Fusion of light halo nuclei

Transcription

1 Fusion of light halo nuclei Alinka Lépine-Szily Instituto de Física-Universidade de São Paulo, São Paulo, Brazil th 118th Intn Few-Body Problems in PhysIcs 8th International IUPAP Conference on Few-Body Problems in PhysIcs 18th International Conference on Few-Body Problems in Physics, august Santos, Brazil

2 1)Nuclear Fusion : Barrier Penetration Model, couplings 2) Halo nuclei: reduction of barrier, break-up 3) Effect of halo on fusion and break-up 4)Continuum Discretized Coupled Channels Calc. 5)Comparison with experiments th 118th Intn Few-Body Problems in PhysIcs 8th International IUPAP Conference on Few-Body Problems in PhysIcs

3 Fusion of two atomic nuclei: quantum mechanical tunneling phenomenon. Barrier Penetration Model (BPM) works surprisingly well.

4 For energies below the Coulomb barrier: Increase in fusion cross section (σ F ) over the BPM result. Decrease in the height of the barrier V N (R)+V C (R) -> coupling to deformations, vibrations, transfer channels or soft dipole resonances Stokstad et al, PRL 41 (1978) 465 Leigh et al, PRC 52 (1995) 3151 M. Beckerman et al, PRL 45 (1980) Barrier Penetration model without couplings

5 Nuclear chart and halo nuclei close Surprises to the drip-line halo nuclei Z Section efficace (u.a.) 18C 19C 18C n Angle Neutron (deg.) Li Li Li Number of neutrons

6 Halo nuclei Borromean nature of 2n halo nuclei

7 Halo Nuclei 11 Li 9 Li+2n 11 Be 10 Be+n 2 neutron halo core of 9 Li 11 Li 4 fm 16 fm S 2n = 0.33 MeV T 1/2 = 8.5 ms T. Nakamura etal. PRL 96, (2006) 11 Li has strong E1 strength at 0.6 MeV, strong two-neutron correlation, <r c, 2n2 > 1/2 = 5.01(32)fm, <θ 12 > = 48(16)

8 Density distribution Nuclear+Coulomb Potential S 2n ( 6 He) = 0.98 MeV T 1/2 = ms S 2n ( 8 He) = 2.14 MeV T 1/2 = 119 ms

9 Neutron halo Projectiles : 11 Li, 11 Be, 6 He etc 6 He ou V Due to the neutron halo the strong force begins to act at larger distances the barrier is lower: an increase is predicted in the fusion probability r

10 Fusion with halo nucleus

11 Fusion with halo nuclei couplings: additional degrees of freedom, that can increase the fusion - Strong low-lying E1 strength in halo projectiles (different spatial distribution of protons and neutrons) reduction in the height of the barrier: an increase is predicted in the fusion probability Break-up of the weakly bound halo projectile : CONFLICTING theoretical predictions about the effect of break-up on fusion: - break-up favours the fusion (treats break-up as an additional channel) Dasso, Vitturi Phys.Rev.C50(1994)R12 - break-up inhibits the fusion (incident flux is reduced) Hussein, Pato, Canto,Donangelo Phys. Rev. C46(1992)377

12 Fusion and Break-up of weakly bound halo nuclei 1 2 T DCF CN A CN = A P +A T DCF CF 1 2 T 2 1 T 1 2 T SCF CN A CN = A P +A T B-Up 1 2 T ICF 1 T 2 ICF 1 CN 2 A CN < A P +A T T 2 1 ICF 2 CN th 118th Intn Few-Body Problems in PhysIcs 8th International IUPAP Conference on Few-Body Problems in PhysIcs Elastic Breakup 1 T 2 Break-up = coupling to the continuum, irreversible,

13 Comparison of break-up, fusion and total reaction cross section for 6 Li + 64 Zn ( 6 Li stable weakly bound) Gomes PLB 601(2204)20 Comparison of break-up, fusion and total reaction cross section for 9 Be Sm ( 9 Be stable weakly bound) σ R = σ CF + σ BU + σ ICF Gomes et al PLB 634(2006) th 118th Intn Few-Body Problems in PhysIcs 8th International IUPAP Conference on Few-Body Problems in PhysIcs Conclusion: for E<V CB break-up is much more probable than fusion. σ BU of halo nuclei > σ BU of stable nuclei for Coulomb break-up

