh, Ujedinjeno Kraljevstvo
  A. C. Shotter, T. Davinson, A. N. Ostrowski
• University of Birmingham, Ujedinjeno Kraljevstvo
  M. Freer, N. M. Clarke, N. Curtis, N. I. Ashwood,
  S. Ahmed, V. A. Ziman, C. J. Metelko, D. Price
• University of Surrey, Guildford, Ujedinjeno
  Kraljevstvo W. N. Catford, S. Pain, D. Mahboub,
  C. Harlin
• Laboratoire de Physique Corpusculaire ISMRA
  Universite de Caen, Francuska N. A. Orr, L.
  Achouri, F. M. Marques, J. C. Angelique, J. C.
  Lecouey, G. Normand, C. Timis, B. Laurent
• Universite Libre de Bruxelles, Belgija F.
  Hanappe, T. Materna, V. Bouchat
• Universite Catholique de Louvain,
  Louvain-la-Neuve, Belgija C. Angulo, E.
  Casarejos, P. Demaret
• Katholieke Universiteit Leuven, Belgija R. Raabe