Studies of Heavy Ion Reactions around Coulomb Barrier

1. 中国原子能科学研究院 China Institute of Atomic Energy Studies of Heavy Ion Reactions around Coulomb Barrier • Part I. Competition between fusion-fission and quasi- fission in 32S+184W reaction • Part II. Sub-barrier fusion enhancement of 32S+90,96Zr • Part III. The study of the surface property of nuclear potential by quasi-elastic scattering • Part IV.The breakup threshold anomaly of 9Be+208Pb,209Bi • Part V. Two-proton emission from 29S,28P excited states after Coulomb excitation. Huanqiao Zhang China Institute of Atomic Energy (CIAE) weihai, 2011. 8. 8-11

2. Part I. Competition between fusion-fission and quasi-fission in 32S+184W reaction Motivation: 1) Fusion-fission dynamics –- Quasi-fission, Pre-equilibrium fission …… 2) Deformation effects in the entrance channels 3) Shell effects in the compound nuclei 32S+184W -> 216Th (N=126)

3. QF DNS T P FF CN ER

4. Experimental Setup Beam:32S ELab: 140，145，150，155, 160，165，170 MeV. Typical beam current: 800-1000 enA. Target: about 200ug/cm2 with 20μg/cm2 carbon foil backing.

5. 32S+184W ELab=170MeV θLab=54.850 32S+184W ELab=170MeV Angle: 76.90---83.980 DF01：54.850 elastic Fission production Energy Spectrometry Correlated two fission fragments

6. The experimental angular distributions of the fission fragments and the fitting with Saddle-Point Transitional State model.

8. Main result The measured capture cross sections and the deduced values of Aexp and K20 for the 32S+184W reaction. The total cross section was deduced from the integration of the differential cross sections.

9. Comparison with theory calculation (DNS) • Dinuclear system is formed at the initial • stage of the reaction, kinetic energy is • transferred into potential and excitation. • Necessary conditions: • presence of a potential pocket; • adequacy of the collision energy Ec.mto overcome the interaction barrier • Characterized by mass (charge) symmetry of its nuclei, rotational energy Vrot and excitation energy E*DNS. Dot-dashed line: the capture path Solid line: potential well

10. The potential energy surface for a dinuclear system leading to the formation of 216Th* The driving potential Udr(Z) is a curve linking minimums corresponding to each charge asymmetry Z in the valley of the potential energy surface from Z = 0 up to Z = ZCN. The dinuclear system formed in the collision of two nuclei evolves to fusion by increasing its mass asymmetry. The evolution of the system along the mass asymmetry degree of freedom is described by the driving potential. A path to fusion is determined by potential energy surface.

11. The quasi-fission spin distributions the value of driving potential Z=16 for the small orientation angle 15° (solid line) and 45°(dashed line).

12. Zhang et.al. Phys. Rev. C 81,034611(2010)

13. The large l contribution leads to quasi-fission The elongated shape leads to quasi-fission The presentation of fusion probability PCN

14. Part II.Sub-barrier fusion enhancement of 32S+90,96Zr Sub-barrier fusion enhancement due to the couplings to the intrinsic degrees of freedom and nucleon transfer channels has been found since 1980s. Research the effect of positive Q-value multi-neutron transfer on the fusion enhancement at sub-barrier energies for 32S+96Zr system.

15. Fusion evaporation residua measurement: Reduced fusion excitation functions of 36S,40,48Ca+90,96Zr systems

16. Experimental setup electrodes MCP target Si(Au) beam The schematic plot of the electrostatic deflector Electrostatic deflector (Separated by electric-rigidity) suppression ratio >108 c.s. down to b level

17. Recoil 12C Contaminator Target recoils Scattering 32S Eva. residua The ΔE-TOF spectrum of the reaction products after separation. The experimental fusion excitation functions of 32S+90,96Zr systems

18. Comparison with Zagrebaev’s theory: assume a successive transfer mechanism of single neutrons (a direct nucleon pair transfer?) Phys. Rev. C 67 061601(R) (2003)

19. Qgg-value for neutron pickup Separation energies of each neutron for 96Zr Dotted line: single-channel Solid line: coupled to inelastic excitation states Dashed line: coupled to inelastic excitation states + neutron transfer

20. Zhang et al., Phys. Rev. C 82 054609(2010)

21. Part III.The research of the surface property of nuclear potential by quasi-elastic scattering • Aim: • Research the difference of the diffuseness parameter extracted from fusion and elastic scattering. • 2. Research the difference of the diffuseness parameter extracted from the spherical and deformed systems by using quasielastic scattering.

