Suppression of Fusion in Heavy Ion Collisions

12th Sep. 2016 Dongyun Jeung

Department of Nuclear Physics, RSPE The Australian National University

2

Outline

§  Background - Fusion & Fusion Suppression - Competing processes

§  Research Project - Experimental observations

- Data analysis & Results

§  Conclusion

Fusion

3

HOT Compound Nucleus

Projectile Target Evaporation Residue

Neutron evaporation

dinuclear

(Statistical Model)

Capture

Fission

4

HOT Compound Nucleus

Projectile Target

Neutron evaporation

dinuclear

(Statistical Model)

Fission fragment

Capture

Energy dissipation prior to capture

5

Nucleons exchange

Fissile Target

CN

Target-like nucleus

> 6 MeV

Transfer-induced fission

Capture

Kinetic energy à Ex (“heating”)

Signature of E-dissipation occurred

Identifying events following E-dissipation

6

Nucleons exchange

Fissile Target

CN

ç 3-body system

é 2-body system

Target-like nucleus

Capture

Fusion Hindrance

7

C. R. Morton, et al., Phys. Rev. C 60, 044608 (1999)

Fusion Suppression at E > VB

0.000001

0.00001

0.0001

0.001

0.01

0.1

1

10

-12 -10 -8 -6 -4 -2 0

101

100

10-1

10-2

10-3

10-4

10-5

10-6 0

500

1000

1500

-10 0 10 20 30 40

a = 1.18 fm

a = 1.65 fm

a = 0.66 fm

16O + 208Pb 16O + 208Pb

a = 1.18 fm

a = 1.65 fm

a = 0.66 fm

EC.M. - B (MeV)EC.M. - B (MeV)

σ fus

(mb)

σ fus

(mb)

(a) (b)

16O + 204Pb

Figure 3. The measured fusion cross-sections as a function of the centre of mass energywith respect to the barrier energy B at deep sub-barrier energies (a) and all energies(b). The calculations using three different values of the diffuseness of the Woods-Saxonnuclear potential are shown. The deep sub-barrier data and above barrier data are notconsistently described using the same value of the parameter.

argued above, both deep sub-barrier and above barrier data probe similar separationdistances, then they are both sensitive to regions inside the average barrier radius wheredissipative effect may be significant. Including such effects in the theoretical formalismmay help in obtaining a simultaneous description of the above barrier and below barrierdata. A related question that must be addressed is whether it is correct to treat thetunnelling of finite sized nuclei on the same footing as that of point particles. This andother aspects, such as multi-nucleon transfer [22], may need to be investigated in orderto obtain a consistent explanation of the fusion process over a range of energies.

5. Fusion with light weakly-bound nuclei

Fusion with weakly-bound light nuclei, both stable and radioactive, is of interest asattested by the large number of contributions to this conference. The challenge, again,is to be able to relate the breakup, transfer, complete- and incomplete-fusion processes,and obtain a consistent description of all these processes in a single framework. The dataare usually described in the framework of the Continuum Discretized Coupled Channels(CDCC) framework [23]. This model is able to predict breakup cross-sections if noneof the fragments are captured by the target, but a major drawback is that it cannotdistinguish complete fusion from fusion of one of the breakup fragments (incompletefusion). This is a major failing which cannot be rectified; and one which critically affectsinterpretation of experimental data. The distinction between complete and incomplete-fusion is possible if the trajectories of the breakup fragments are followed. A 3-dimensionalclassical trajectory model, which can relate breakup well below the barrier to complete

M. Dasgupta et al. / Nuclear Physics A 787 (2007) 144c–149c148c

CC approach

EC.M –VB (MeV)

Fusion Hindrance

8

Fusion Suppression at E > VB

0.000001

0.00001

0.0001

0.001

0.01

0.1

1

10

-12 -10 -8 -6 -4 -2 0

101

100

10-1

10-2

10-3

10-4

10-5

10-6 0

500

1000

1500

-10 0 10 20 30 40

a = 1.18 fm

a = 1.65 fm

a = 0.66 fm

16O + 208Pb 16O + 208Pb

a = 1.18 fm

a = 1.65 fm

a = 0.66 fm

EC.M. - B (MeV)EC.M. - B (MeV)