14 ContinuumCContinuum Continuum Discretized Coupled Channels Calculation (CDCC) Since the weakly bound nuclei break-up, it is necessary to include at least 3 body effects in the description of the collision. The continuum must be considered. One has to truncate the number of states (CDCC). CDCC calculations describing the break-up of the projectile P are performed replacing the continuum by a finite number of configurations of the P = F1 + F2 system. CDCC calculations require the inclusion of both Coulomb and nuclear couplings, a large set of continuum states and multi-step processes th 118th Intn Few-Body Problems in PhysIcs 8th International IUPAP Conference on Few-Body Problems in PhysIcs

15 Schematic representation of bound and continuum states and their couplings in CDCC calculations th 118th Intn Few-Body Problems in PhysIcs 8th International IUPAP Conference on Few-Body Problems in PhysIcs Full lines: Hagino Dashed lines: additional couplings by Diaz-Torres and Thompson 2002

16 Full CDCC calculation for 11 Be Pb Diaz-Torres, Thompson- PRC65 (2002), a) Complete fusion (solid) only couplings to bound states b) complete fusion (solid) also continuum continuum couplings are included th 118th Intn Few-Body Problems in PhysIcs 8th International IUPAP Conference on Few-Body Problems in PhysIcs

17 Comparison of CDCC calculations for 6 He+ 12 C at 18MeV using 3 body nuclear break-up process ( 4 He+dineutron+ 12 C) with 4 body description ( 4 He+n+n+ 12 C). The 4-body CDCC calculation takes the Borromean nature of 6 He explicitly into account. The 4-body calculation reproduces well the total reaction cross section. Conclusion: 4-body CDCC calculation yields higher reaction cross section by 8%, than the 3-body calculation for light targets. Matsumoto and Kamimura et al Phys. Rev. C 70, (R) (2004) PhysIcs

18 Experimental results on fusion of neutron halo projectiles with heavy targets --M. Trotta et al PRL 84 (2000) 2342 compared 4,6He + 238U fusion cross sections and found enhancement --J.J.Kolata et al P. R. L. 81 (1998) 4580 compared 4,6 He Bi fusion cross sections and found enhancement for the 6 He halo nucleus --C.Signorini et al Nucl. Phys. A735 (2004)329 compared 9,10,11 Be Bi fusion cross sections and found no enhancement for the 11 Be halo nucleus

19 6 He U experiment in Louvain-la Neuve Fission as signature Experiments in Louvain-la-Neuve Beam intensity of 6 He pps 238 U target 500 μg/cm 2 Detection of back-to-back fission fragments in an array of Si detectors (angular coverage about 70% of 4π) Fission induced by transfer or inelastic excitation channels: a quasi-projectile particle is detected in coincidence

20 6 He U in Louvain-la-Neuve Transfer and fusion 2 fission fragments and a 3rd particle in coincidence. Angular distribution and energy spectrum indicates the direct transfer of 2n. 238 U( 6 He, 4 He) 240 U Q (transferencia) = MeV E*( 240 U) >20MeV fissão DWBA calculation with FRESCO including the continuum

21 R. Raabe et al, Nature, 431 (2004) 823 No enhancement of the fusion cross section

22 Conclusions The additional degrees of freedom in the halo nuclei 6 He and 11 Be enhance the break-up and reaction cross section on heavy and medium mass targets at energies around and below the potential barrier. The results on Fusion are still controversial: The additional degrees of freedom in the halo nuclei do not enhance the fusion cross section for 6 He U and 11 Be Bi, while the 6 He Bi seems to exhibit some enhancement at energies around and below the potential barrier.

23 Outlook : more experimental data on fusion and break-up of halo nuclei is needed theory has to take into account Coulomb and nuclear interaction, Borromean nature (3-4 body problem),coupling to and into the continuum RIBRAS (Radioactive Ion Beams Brasil) installed in the Pelletron Laboratory of IF - USP allows the study of this kind of experiments