22. The values of the diffuseness parameter a as a function of Z1Z2 extracted from the fusion excitation functions above the barrier energies. The values of the diffuseness parameter a are different for spherical and deformed systems. Deformed systems • Larger than the commonly accepted value; • Increase with the increase of Z1Z2. The open symbols represent the values deduced from fusion cross section. Phys. Rev. C 70 024605(2004) Phys. Rev. C 73 034607 (2006)

23. Extract the diffuseness parameter using the backward quasi-elastic scattering at deep sub-barrier energies. Phys. Rev. C 76 024612 (2007)

24. Quasi-elastic (QEL) scattering is sensitive to the surface property of the nuclear potential at deep sub-barrier energy region. small deviationdue to VN A way to extract the a parameter Phys. Rev. C 69 054610 (2004)

25. Experimental setup in order to effectively reduce the scattered electrons and projectiles into backward detectors. Energy spectrum of the projectile-like particles at θlab=175°

26. Energy spectrum of the projectile-like particles at θlab=175° reaction mechanism? Proton transfer? Multi-nucleon transfer or deep inelastic? low inelastic states Z and A identification! ANU group As also reported in PRC78, 034614 (08) More complicated than transfer mechanism. More exit channels populated than what is included in the CC calculations.

27. Excitation functions of quasi-elastic scatterings at 175

28. Data analysis: using a modified CCFULL code CQUEL by K. Hagino confine the analysis data to dσqel/d σRu>0.94 (expect the coupling effect is negligible in this range) the parameters of optical potential I.a short range imaginary potential (produces absorption): W= 30 MeV, aw= 0.1 fm, and rw = 0.8 fm II. real potential (produces a deflection) Keep V0 = 100 MeV fixed Constraint: reproduce the expected average fusion barrier energy using the 3 parameters. Coupling to the low inelastic states of the targets was included; without coupling to the inelastic states of projectile. Phys. Rev. C 71 044612 (2005); 76 024612( 2007); 78 034614 (2008)

29. Lin et al., Phys. RevC79_064603_(2009)

30. Part IV.The breakup threshold anomaly of 9Be+208Pb,209Bi. The threshold anomaly (TA) comes from the coupled-channels (CC) effects and plays an important role in heavy ion reactions at the energies around Coulomb barrier. How does the breakup of the weakly bound projectile affect the TA ?

31. First observed in J. S. Lilley, et al., Phys. Lett. B 151,181, (1985).

32. Two different results: C. Signorini, et al., 9Be+209Bi unusual optical behavior Phys. Rev. C, 61, 061603, (2001). Woolliscroft, et al., 9Be+208Pb. threshold anomaly, Phys. Rev. C 69, 044612, (2004).

33. Elastic scattering angular distributions for the 9Be+208Pb,209Bi systems and the optical model fit with PTOLEMY.

34. The breakup/unusual threshold anomaly The real and imaginary parts of optical potential for the two systems. N. Yu et.al., J. Phys. G. 37(2010) 075108

35. Quasi-Elastic excitation function and barrier distribution for 9Be+208Pb H. M. Jia, et.al., Phys. Rev. C82, 027602(2010)

36. 17F+12C弹性散射

41. Part V. Two-proton emission from 29S,28P excited states after Coulomb excitation. • Two-proton radioactivity: • Two-body sequential emission; • 2) Three-body simultaneously democratic emission; • 3) 2He cluster emission and following breakup. Decay Dynamics of two-proton emission from excited states Experiment Theory Invariant Mass, qpp=|p1-p2|/2,the relative momentum, ppcm ,the opening angle, the relative energy, ....... Three body models, The extended R-matrix theory, The Faddeev equations, .......

42. Complete-kinematics measurements Secondary target:197Au,100 µm SD: Silicon detectors, 325 ,1000 µm SSSD: Single sided Silicon Strip Detectors, 300 µm, 24 strips with 2 mm in the width and 0.1 mm in the intervalforthe construction of the particle trajectories CsI(Tl) detectors: 6×6 lattices ,each 15×15×20mm, read out through PIN photodiodes

43. Two-proton correlation for 7.4MeV state • 7.0<Ex<7.8MeV • The maximum at qpp=35 MeV and the opening angle of sinθ indicates the branching ratio of 2He emission less than 10% with MC simulations (for three-body democratic decay, no FSI)

44. Two-proton correlation for 10.0MeV state of 29S • 9.6<Ex<10.4MeV • The enhanced peaks at qpp=20MeV/c and θpp=35o According to MC simulations (for three-body democratic decay, no FSI) the branching ratio of 2He emission is 29 % . C.J. Lin et al. Phys. Rev. C 80, 014310 (2009)

45. Excitation-energy spectrum of 29S reconstructed from 27Si+p+p events where Ei and Pi are the total energy and the momentum of each fragment including heavy ions and light protons, Mgr is the ground-state mass of mother nuclei. The configurations (J π) of these levels are still unknown and information is not available in the literature at all. The experimental excitation-energy resolution was estimated as 400 keV.

46. Two-proton correlation for 28P Ex<17MeV No obvious 2He emission! X.X. Xu et al. Phys. Rev. C 81, 054317 (2010) X. X. Xu et al. Phys. Rev. C82, 064316 (2010)