σ fus

(mb)

σ fus

(mb)

(a) (b)

16O + 204Pb

Figure 3. The measured fusion cross-sections as a function of the centre of mass energywith respect to the barrier energy B at deep sub-barrier energies (a) and all energies(b). The calculations using three different values of the diffuseness of the Woods-Saxonnuclear potential are shown. The deep sub-barrier data and above barrier data are notconsistently described using the same value of the parameter.

argued above, both deep sub-barrier and above barrier data probe similar separationdistances, then they are both sensitive to regions inside the average barrier radius wheredissipative effect may be significant. Including such effects in the theoretical formalismmay help in obtaining a simultaneous description of the above barrier and below barrierdata. A related question that must be addressed is whether it is correct to treat thetunnelling of finite sized nuclei on the same footing as that of point particles. This andother aspects, such as multi-nucleon transfer [22], may need to be investigated in orderto obtain a consistent explanation of the fusion process over a range of energies.

5. Fusion with light weakly-bound nuclei

Fusion with weakly-bound light nuclei, both stable and radioactive, is of interest asattested by the large number of contributions to this conference. The challenge, again,is to be able to relate the breakup, transfer, complete- and incomplete-fusion processes,and obtain a consistent description of all these processes in a single framework. The dataare usually described in the framework of the Continuum Discretized Coupled Channels(CDCC) framework [23]. This model is able to predict breakup cross-sections if noneof the fragments are captured by the target, but a major drawback is that it cannotdistinguish complete fusion from fusion of one of the breakup fragments (incompletefusion). This is a major failing which cannot be rectified; and one which critically affectsinterpretation of experimental data. The distinction between complete and incomplete-fusion is possible if the trajectories of the breakup fragments are followed. A 3-dimensionalclassical trajectory model, which can relate breakup well below the barrier to complete

M. Dasgupta et al. / Nuclear Physics A 787 (2007) 144c–149c148c

CC approach

Fusi

on s

uppr

essi

on

com

pare

d to

m

odel

exp

ecta

tion

Increasing reduction in fusion with Z1Z2

J. O. Newton, et. al., Phys. Rev. C 70, 024605 (2004)

Z1Z2 = 656

656

EC.M –VB (MeV)

Fusion Hindrance

9

Fusion Suppression at E > VB

0.000001

0.00001

0.0001

0.001

0.01

0.1

1

10

-12 -10 -8 -6 -4 -2 0

101

100

10-1

10-2

10-3

10-4

10-5

10-6 0

500

1000

1500

-10 0 10 20 30 40

a = 1.18 fm

a = 1.65 fm

a = 0.66 fm

16O + 208Pb 16O + 208Pb

a = 1.18 fm

a = 1.65 fm

a = 0.66 fm

EC.M. - B (MeV)EC.M. - B (MeV)

σ fus

(mb)

σ fus

(mb)

(a) (b)

16O + 204Pb

Figure 3. The measured fusion cross-sections as a function of the centre of mass energywith respect to the barrier energy B at deep sub-barrier energies (a) and all energies(b). The calculations using three different values of the diffuseness of the Woods-Saxonnuclear potential are shown. The deep sub-barrier data and above barrier data are notconsistently described using the same value of the parameter.

argued above, both deep sub-barrier and above barrier data probe similar separationdistances, then they are both sensitive to regions inside the average barrier radius wheredissipative effect may be significant. Including such effects in the theoretical formalismmay help in obtaining a simultaneous description of the above barrier and below barrierdata. A related question that must be addressed is whether it is correct to treat thetunnelling of finite sized nuclei on the same footing as that of point particles. This andother aspects, such as multi-nucleon transfer [22], may need to be investigated in orderto obtain a consistent explanation of the fusion process over a range of energies.

