NUCLEAR CLUSTERS AND NUCLEAR MOLECULES IN LIGHT NUCLEI
- Slides: 36
NUCLEAR CLUSTERS AND NUCLEAR MOLECULES IN LIGHT NUCLEI Neven Soić Ruđer Bošković Institute Zagreb, Croatia
• Research programme: cluster structure of light nuclei and reactions between light deformed nuclei • 8, 9, 10 Be, 10, 11, 12 B, 11, 12, 13, 14 C, 16, 17, 18 O, 20, 22 Ne • 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 10 Be: Zagreb experiment • Further studies of 10 Be at LNS Catania, CRC UCL Louvain-la-Neuve and Ganil • Experimental evidence for the first molecular structure in 10 Be: experiments at Louvain-le-Neuve • Current and future research: 12 Be, 10 B, 10 C, 14 C, 16 C
Introduction - Light nuclei • Light nuclei: 3 < A < 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
• Strange bindings of three-body system that have no bound two-body subsystems: Borromean nuclei: – 6 He: α+2 n, binding energy 0. 97 Me. V – 9 Be: 2α+n, binding energy 1. 57 Me. V – 8 He: 6 He+2 n, binding energy 2. 14 Me. V – 14 Be: 12 Be+2 n, binding energy 1. 12 Me. V • “Super-Borromean” nucleus 10 C: four-body system with no bound threeand two-body subsystems – 10 C: 2α+2 p, binding energy 3. 73 Me. V • 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
• Deformations are pronounced in light nuclei – ground and excited states - clusterization increase binding energy • Basic subunit: α-particle (4 He) – very stable and strongly bound nucleus – doubly magic (1 st excited state at 20. 21 Me. V) • Strong repulsive force (Pauli principle) between nucleons in 2 α-particles holds deformed structure 6 Li 7 Be 12 C 8 Be 16 O
Ikeda diagram for nα nuclei Cluster structures appear mainly at excitation energies close to the thresholds for nucleus decomposition into clusters – these excitations are labeled in Ikeda diagram
Nuclear molecules • Structures formed by two or more strongly bound clusters (e. g. α-particles) surrounded with valence neutrons 9 Be • Additional neutrons don’t destroy cluster structure, they actually enhance it • This idea was introduced already in 30’s, discussed by Seya in early 80’s, in the mid 90’s 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
• 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 α-particles (8 Be) • 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)
• Weakly bound single-particle orbitals of valence particles (neutrons in porbital around α-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 σ orbital: s. p. w. f. parallel with z axis π orbital: s. p. w. f. vertical to z axis
• 9 Be i 10 Be nuclei (valence neutrons in p 3/2 orbit around α–particle) are crucial for understanding of nuclear molecular phenomena 3 -D view of p=1 molecular orbit for m=1 (π orbit) and m=0 (σ 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
• Expanded Ikeda diagram (von Oertzen diagram): neutron-rich nuclei with valence neutrons in covalent molecular orbitals around 4 He and 16 O 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. En’yo, Physics Reports 432 (2006) 43
Experimental studies of 1988 2004 10 Be nucleus
RBI accelerator facility
CRC Louvain-la-Neuve radioactive ion beam facility
• Our first experiment (1994) on 10 Be cluster structure • Measurement of the 7 Li+7 Li → α+α+6 He (Q= 7. 37 Me. V) reaction at the RBI EN tandem Van de Graaff accelerator, beam energy 8 Me. V • Idea of the experiment: use of the well developed cluster structure of 7 Li to excite possible deformed structure in 10 Be • Only 8 Be contribute to the excitation spectra
Q=7. 37 Me. V Relative energy spectra for various pairs of reaction products Q-value spectra (reaction total energy) for 7 Li(7 Li, αα)6 He and 7 Li(7 Li, α 6 He)4 He reactions
Results interpreted in terms of extremely deformed structure rotational band: 0+ at 6. 18 Me. V, 2+ at 7. 54 Me. V, (4+) at 10. 2 Me. V New excited state at 10. 2 Me. V which decays exclusively by α-particle emission
• Further experiments: 7 Li+7 Li → α+α+6 He E =30 Me. V at tandem Van de Graaf accelerator of 0 Laboratori Nazionali del Sud, Catania 9 Be+7 Li → α+6 Li+6 He E =52 Me. V at tandem Van de Graaf accelerator of 0 Laboratori Nazionali del Sud, Catania 7 Li+7 Li → α+α+6 He E =8 Me. V at RBI tandem Van de Graaf 0 Fizika B 10 235 (2001)
