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)

中国原子能科学研究院China Institute of Atomic Energy

weihai, 2011. 8. 8-11

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)

T DNS P

ER

CN

QF

FF

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.

32S+184WELab=170MeVθLab=54.850 elastic

Energy Spectrometry

32S+184WELab=170MeVAngle: 76.90---83.980

DF01 ： 54.850

Fission production

Correlated two fission fragments

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

2/1

a

ET x

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.

Main result

Comparison with theory calculation (DNS)

Dinuclear system is formed at the initialstage of the reaction, kinetic energy istransferred into potential and excitation.

Necessary conditions:1. presence of a potential pocket;2. adequacy of the collision energy Ec.m to

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

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.

The potential energy surface for a dinuclear system leading to the formation of 216Th*

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 PCN

The elongated shape leads to quasi-fission

The large l contribution leads to quasi-fission

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.

Reduced fusion excitation functions of 36S,40,48Ca+90,96Zr systems

Fusion evaporation residua measurement:

Experimental setup

Electrostatic deflector (Separated by electric-rigidity)

suppression ratio >108 c.s. down to b level

beam target Si(Au)electrodes

MCP

The schematic plot of the electrostatic deflector

Eva. residua

Contaminator

Target recoils

Scattering 32S

Recoil 12C

The ΔE-TOF spectrum of the reaction products after separation.

The experimental fusion excitation functions of 32S+90,96Zr systems

70 75 80 85 90 95

10-1

100

101

102

103

fus (

mb)

Ec.m.

(MeV)

32S+90Zr

32S+96Zr

Comparison with Zagrebaev’s theory:

Phys. Rev. C 67 061601(R) (2003)

assume a successive transfer mechanism of single neutrons

(a direct nucleon pair transfer?)

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

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

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 70 024605(2004)

The values of the diffuseness parameter a as a function of Z1Z2 extracted from the fusion excitation

functions above the barrier energies.

1. Larger than the commonly accepted value;

2. Increase with the increase of Z1Z2.

Phys. Rev. C 73 034607 (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 76 024612 (2007)

A way to extract the a parametersmall deviation due to VN

Quasi-elastic (QEL) scattering is sensitive to the surface property of the nuclear potential at deep sub-barrier energy region.

Phys. Rev. C 69 054610 (2004)

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°

As also reported in PRC78, 034614 (08)

Energy spectrum of the projectile-like particles at θlab=175°

More complicated than transfer mechanism.

More exit channels populated than what is included in the CC calculations.

reaction mechanism?

low inelastic statesProton 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 potentialI. 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.

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)

Data analysis:

Phys. Rev. C 71 044612 (2005); 76 024612( 2007); 78 034614 (2008)

Coupling to the low inelastic states of the targets was included;

without coupling to the inelastic states of projectile.

Lin et al., Phys. RevC79_064603_(2009)

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 ?

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, 061603, (2001).

Woolliscroft, et al., 9Be+208Pb. threshold anomaly, Phys. Rev. C 69, 044612, (2004).

Elastic scattering angular distributions for the 9Be+208Pb,209Bi 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) 075108

25 30 35 40

10-2

10-1

100

9Be+208Pb

dqe

l/d R

u

Ec.m.

(MeV)

Quasi-Elastic excitation function and barrier distribution for 9Be+208Pb

H. M. Jia, et.al., Phys. Rev. C82, 027602(2010)

17F+12C弹性散射

Part V. Two-proton emission from 29S,28P excited states after Coulomb excitation.

Two-proton radioactivity:

1) 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

Invariant Mass,qpp=|p1-p2|/2,the relative momentum,

ppcm ,the opening angle, the relative energy,

.......

Experiment Theory

Three body models,The extended R-matrix theory,

The Faddeev equations,.......

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 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<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)

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 % .

1011

C.J. Lin et al. Phys. Rev. C 80, 014310 (2009)

Excitation-energy spectrum of 29S reconstructed from 27Si+p+p events

grii McPEE 22* )()(

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.

Two-proton correlation for 28P Ex<17MeV

X.X. Xu et al. Phys. Rev. C 81, 054317 (2010)

X. X. Xu et al. Phys. Rev. C82, 064316 (2010)

No obvious 2He 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 32S+96Zr 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 9Be+208Pb and 9Be+209Bi. It shows the breakup threshold anomaly.

5. It is found that 2He emission exists in the excited state of 29S but sequential proton emission in 28P and sequential alpha emission in 18Ne .

Challenge:

comprehensive description of the dynamical processes in the reactions!

Thank you !