5. Fusion with light weakly-bound nuclei

Fusion with weakly-bound light nuclei, both stable and radioactive, is of interest asattested by the large number of contributions to this conference. The challenge, again,is to be able to relate the breakup, transfer, complete- and incomplete-fusion processes,and obtain a consistent description of all these processes in a single framework. The dataare usually described in the framework of the Continuum Discretized Coupled Channels(CDCC) framework [23]. This model is able to predict breakup cross-sections if noneof the fragments are captured by the target, but a major drawback is that it cannotdistinguish complete fusion from fusion of one of the breakup fragments (incompletefusion). This is a major failing which cannot be rectified; and one which critically affectsinterpretation of experimental data. The distinction between complete and incomplete-fusion is possible if the trajectories of the breakup fragments are followed. A 3-dimensionalclassical trajectory model, which can relate breakup well below the barrier to complete

M. Dasgupta et al. / Nuclear Physics A 787 (2007) 144c–149c148c

CC approach

Fusi

on s

uppr

essi

on

com

pare

d to

m

odel

exp

ecta

tion

Increasing reduction in fusion with Z1Z2

J. O. Newton, et. al., Phys. Rev. C 70, 024605 (2004)

Z1Z2 = 656

656

EC.M –VB (MeV)

Density Overlap at the Barrier Radius

10

Dasgupta M, et al., Annu. Rev. Part. Sci. 48: 401-61 (1998)

Z1Z2 64 400 2500

The density overlap increases at the barrier

ρ (1

044 m

-3)

Research Project

11

•  Investigate an influence of the density overlap of the colliding nuclei to energy dissipative processes and explore the role of energy dissipation.

Projectile Target Z1Z2 Density overlap E / VB 18O

232Th

720 0.93 – 1.14 30Si 1260 0.94 – 1.08 34S 1440 0.87 – 1.07

40Ca 1800 0.93 – 1.14 54Cr 2160 0.93 – 1.02 64Ni 2520 1.05 – 1.13

Increasing density overlap at the barrier

VB = Fusion barrier W.J. Swiateck, et. al., Phys. Rev. C 71, 04602 (2005)

MAPPING QUASIFISSION CHARACTERISTICS AND . . . PHYSICAL REVIEW C 88, 054618 (2013)

x

y

MWPC 1

x

z

MW

PC 2

θ

d

Target Beam

MW

PC 1

(a)

MW

PC 2d

φ

(b)

FIG. 1. (Color online) Schematic view of the experimental setupof the CUBE detectors. Panel (a) shows the azimuthal positioningwhere the beam is going into the page and (b) illustrates theconfiguration as seen from above. The placement of the detectorsis defined by the distance d of their centers to the target, togetherwith their azimuthal angle (φ), and polar angle (θ ) relative to thebeam axis.

the atomic number of the compound nucleus ZCN, whoseidentities are given in black next to the left-hand axis. Sincethe bombarding energies are generally less than 10% above thecapture barrier, where pre-equilibrium emission is expected tobe negligible, it is safe to assign ZCN as simply the sum of theprojectile and target atomic numbers Zp + Zt . The horizontaldotted black lines join reactions forming compound nucleiof the same element, which is identified in black next to theaxis on the left. In many cases, reactions forming the sameelement also formed the same isotope (see Table I).

The locus of reactions with target nuclei having the sameatomic number Zt are diagonals in the graph, indicated bythe blue dotted lines. The target elements are identified inblue next to the right-hand axis. At the intersection betweenthe three dotted lines, the MAD plot for the correspondingreaction, whose details are given in Table I, is presented.

For clarity, the axis labels have been removed from thesesmaller MAD plots within the figure; they all correspond informat to the larger MAD example shown in the top left ofthe figure.

IV. FEATURES OF EXPERIMENTALMASS-ANGLE DISTRIBUTIONS

In this section, important aspects of the experimentalMADs presented here will be discussed. First, the effects thatinstrumental properties have on MADs will be reviewed. Next,the reaction mechanisms that contribute to the MAD will beidentified, and the physical mechanisms responsible for theirdiffering contributions to each MAD will be described. Finally,three categories of MADs will be defined, in order to simplifythe subsequent discussion of the systematic trends observed inthe mass-angle distribution characteristics presented in Fig. 2.

