Mapping timescales of quasifission Dr. Elizabeth Williams, Australian National University Humboldt Kolleg, ECT*, Trento, Italia, 1 September 2015

Mapping timescales of quasifission

Dr. Elizabeth Williams, Australian National UniversityHumboldt Kolleg, ECT*, Trento, Italia, 1 September 2015

E. Williams, Humboldt Kolleg, 1 September 2015

Outline

Quasifission and superheavy element formation

ANU’s quasifission mapping program

New technique: High angular momentum mass angle distributions


40Ca

Quasifission238U

TDHF calculation of 40Ca+238U reaction (Cedric Simenel, Aditya Wakhle)


Quasifission: PCN = 1 - PQF

The ANU Quasifission Program

Aims to examine the dependence of quasifission probability and characteristics on collision variables (related to PCN):

• Compound nucleus fissility (Z2/A);

• Coulomb repulsion in the entrance channel (Z1Z2);

• Angular momentum;

• Nuclear structure of the colliding nuclei:o deformation (alignment with projectile)o closed shells (magic numbers) in the colliding nuclei

E. Williams, Humboldt Kolleg, 1 September

2015

Aim of the ANU Quasifission Program

Ultimate goal: Reliable model including all relevant physics to predict PCN

Model should allow direct comparison to experimental data

Model should predict quasifission probability, since PCN = 1 – PQF


2015

Means of creating this model

Start with experimental data

Define smooth trends in quasifission dynamics Fissility Coulomb repulsion Angular momentum

Then take into account the influence of shell effects on quasifission outcomes Magicity Collective structure Valence nucleon number

Work closely with theorists to develop models that provide insight into the physics driving quasifission probabilities E. Williams, Humboldt Kolleg, 1

September 2015



Define smooth trends in quasifission dynamics Fissility Coulomb repulsion Angular momentum



September 2015

E.

Wil

liam

s,

Hu

mb

ol

dt

Koll

eg

, 1

Sep

tem

ber

20

15The MAD Map

Identifying smooth trends in quasifission dynamics

How do we identify smooth trends in quasifission dynamics experimentally?

• Minimize shell effects – high E*

• Minimize effects of angular momentum – low E/Vb

• Compromise: choose E/Vb = 1.05-1.10o Effects of spherical magic numbers

attenuated by E*

o Effects of deformation alignment averaged out

o Angular momentum not too high (but still relevant to SHE production)


2015


2015

The MAD Map

R. du Rietz, E. Williams et al.,

PRC 88 (2013) 054618

Z = 6 Z = 28Projectile Z

Z = 82

Z = 92

Z = 102

Z = 112

Com

pound n

ucl

eus

ZTa

rget

Z

Hg

No

Ti

0

45

90

135

180

0 0.5 1MR

q (d

eg.)

q

(deg

.)

0

0.5

1

0 20 40 60 80Time

MR

04590

135180

0 20 40 60 80Time

q (d

eg.)

Miminal mass-angle correlationStrong mass-angle correlation

160o

20o Scission

R. Bock et al., NP A388 (1982)

334

J. Toke et al., NP A440 (1985)

327

W.Q. Shen at al., PRC 36 (1987)

115

B.B. Back et al., PRC 53 (1996)

1734

10 20 30 40

10 20 30 40

MADs: Mass equilibration and rotation

Slide courtesy of D. J. Hinde

Quasisim: A simple Monte Carlo model for quasifission timescales

Ingredients:

Reaction timescale determined by:• Angular velocity ω = L/I

Angular momentum L moment of inertia I

• Center-of-mass scattering angle θc.m.

• θi,f: ½ Coulomb deflection angles for the initial and final trajectories

Angle of rotation of the dinuclear system during reaction: Δθ = π-θi-θf-θc.m

trxn = Δθ/ω

Mass equilibration: 1-exp(trxn / τm), τm ~ 5.2 zs

[1] J. Tōke et al. Nucl. Phys. A 440, 327 (1985)[2] R. du Rietz et al. PRL 106, 052601 (2011)


2015

QF Timescales

<t> 5x10-21s <t> 10x10-21s <t> >> 10x10-

21s

186W

Experimental MAD

Simulated MAD

R. du Rietz et al. PRL 106 (2011)

052701 MAD1 MAD2 MAD3


MAD Classes: Distinguishing features

Class Mass distribution Mass-angle correlation?

MAD 1 (<τ> < 5 zs)

Minimum at Mr=0.5 Yes

MAD 2 (<τ> ~ 10 zs)

Maximum at Mr=0.5;

Significantly wider than that predicted

for fusion-fission

Yes

MAD 3 (<τ> >> 10 zs)

Maximum at Mr=0.5; may be

slightly wider than that predicted for

fusion-fusion

No

For MAD class 3, quasifission can be identified using other observations (e.g. angular anisotropies in comparison to Standard Model predictions).