Inclusive 10 Be excitation energy spectra for two measured reactions 10 Be excitation spectra for coincidence events for three measured reactions
• Idea for following experiments: use of radioactive 6 He ion beam to pickup α-particle and excite 10 Be states with deformed structure β-decays with half-life of 800 ms • structure: compact α-core with two weakly bound valence neutrons • 6 He • Measurements of the 6 He+6 Li → 6 He+α+d and 6 He+7 Li → 6 He+α+t reactions in two experiments in 1998. and 1999. at Louvain-la-Neuve • Beam energies: 17 and 18 Me. V, beam intensity: 3 x 106 p/s • Particle identification: time of flight, reaction kinematics
Be+d coincidence events for 6 Li(6 He, d)10 Be reaction Experimental angular distributions for 6 Li(6 He, 10 Be)2 H reaction Theory: disturbed wave Born approximation (FRDWBA) Large cross-section for α-particle transfer for doublet of states at 7. 5 Me. V → well developed α+6 He structure
• results confirmed in measurements at higher beam energy • α-spectroscopic factor for 7. 5 Me. V 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 Me. V state 10 Be excitation energy spectrum for 6 He+6 Li →α+6 He+d (triple coincidence)
Measurements of the 7 Li(7 Li, α 6 He)4 He reaction Eb=58 Me. V at tandem Van de Graaf accelerator ANU Canberra Results indicate Jπ=3 for 10. 15 Me. V state, for state at 11. 8 Me. V possible are 4+, 6+ Experimental correlation function
Experiment with radioactive 10 Be ion beam at GANIL Measurements of 12 C(10 Be, α 6 He)12 C reaction beam energy 302 Me. V
In this experiment we used neutron detector DEMON – array of 81 modules with liquid scintillator NE 213 Reactions 12 C(10 Be, αα) i 12 C(10 Be, ααn) Two neutrons removal from 10 Be mainly excite 8 Be 2+ state Two (or more) steps complex process, excitation of various 9 Be states, core excitation
10 Be excitation energy spectra: peaks at 7. 54 (130 ke. V above threshold) and 10. 15 Me. V
Angular correlation analysis for 10. 15 Me. V state Angular correlations between decay products may provide information on spin and parity of decaying state (if both products are Jπ= 0+) Ex=10. 15 Me. V, Jπ=4+
• Results: state at 10. 15 Me. V has spin and parity Jπ=4+ (with assumption of the reaction mechanism), confirmed its well developed cluster structure α+6 He ; for 7. 54 Me. V state (Jπ=2+) confirmed its well developed α+6 He structure • These results confirm our previous speculation of very deformed structure for 10 Be excited states and rotational band 0+ (6. 18 Me. V), 2+ (7. 54 Me. V), 4+ (10. 15 Me. V) • Band rotational parameter: ℏ/2 I = 200 ke. V 2. 5 times larger than for 8 Be ground state band ! • Two neutrons move along symmetry axis between two separated αparticles → σ-orbitals
Final evidence and confirmation of results
• Experiment: 6 He beam and 4 He 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 Me. V state can be described only with 4+ (coincidence events)
• Γα = 0. 10 – 0. 13 Me. V ; Γα /Γ = 0. 35 – 0. 46 • Extremely large value for spectroscopic factor for α-cluster • These results are final confirmation of our previous claims Singles data compared with non-resonant elastic scattering
Results of antisymmetrized molecular dynamics (AMD) calculations for structure of 10 Be nucleus, Y. Kanada. En’yo, H. Horiuchi, A. Dote, Phys. Rev. C 60 064304 (1999) Results of molecular orbital model calculations for 0+ levels in 10 Be, N. Itagaki, S. Okabe, Phys. Rev. C 61 044306 (2000)
Conclusion and outlook • Importance of results: experimentally confirmed extremely deformed structure in 10 Be – the first nuclear molecule • Two neutrons move along symmetry axis between two separate αparticles → σ-orbitals • Studies of light nuclei still and again provide unexpected new results • Further studies: 10 Be complete spectroscopy, isospin analog states in 10 B i 10 C (very exotic nucleus) • Neutron-rich beryllium nuclei: 12 Be (2α + 4 neutrons) • Thee-centre nuclear molecules: neutron-rich carbon nuclei: 13 C, 14 C, 16 C • 16 C: three α chain state stabilized by valence neutrons
Collaborators • RBI: M. Milin, Đ. Miljanić, M. Zadro, S. Blagus, M. Bogovac, S. Fazinić, D. Rendić, T. Tadić • 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
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