A. Instrumental features

A key feature of the two MWPC detectors used in this studyis their wide angular acceptance, allowing a large part of thefull angular range of the MAD (0◦ to 180◦) to be captured ina single measurement. The scattering angle coverage of theexperimental MAD is determined by the size and placementof the two MWPC detectors, each of which covered 75◦ inscattering angle. Figure 3 shows the measured MAD for thereaction Ebeam = 111 MeV 16O + 196Pt. The gray shaded areaindicates the region that was not visible due to the binaryfragment acceptance of the detectors. As described in detail inRef. [29], the binary events are simultaneously mirrored aboutMR = 0.5 and θc.m. = 90◦. The two white points indicate howa single binary event populates the MAD at the complementarymass ratios and center-of-mass scattering angles.

All MADs presented have a constant acceptance in φ,independent of θ , arising either from the placement of thebackward angle detector or from gating applied in the analysis.Thus the intensity scale in all the MADs is proportional todσ/dθc.m., rather than dσ/d$c.m.. A distribution of dσ/d$c.m.

proportional to 1/ sin θc.m. would thus appear uniformlydistributed in θc.m. within the MADs.

Inside the angular range covered by the measurements, themajor features seen in the MAD plots in Fig. 2 result from anumber of different processes. These are elastic and inelasticscattering and transfer reactions (collectively referred to asquasielastic events), deep inelastic reactions, quasifission, andfusion-fission. Ideally all the patterns observed in a given MADshould relate only to the probabilities of these conceptuallydistinct [35] reaction outcomes. However, there are twoinstrumental effects that can distort this idealized picture. Itis important to note these do not affect the fusion-fission orquasifission components of the MAD.

The first instrumental effect comes through the deliberaterejection of elastically scattered particles where possible.For the reactions with the lighter projectiles 12C and 16,18O,essentially the only events visible in the MAD come fromfission. For reactions involving these projectiles only, thedetector electronic thresholds could be set to exclude the smallsignals from beam-like particles without excluding any fissionevents, helped by the small center-of-mass velocity of thefissioning system. This is why no such events are seen inFig. 3. For all other projectiles, beam-like particles were notrejected, in order that any fission-like events with low pulseheight would not also be rejected, thus avoiding distortion

054618-5

MAPPING QUASIFISSION CHARACTERISTICS AND . . . PHYSICAL REVIEW C 88, 054618 (2013)

x

y

MWPC 1

x

z

MW

PC 2

θ

d

Target Beam

MW

PC 1

(a)

MW

PC 2d

φ

(b)

FIG. 1. (Color online) Schematic view of the experimental setupof the CUBE detectors. Panel (a) shows the azimuthal positioningwhere the beam is going into the page and (b) illustrates theconfiguration as seen from above. The placement of the detectorsis defined by the distance d of their centers to the target, togetherwith their azimuthal angle (φ), and polar angle (θ ) relative to thebeam axis.

the atomic number of the compound nucleus ZCN, whoseidentities are given in black next to the left-hand axis. Sincethe bombarding energies are generally less than 10% above thecapture barrier, where pre-equilibrium emission is expected tobe negligible, it is safe to assign ZCN as simply the sum of theprojectile and target atomic numbers Zp + Zt . The horizontaldotted black lines join reactions forming compound nucleiof the same element, which is identified in black next to theaxis on the left. In many cases, reactions forming the sameelement also formed the same isotope (see Table I).

The locus of reactions with target nuclei having the sameatomic number Zt are diagonals in the graph, indicated bythe blue dotted lines. The target elements are identified inblue next to the right-hand axis. At the intersection betweenthe three dotted lines, the MAD plot for the correspondingreaction, whose details are given in Table I, is presented.

For clarity, the axis labels have been removed from thesesmaller MAD plots within the figure; they all correspond informat to the larger MAD example shown in the top left ofthe figure.

IV. FEATURES OF EXPERIMENTALMASS-ANGLE DISTRIBUTIONS

In this section, important aspects of the experimentalMADs presented here will be discussed. First, the effects thatinstrumental properties have on MADs will be reviewed. Next,the reaction mechanisms that contribute to the MAD will beidentified, and the physical mechanisms responsible for theirdiffering contributions to each MAD will be described. Finally,three categories of MADs will be defined, in order to simplifythe subsequent discussion of the systematic trends observed inthe mass-angle distribution characteristics presented in Fig. 2.