Class 1

Class 2Class 3MADs for reactions in this energy regime (E/VB ~ 1.05 – 1.10) show a smooth evolution from long to short timescales as a function of entrance channel parameters.

Primarily fusion-fission


2015

Class 1

Class 2Class 3MADs for reactions in this energy regime (E/VB ~ 1.05 – 1.10) show a smooth evolution from long to short timescales as a function of entrance channel parameters.

Based on entrance channel quantities (charge product, effective fissility, etc.) and compound nucleus properties, can we predict which MAD class a given reaction is likely to conform with?



2015

R. du Rietz, E. Williams et al., PRC 88 (2013) 054618

Smooth trends: Coulomb repulsion

Smooth trends: Fissility

W. J. Swiatecki, Phys. Scr. 24, 113 (1981)

Compound nucleus fissility

Effective fissility

R. du Rietz, E. Williams et al., PRC 88 (2013) 054618

Smooth trends: Fissility


What conclusions can we draw from the MAD Map?

Using ZpZt and ZCN, or Xeff and XCN, we can roughly estimate the average timescale of a given reaction at ~1.05-1.10 VB.

We can use the same parameters to determine whether quasifission is likely to dominate in a given reaction in this energy range.

We have observed a smooth evolution in the MADs as a function of two categories of reaction parameters; this smooth evolution provides a first test of future dynamic models of reactions.

E.

Wil

liam

s,

Hu

mb

ol

dt

Koll

eg

, 1

Sep

tem

ber

20

15

Mapping MADs for high angular momentum collisionsA new method of extracting more from experimental data



Define smooth trends in quasifission dynamics Fissility Coulomb repulsion

Angular momentum



September 2015

MAD Map


The angular momentum degree of freedom

This is a difficult thing to study directly:

Each observation represents the sum of many reaction outcomes, reflecting the angular momentum distribution of the reaction in question.

We cannot select out reactions corresponding to a single angular momentum (L) value.

But can we restrict the angular momentum range we examine, using observations from complementary reactions?


Complementary reactions

We’ll define complementary reactions based on fusion angular momentum distributions.

0 20 40 60 800

2

4

6

8CCFULL [1]No couplinga = 1 fmr0 = 1 fmVB reproduced

52Cr + 198Pt (Elab = 264.8 MeV; E*=42.9 MeV)

[1] K. Hagino, N. Rowley, and A.T. Kruppa, Comp. Phys. Comm. 123 (1999) 143




CCFULL [1]No couplinga = 1 fmr0 = 1 fmVB reproduced



0 20 40 60 800

2

4

6

8





CCFULL [1]No couplinga = 1 fmr0 = 1 fmVB reproduced



0 20 40 60 800

2

4

6




2015



0 20 40 60 800

2

4

6

8


0 20 40 60 800

400

800

1200



2015




0 20 40 60 800

400

800

1200



54Cr + 196Pt 250Nb; E*~42.6 MeV

0 20 40 60 800

400

800

1200

0 20 40 60 800

400

800

1200

Subtract the two complementary distributions to isolate the high angular momentum component:

52Cr + 198Pt (Elab = 264.8 MeV; E*=42.9 MeV)54Cr + 196Pt (Elab = 272.2 MeV; E*=42.3 MeV)

Class 1

Class 2Class 3



2015 Cr

Pt

Complementary reaction:- Same reaction

(and therefore, same CN), different E*.

- Same CN and E*, different projectile / target combinations leading to the same MAD class.


High angular momentum MAD

How can we use this concept to extract high angular momentum MADs?

52C

r +

19

8Pt

(Ela

b =

26

4.8

MeV

)54C

r +

196P

t (E

lab =

272.2

MeV

)



MA

D1

: 52C

r +

19

8Pt

(Ela

b =

26

4.8

MeV

)M

AD

2:

54C

r +

196P

t (E

lab =

272.2

MeV

)

MAD 2’ - MAD 1’ = ΔMAD



0 20 40 60 800

400

800

1200


Correspondingangular momentum

distribution

54Cr + 196Pt 250Nb; E*~42.6 MeV

Cr + Pt reaction energies


2015

ANU 14UD tandem accelerator + LINAC + CUBE

CUBE

CN Reaction Elab (MeV)

E/VB E* (MeV)

250Nb

52Cr + 198Pt

264.84 1.03 42.9

54Cr + 196Pt

272.15 1.06 42.3

52Cr + 198Pt

272.67 1.06 49.1

54Cr + 196Pt

278.22 1.08 47.0

52Cr + 198Pt

276.85 1.08 52.4

54Cr + 196Pt

284.29 1.10 51.8

52Cr + 198Pt

282.85 1.10 57.2

54Cr + 196Pt

288.68 1.12 55.2

Preliminary

Mass-Angle Distributions (elastics / recoils excluded)


Preliminary

Loss of efficiency due to pulse height in back detector X-position delay line

Detector effects


Preliminary

Loss of efficiency due to pulse height in back detector X-position delay line

Exclusion of events due to poor front detector timing resolution

Detector effects


Preliminary

Mass-Angle Distributions (elastics / recoils excluded)


Preliminary

CCFULL (no coupling, a = 1 fm, r0 = 1 fm, VB reproduced)K. Hagino, N. Rowley, and A.T. Kruppa, Comp. Phys. Comm. 123 (1999) 143

High Angular Momentum Mass-Angle Distributions

0 20 40 60 80 100

0

500

1000

1500

What can we learn from this?


