Studies of Heavy Ion Reactions around Coulomb Barrier Part I. Competition between fusion-fission and quasi- fission in 32 S+ 184 W reaction Part II. Sub-barrier fusion enhancement of 32 S+ 90,96 Zr Part III. The study of the surface property of nuclear potential by quasi-elastic scattering Part IV. The breakup threshold anomaly of 9 Be+ 208 Pb, 209 Bi Part V. Two-proton emission from 29 S, 28 P excited states after Coulomb excitation. Huanqiao Zhang China Institute of Atomic Energy (CIAE) 中国原子能科学研究院 China Institute of Atomic Energy Dubna, June 29, 2010

Part I. Competition between fusion-fission and quasi- fission in 32 S+ 184 W 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 32 S+ 184 W -> 216 Th (N=126)

Experimental Setup Beam: 32 S E Lab : 140 ， 145 ， 150 ， 155, 160 ， 165 ， 170 MeV. Typical beam current: enA. Target: about 200ug/cm 2 with 20μg/cm 2 carbon foil backing.

32 S+ 184 W E Lab =170Me V θ Lab = elastic Energy Spectrometry 32 S+ 184 W E Lab =170MeV Angle: DF01 ： Fission production Correlated two fission fragments

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

The measured capture cross sections and the deduced values of A exp and K 2 0 for the 32 S+ 184 W reaction. The total cross section was deduced from the integration of the differential cross sections. Main result

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: 1.presence of a potential pocket; 2.adequacy of the collision energy E c.m to overcome the interaction barrier Characterized by mass (charge) symmetry of its nuclei, rotational energy V rot and excitation energy E * DNS. Dot-dashed line: the capture path Solid line: potential well

The driving potential U dr (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 = Z CN. 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. The potential energy surface for a dinuclear system leading to the formation of 216 Th*

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

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

The presentation of fusion probability P CN The elongated shape leads to quasi-fission The large l contribution leads to quasi-fission

Part II. Sub-barrier fusion enhancement of 32 S+ 90,96 Zr 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 32 S+ 96 Zr system.

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

Experimental setup Electrostatic deflector (Separated by electric-rigidity) suppression ratio >10 8 c.s. down to  b level beam targetSi(Au) electrodes MCP The schematic plot of the electrostatic deflector

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

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

Q gg -value for neutron pickup 32 S+ 90 Zr Separation energies of each neutron for 96 Zr Dotted line: single-channel Dashed line: coupled to inelastic states Solid line: coupled to inelastic states + neutron transfer Dotted line: single-channel Solid line: coupled to inelastic states

Part III. The research of the surface property of nuclear potential by quasi-elastic scattering Aim: 1.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.

Phys. Rev. C (2004) The values of the diffuseness parameter a as a function of Z 1 Z 2 extracted from the fusion excitation functions above the barrier energies. 1.Larger than the commonly accepted value; 2.Increase with the increase of Z 1 Z 2. Phys. Rev. C (2006) The values of the diffuseness parameter a are different for spherical and deformed systems. Deformed systems The open symbols represent the values deduced from fusion cross section.

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

A way to extract the a parameter small deviation due to V N Quasi-elastic (QEL) scattering is sensitive to the surface property of the nuclear potential at deep sub-barrier energy region. Phys. Rev. C (2004)

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

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

Excitation functions of quasi-elastic scatterings at 175 

the parameters of optical potential I. a short range imaginary potential (produces absorption): W= 30 MeV, a w = 0.1 fm, and r w = 0.8 fm II. real potential (produces a deflection) Keep V 0 = 100 MeV fixed Constraint: reproduce the expected average fusion barrier energy using the 3 parameters. using a modified CCFULL code CQUEL by K. Hagino confine the analysis data to dσ qel /d σ Ru >0.94 (expect the coupling effect is negligible in this range) Data analysis: Phys. Rev. C (2005); ( 2007); (2008) Coupling to the low inelastic states of the targets was included; without coupling to the inelastic states of projectile.

PhysRevC79_064603_(2009)

Part IV. The breakup threshold anomaly of 9 Be+ 208 Pb, 209 Bi. 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 ?

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

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

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

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

Quasi-Elastic excitation function and barrier distribution for 9 Be+ 208 Pb H. M. Jia, et.al., Submitted to PRC

Part V. 17 F+ 12 C elastic scattering.

Optical model parameters for 16 O elastic scattering on 12 C. Averaged 16 O- 12 C optical potential and its comparison with experimental data.

Optical model calculation and comparison with experimental data of proton elastic scattering from 12 C at 3.53 MeV. Contributions of each j π states to the CDCC breakup cross sections.

Theoretical angular distributions of differential cross sections of elastic, inelastic and breakup for 60 MeV 17 F+ 12 C.

Experimental and CDCC calculated angular distribution of 60 MeV 17 F+ 12 C

Part VI. Two-proton emission from 29 S, 28 P excited states after Coulomb excitation. Two-proton radioactivity: 1)Two-body sequential emission; 2) Three-body simultaneously democratic emission; 3) 2 He cluster emission and following breakup. Decay Dynamics of two-proton emission from excited states Invariant Mass, q pp =|p 1 -p 2 |/2,the relative momentum,  pp cm,the opening angle, the relative energy, ExperimentTheory Three body models, The extended R-matrix theory, The Faddeev equations,

Complete-kinematics measurements Secondary target: 197 Au,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 interval for the construction of the particle trajectories CsI(Tl) detectors: 6×6 lattices,each 15×15×20mm, read out through PIN photodiodes

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

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

Excitation-energy spectrum of 29 S reconstructed from 27 Si+p+p events where E i and P i are the total energy and the momentum of each fragment including heavy ions and light protons, M gr 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.

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

summary: 1.Both the elongated shape with small orientation angle at low energies and the large angular momentum partial waves at high energies lead to the quasi-fission (fusion hindrance). The contributions of fusion-fission and quasi-fission fragments are comparable. 2.The positive Q-value multi-neutron transfer really enhances the fusion cross section besides the couplings to the low inelastic states at sub- barrier energy region for 32 S+ 96 Zr system. But the transfer mechanism remains unknown.

3. For near-spherical systems, both single-channel and coupled-channels calculations give almost the same diffuseness parameters. The coupling effect is negligible. But For well-deformed systems, coupling effect is important. Coupled-channels calculations give smaller diffuseness parameters than the single-channel calculations and a better fitting to the experimental data. 4.Resolving the different threshold anomaly behaviors between 9 Be+ 208 Pb and 9 Be+ 209 Bi. It shows the breakup threshold anomaly. 5. It is found that 2 He emission exists in the excited state of 29 S but sequential proton emission in 28 P. Challenge: comprehensive description of the dynamical processes in the reactions!

Thank you !