A. Instrumental features

A key feature of the two MWPC detectors used in this studyis their wide angular acceptance, allowing a large part of thefull angular range of the MAD (0◦ to 180◦) to be captured ina single measurement. The scattering angle coverage of theexperimental MAD is determined by the size and placementof the two MWPC detectors, each of which covered 75◦ inscattering angle. Figure 3 shows the measured MAD for thereaction Ebeam = 111 MeV 16O + 196Pt. The gray shaded areaindicates the region that was not visible due to the binaryfragment acceptance of the detectors. As described in detail inRef. [29], the binary events are simultaneously mirrored aboutMR = 0.5 and θc.m. = 90◦. The two white points indicate howa single binary event populates the MAD at the complementarymass ratios and center-of-mass scattering angles.

All MADs presented have a constant acceptance in φ,independent of θ , arising either from the placement of thebackward angle detector or from gating applied in the analysis.Thus the intensity scale in all the MADs is proportional todσ/dθc.m., rather than dσ/d$c.m.. A distribution of dσ/d$c.m.

proportional to 1/ sin θc.m. would thus appear uniformlydistributed in θc.m. within the MADs.

Inside the angular range covered by the measurements, themajor features seen in the MAD plots in Fig. 2 result from anumber of different processes. These are elastic and inelasticscattering and transfer reactions (collectively referred to asquasielastic events), deep inelastic reactions, quasifission, andfusion-fission. Ideally all the patterns observed in a given MADshould relate only to the probabilities of these conceptuallydistinct [35] reaction outcomes. However, there are twoinstrumental effects that can distort this idealized picture. Itis important to note these do not affect the fusion-fission orquasifission components of the MAD.

The first instrumental effect comes through the deliberaterejection of elastically scattered particles where possible.For the reactions with the lighter projectiles 12C and 16,18O,essentially the only events visible in the MAD come fromfission. For reactions involving these projectiles only, thedetector electronic thresholds could be set to exclude the smallsignals from beam-like particles without excluding any fissionevents, helped by the small center-of-mass velocity of thefissioning system. This is why no such events are seen inFig. 3. For all other projectiles, beam-like particles were notrejected, in order that any fission-like events with low pulseheight would not also be rejected, thus avoiding distortion

054618-5

12

Back

Front Bac

k

5°

80°

55° 130°

225°

135°

CUBE Detector

-80° (280°)

80°

Azimuthal Angle (ϕ) Polar Angle (θ)

MWPC: 357x270 mm

Lab frame

Beam

Target

Binary Kinematic Reconstruction

13

x

yz

Beam axis

•  V|| = VCN •  Coplanar with the beam axis

CN

VCN

LAB Frame

3-body Kinematic

14

x

y

z

Beam axis

•  V|| > VCN or V|| < VCN •  Vperp

vperp

LAB Frame

Signature of 3-body and 2-body systems

15

3-body vperp

Beam-like nucleus

Beam 2-body

34S + 232Th at E/VB = 1.02

Vpe

rp [m

m/n

s]

Vpar – Vcn [mm/ns] x

y

16

Velocity Vector Plots

103

102 10 1

1

Vpar - Vcn [cm/ns]

-0.48 -0.16 0.16 0.48

0.4

0

-0.4

Vpe

rp [c

m/n

s]

16O + 232Th 30Si + 232Th 34S + 232Th

-0.48 -0.16 0.16 0.48 -0.48 -0.16 0.16 0.48

2

-0.8

0.8

~ 2% above VB

Example: 34S + 232Th

34S + 232Th

103

102 10 1

Vpar - Vcn [cm/ns] -0.48 -0.16 0.16 0.48 -0.48 -0.16 0.16 0.48 -0.48 -0.16 0.16 0.48 -0.48 -0.16 0.16 0.48 -0.48 -0.16 0.16 0.48 -0.48 -0.16 0.16 0.48

0.4

0

-0.4 Vpe

rp [c

m/n

s]