A first estimate of timescales

Ingredients:

Reaction timescale determined by:• Angular velocity ω = L/I

Angular momentum L moment of inertia I

• Center-of-mass scattering angle θc.m.

• θi,f: ½ Coulomb deflection angles for the initial and final trajectories

Angle of rotation of the dinuclear system during reaction: Δθ = π-θi-θf-θc.m

trxn = Δθ/ω

Mass equilibration: 1-exp(trxn / τm), τm ~ 5.2 zs

[1] J. Tōke et al. Nucl. Phys. A 440, 327 (1985)[2] R. du Rietz et al. PRL 106, 052601 (2011)


2015

Preliminary

45 55 65 75 850

5

10

15

Tim

e [z

s]

Moment of inertia –TDHF (tip collision):K. Vo-Phuok


Cr + Pt: Summary of findings

Preliminary

• High angular momentum mass angle distributions have been extracted for reactions leading to 250No

• Simple model suggests quasifission timescales decrease with increasing angular momentum

• Next steps: Improve the model, cross check with TDHF Apply method more broadly




Define smooth trends in quasifission dynamics Fissility Coulomb repulsion

Angular momentum



September 2015

MAD Map

High angular momentum MADs

0.5 0.6 0.7 0.80.75

0.80

0.85

0.90

0.95

Ca+Pb Simenel et al.Cr+W Hammerton et al.

Xeff

XC

.N.

Entrance channel magicity, isospin: C. Simenel et al., PLB 710 (2012) 607N/Z ratio: K. Hammerton et al., PRC 91 (2015) 041602Shell effects: G. Mohanto et al., ANU, in preparation

Additional measurements


Collaborators

Heavy Ion Accelerator Facility (HIAF)

E. Williams, D.J. Hinde, C. Simenel, M. Dasgupta, A. Wakhle, I.P. Carter, K.J. Cook, D.Y. Jeung, D.H. Luong, G. Mohanto, C.S. Palshetkar, E. Prasad, D.C. Rafferty and R. du Rietz (ANU)

The ANU Accelerator and Technical Staff

Research made possible by the Australian Research Council Grants and Fellowships DP110102858, DP110102879, DP130101569, FL110100098, FT120100760, and DE140100784.

Thank you!

Thank you!



ANU Experiments

Hinde et al., PRC 53 (1996) 1290 Rafiei et al., PRC 77 (2008) 024606 Thomas et al., PRC 77 (2008) 034610 Hinde et al., PRL 100 (2008) 202701 Hinde et al., PRL 101 (2008) 092701 du Rietz et al., PRL 106 (2011) 052701 Lin et al., PRC 85 (2012) 014611 Simenel et al., PLB 710 (2012) 607 Williams et al., PRC 88 (2013) 034611 du Rietz et al., PRC 88 (2013) 054618 Wakhle et al., PRL 113 (2014) 182502

Designed to study two-body fission.

• Composed of two large-area multiwire proportional counters (MWPC).

• MWPCs are position sensitive in X,Y coordinates.

• Position resolution: ~ 1mm

• Relative positions of the MWPCs can be adjusted to suit the experimental aims.

• Pulsed beam allows time-of-flight measurement.

• Resolution ~1 ns

• Angular coverage ~ 1.2π sr

Hinde et al., PRC 53 (1996) 1290

Rafiei et al., PRC 77 (2008)

024606

Thomas et al., PRC 77 (2008)

034610

Williams 50

The ANU CUBE detector

Hinde et al., PRC 53 (1996) 1290

Rafiei et al., PRC 77 (2008) 024606

Thomas et al., PRC 77 (2008) 034610

Position and time-of-flight information provide:- scattering angle θC.M. in the

center of mass frame,

- differential cross sections, and

- angular anisotropies.

Williams


51

V1

V2

V1cm

V2cm

Hinde et al., PRC 53 (1996) 1290

Rafiei et al., PRC 77 (2008) 024606

Thomas et al., PRC 77 (2008) 034610

Kinematic coincidence:Position and time-of-flight information allow us to determine the mass ratio MR of the two fission fragments:

MR1 = AF1/(AF1+AF2)

= V2cm/(V1cm+V2cm)

Williams


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Documents

Mapping timescales of quasifission Dr. Elizabeth Williams, Australian National University Humboldt Kolleg, ECT*, Trento, Italia, 1 September 2015