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This presentation discusses the role of coupling effects with the continuum in nuclear reactions involving exotic nuclei, including measurements of coupling effects, correlations of valence neutrons, and virtual couplings. Examples are presented to illustrate the importance of these couplings in various experimental scenarios.
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Nuclear reactions with exotic nuclei: role of the continuumKrzysztof Rusek Heavy Ion Laboratory of the University of Warsaw, Poland
Overview Coupling effects with the continuum have a long history going back nearly forty years. This contribution is not meant as a review but rather as a presentation of a few examples where the couplings play an important role. : • coupling effects can be measured • coupling and fusion • correlations of valence neutrons
Virtual couplings (polarizability) – processes whereby, for example, the projectile nucleus is raised to an excited state which then decays back to the ground state before the projectile has traversed the field of the target nucleus. Real and virtual processes core Target N.Keeley et al. Progress in Particle and Nuclear Physics 63,396 (2009)
Experiments with polarized 6,7Li beams(1970’s – 1990’s) Dieter Fick, Univ. Marburg Garry Tungate, Univ. Birmingham Kirby W. Kemper, Florida State Univ.
Coupling with 3+ resonance 6Li + 58Ni ? resonance α+d breakup threshold g.s. 6Li H. Nishioka et al., NPA 415, 230 (1984)
Comparison of 6Li-6He No E1 Reduced Coulomb-nuclear interference peak for 6He, caused by Coulomb dipole couplings to the continuum
Model: Continuum-Discretized Coupled-Channels An extension of coupled channels technique to allow the treatment of couplings to unbound states, both resonances and non-resonant continuum. It was first developed to describe the effect of breakup couplings on deuteron elastic scattering.
CDCC in action L. Acosta et al. Dot-dashed: no dipole couplings Dashed: no couplings to the continuum
V = Vo + i W +DPP Dynamic Polarization Potential N. Keeley et al. PRC88, 017602 (2013)
6He + 206Pb at 18 MeV L. Standylo et al. PRC87 064603 (2013) R. Wolski et al. EPJA 47, 111 (2011) 6He beam 6He + 206Pb experiment at the Cyclotron Research Centre in Louvain-la-Neuve
Spectra Forward angle Backward angle elastic alphas Optimum Q-values are different from expected!
Coulomb post-acceleration D Z1 6He energy at D: ED = E – Z1Z2e2/D Z2 αenergy at the detector: 4/6 ED + Z1Z2e2/D = 4/6 E + 1/3 Z1Z2e2/D Breakup occurs at D ~ 25 fm ~3 MeV
6He+208Pb – a very few reaction channels open 6He 4He L.Acosta et al. PRC84, 044604
CRC result: elastic scattering CRC, one parameter varied („2n coupling strength”)
CRC result: alpha – particles yield 2n- transfer c.c. – 149 mb 1n – transfer c.s. – 155 mb Breakup c.c. – 151 mb Fusion – 32 mb (exp. 48+-5 mb) breakup and n-transfer channels dominate below the barrier (not fusion)
Pairing in 6He Enhancement factor – a measure of pairing
Summary • Everything is coupled! • Analysing powers are very sensitive to the coupling effects (new polarized ion sources?) • Elastic scattering data sets bring a lot of info about couplings (exp. must be precise!) • Separation of different contributions important (neutron detectors needed) • Direct reaction models (CDCC, CRC) are well-suited to describe experimental results obtained with light, weakly bound, projectiles at energies around the Coulomb barrier (for limited number of open channels)