E/VB = 0.927 Ecm = 143.6 MeV

E/VB = 0.941 Ecm = 145.7 MeV

E/VB = 0.955 Ecm = 147.9 MeV

E/VB = 0.972 Ecm = 150.5 MeV

E/VB = 1.023 Ecm = 158.3 MeV

E/VB = 1.077 Ecm = 166.7 MeV

17

Results

18

Total fission = 3-body + 2-body

2-B

ody/

Tota

l Fis

sion

E/VB

Results: E>VB

19

Total fission = 3-body + 2-body

2-B

ody/

Tota

l Fis

sion

E/VB

Fusion Suppression at E>VB

20

Fusi

on S

uppr

essi

on

E/VB

2-B

ody/

Tota

l Fis

sion

Fusion Suppression at E>VB

21

Z1Z2

Fusi

on S

uppr

essi

on

E/VB

2-B

ody/

Tota

l Fis

sion

σ mea

usre

d ca

ptur

e/σca

lcul

ated

720 810 1260 1440 1800

Fusion Suppression at E>VB

22

Z1Z2

Fusi

on S

uppr

essi

on

E/VB

2-B

ody/

Tota

l Fis

sion

σ mea

usre

d ca

ptur

e/σca

lcul

ated

720 810 1260 1440 1800

Light system reactions

23

Nucleons exchange

Fissile Target

CN

Target-like nucleus

> 6 MeV

Fusion-fission

Transfer-induced fission

Capture

Quasi-Fission

24

Fissile Target

CN

Quasi-fission

Capture

Mass Angle Distribution

MAPPING QUASIFISSION CHARACTERISTICS AND . . . PHYSICAL REVIEW C 88, 054618 (2013)

180

135

90

45

0

R

0.2 0.4 0.6 0.8 1.00.01

10

10

10

10

10

2

3

5

4

M

θ c.m

.[de

g]

FIG. 3. (Color online) Measured mass-angle distribution for the16O + 196Pt reaction, expected to lead to fusion-fission. The whitesquares illustrate how the MAD is populated at two points for eachbinary event measured. For fusion-fission, the distribution is expectedto be symmetric in angle about the blue line, and symmetric in massabout the yellow line. Elastic events are not seen for the 16O beam asthey were below the MWPC timing thresholds (see text).

of the fission-like components of the MAD. The softwaregates applied in generating all the MAD are those shown inFigs. 11(b) and 12.

The second instrumental effect involves target nucleusrecoils following elastic scattering, and is relevant to thosereactions involving beams heavier than 12C and 16,18O. Kine-matic considerations show that as the elastic scattering angle ofbeam particles approaches 0◦, the corresponding target recoilsare ejected from the target with laboratory angles approaching90◦ and energies approaching zero. Clearly there is an energythreshold below which the recoils will not penetrate thegas window and cathode foil. Thus, they will not appear incoincidence with the corresponding elastic scattering event,and will consequently be absent from the MAD. This is whythe elastic-recoil coincidences corresponding to forward angleelastically scattered beam particles are absent or suppressed inthe MAD, principally for the Mg and Si beams, for which theenergies of the recoils at a given angle are low. The distributionwith angle of fission-like events in the MAD spectra show thatthis instrumental low-energy threshold has essentially no effecton the mass-angle distribution of the fission and quasifissionevents, since unlike target recoils, their laboratory energies donot approach zero.

B. Reaction outcome features

Having covered the instrumental influences on the experi-mental MADs, the physical mechanisms responsible for theappearance of each MAD can be explored. An idealizeddiagram of the features seen in an experimental MAD ispresented in Fig. 4. It shows the location in mass and angle ofthe different reaction outcomes, and also illustrates the originof the correlation between the mass and angle seen in thequasifission events. Each of these features is discussed below.

1. Quasielastic and deep inelastic collisions

The two green shaded regions marked QE in Fig. 4 aremainly populated through elastic and inelastic scattering of the

0.2 0.4 0.6 0.8 0.10.0

45

90

135

0

180

c.m

. θ

[deg

]

MR

I

II

III

EQEQ

QF

QF

FF

FIG. 4. (Color online) Schematic illustration of the MAD show-ing the regions corresponding to different reaction processes.Quasielastic and deep inelastic scattering are denoted by QE, fusion-fission by FF, and quasifission by QF. The curved red and blue linescorrespond to average quasifission trajectories for a single angularmomentum (see text).

projectile and target nuclei, with some contribution from trans-fer and deep inelastic reactions. By deep-inelastic reactions,we mean that class of events where (i) energy is increasinglydissipated into heating the colliding nuclei until completedamping of relative motion occurs, (ii) mass exchange canoccur, but without significant mass drift away from theentrance channel mass-asymmetry [36,37], and (iii) deviationsfrom Coulomb trajectories occur due to nuclear orbiting. Theangular dependence of the elastic and quasielastic scatteringyield essentially follows Rutherford scattering, except atthe backward angles, where scattering events are depleted,being transformed into fission-like events as a result ofcapture, sticking, and mass flow towards symmetry. For elasticscattering, the expected mass ratio MR should correspond tothe value expected from the entrance channel projectile andtarget masses mp and mt ; i.e., MR = mp/(mp + mt ) if theprojectile is detected in MWPC1, or MR = mt/(mp + mt ) ifthe target nucleus reaches MWPC1.

2. Fusion-fission and quasifission

The processes of interest in this work are quasifission andfusion-fission. Both result in fission fragments consistent witha two-body full-momentum transfer (FMT) fission event [9].Most kinematic properties of the fission fragments resultingfrom these two processes will be similar, if not identical. How-ever, heavy ion fusion-fission generally shows the peak yield atmass symmetry (MR = 0.5), and a standard deviation in MR of∼0.05 to 0.08. Low-energy mass-asymmetric actinide fissiontypically has a peak mass yield around A = 139; thus the massratio is typically not more asymmetric than MR = 0.60/0.40.The gray shaded fusion-fission band (marked FF in Fig. 4) thusis drawn as extending from MR = 0.4 to 0.6. Although bothfusion-fission and quasifission can contribute to this shadedregion, quasifission alone gives significant population to the

054618-7

25

Sticking time (sec)

~ 5×10-21 ~10-19

Mass Angle Distribution

MAPPING QUASIFISSION CHARACTERISTICS AND . . . PHYSICAL REVIEW C 88, 054618 (2013)

180

135

90

45

0

R

0.2 0.4 0.6 0.8 1.00.01

10

10

10

10

10

2

3

5

4

M

θ c.m

.[de

g]

FIG. 3. (Color online) Measured mass-angle distribution for the16O + 196Pt reaction, expected to lead to fusion-fission. The whitesquares illustrate how the MAD is populated at two points for eachbinary event measured. For fusion-fission, the distribution is expectedto be symmetric in angle about the blue line, and symmetric in massabout the yellow line. Elastic events are not seen for the 16O beam asthey were below the MWPC timing thresholds (see text).

of the fission-like components of the MAD. The softwaregates applied in generating all the MAD are those shown inFigs. 11(b) and 12.

The second instrumental effect involves target nucleusrecoils following elastic scattering, and is relevant to thosereactions involving beams heavier than 12C and 16,18O. Kine-matic considerations show that as the elastic scattering angle ofbeam particles approaches 0◦, the corresponding target recoilsare ejected from the target with laboratory angles approaching90◦ and energies approaching zero. Clearly there is an energythreshold below which the recoils will not penetrate thegas window and cathode foil. Thus, they will not appear incoincidence with the corresponding elastic scattering event,and will consequently be absent from the MAD. This is whythe elastic-recoil coincidences corresponding to forward angleelastically scattered beam particles are absent or suppressed inthe MAD, principally for the Mg and Si beams, for which theenergies of the recoils at a given angle are low. The distributionwith angle of fission-like events in the MAD spectra show thatthis instrumental low-energy threshold has essentially no effecton the mass-angle distribution of the fission and quasifissionevents, since unlike target recoils, their laboratory energies donot approach zero.

B. Reaction outcome features

Having covered the instrumental influences on the experi-mental MADs, the physical mechanisms responsible for theappearance of each MAD can be explored. An idealizeddiagram of the features seen in an experimental MAD ispresented in Fig. 4. It shows the location in mass and angle ofthe different reaction outcomes, and also illustrates the originof the correlation between the mass and angle seen in thequasifission events. Each of these features is discussed below.

1. Quasielastic and deep inelastic collisions

The two green shaded regions marked QE in Fig. 4 aremainly populated through elastic and inelastic scattering of the

0.2 0.4 0.6 0.8 0.10.0

45

90

135

0

180

c.m

. θ

[deg

]

MR

I

II

III

EQEQ

QF

QF

FF

FIG. 4. (Color online) Schematic illustration of the MAD show-ing the regions corresponding to different reaction processes.Quasielastic and deep inelastic scattering are denoted by QE, fusion-fission by FF, and quasifission by QF. The curved red and blue linescorrespond to average quasifission trajectories for a single angularmomentum (see text).

projectile and target nuclei, with some contribution from trans-fer and deep inelastic reactions. By deep-inelastic reactions,we mean that class of events where (i) energy is increasinglydissipated into heating the colliding nuclei until completedamping of relative motion occurs, (ii) mass exchange canoccur, but without significant mass drift away from theentrance channel mass-asymmetry [36,37], and (iii) deviationsfrom Coulomb trajectories occur due to nuclear orbiting. Theangular dependence of the elastic and quasielastic scatteringyield essentially follows Rutherford scattering, except atthe backward angles, where scattering events are depleted,being transformed into fission-like events as a result ofcapture, sticking, and mass flow towards symmetry. For elasticscattering, the expected mass ratio MR should correspond tothe value expected from the entrance channel projectile andtarget masses mp and mt ; i.e., MR = mp/(mp + mt ) if theprojectile is detected in MWPC1, or MR = mt/(mp + mt ) ifthe target nucleus reaches MWPC1.

2. Fusion-fission and quasifission

The processes of interest in this work are quasifission andfusion-fission. Both result in fission fragments consistent witha two-body full-momentum transfer (FMT) fission event [9].Most kinematic properties of the fission fragments resultingfrom these two processes will be similar, if not identical. How-ever, heavy ion fusion-fission generally shows the peak yield atmass symmetry (MR = 0.5), and a standard deviation in MR of∼0.05 to 0.08. Low-energy mass-asymmetric actinide fissiontypically has a peak mass yield around A = 139; thus the massratio is typically not more asymmetric than MR = 0.60/0.40.The gray shaded fusion-fission band (marked FF in Fig. 4) thusis drawn as extending from MR = 0.4 to 0.6. Although bothfusion-fission and quasifission can contribute to this shadedregion, quasifission alone gives significant population to the

054618-7

26

180

MR

Θcm

[deg

]

40Ca + 232Th at E/VB=1.016

135

90

45

0 0.2 0.4 0.6 0.8 1

10

102

103

104

27

Mass Angle Distribution plots

0.2 0.4 0.6 0.8 0.2 0.4 0.6 0.8 0.2 0.4 0.6 0.8 0.2 0.4 0.6 0.8

135

90

45

Θc.

m [d

eg]

104 103 102 10 1

18O + 232Th E/VB = 1.024

30Si + 232Th E/VB = 1.027

34S + 232Th E/VB = 1.023

40Ca + 232Th E/VB = 1.016

MR 0

28

Effects of QF

0.2 0.4 0.6 0.8 0.2 0.4 0.6 0.8 0.2 0.4 0.6 0.8 0.2 0.4 0.6 0.8

135

90

45

Θc.

m [d

eg]

104 103 102 10 1

18O + 232Th E/VB = 1.024

30Si + 232Th E/VB = 1.027

34S + 232Th E/VB = 1.023

40Ca + 232Th E/VB = 1.016

MR 0

Increasing in Z1Z2

σcapture ≠ σfusion

σ3-body system ≠ σtrasnfer

Conclusions §  Study E-dissipation before capture

à Transfer followed by fission increases with Z1Z2

à Correlates with increasing fusion suppression with Z1Z2 Promising signature that E-dissipation before capture suppresses fusion

§  Quantitative results for projectiles heavier than 34S

à A significant contribution of Quasi fission

29

30

A possible problem – sequential fission

31

Nucleons exchange

Primary Fission

Sequential Fission

CN

Heavy target-like

nucleus