Recent results on ternary fission Dr. Olivier SEROT CEA-Cadarache, DEN/DER/SPRC/LEPh,

1Workshop Espace de Structure Nucléaire Théorique / 12-16 April, 2010

Recent results on ternary fission

Dr. Olivier SEROTCEA-Cadarache,

DEN/DER/SPRC/LEPh, F-13108 Saint Paul lez Durance,

France

University of Gent, (Belgium) : C. Wagemans, S. Vermote EC-JRC, Institute for Reference Materials and Measurements: J. Heyse Institut Laue-Langevin (France): T. Soldner, P. Geltenbort CEA-Cadarache, DEN/DER/SPRC/LEPh : O. Serot CENBG: Nicolae Carjan Lawrence Berkeley National Laboratory, Berkeley CA 94720, USA: I. AlMahamid

Collaborations


Experimental Procedure

What did we measure?

Main results:

Energy distributions Influence of the fission modes Influence of the spin of the resonances 239Pu(n,f)

reaction Influence of the alpha clusters Influence of the excitation energy of the fissioning

nucleus

Emission mechanism using the sudden approximation

Conclusion and outlook

Content


Binary Fission Ternary Fission

Long Range Alpha (LRA)

FFL

FFH

LRA

Phys. Rev. 71, 382 - 383 (1947)

Observed for the first time in 1946

The two heavy fragments are sometimes accompanied by a Light Charged Particle (LCP): Ternary fission

(roughly 2 to 4 times every thousand events depending on the mass of the fissioning nucleus)

Ternary Fission / Introduction (1/4)


The ternary particles: important source of helium and tritium

production in nuclear reactors and in used fuel elements

Data concerning this production are therefore requested by nuclear safety specialists

Applied Research


Tritium Formation in UOx fuel element Contribution

Ternary Fission: 3H 81.94 %

Ternary Fission: 6He 6He 6Li + - 6Li + n 4He + 3H (941.3b) 11.50 %

3H 3He + - 3He + n p + 3H (5317b) 4.82 %

n + 16O 14N + 3H 0.96 %

n + 16O 14C + 3He 3He + n p + 3H 0.05 %

Calculations performed

for UOx (3.25%) at 33GWd/t

From J. Pavageau, NT-SPRC-Lecy

00329


Ternary Particle = unique probe of the nucleus at the scission point

Fundamental Research

Since ternary particles are emitted in space and time close to the scission point, it is expected to infer information on scission point configuration and on the fission process itself

From Theobald et al., 1985

10

15

20

25

30

ELR

A /

MeV



Ternary Fission / Experimental Procedure

Telescopes used for the

ternary particles

detection

Sample placed in

between the two

telescopes

Vacuum chamber


Vacuum chamber

Neutron flux

Al foil put in front of the telescope: used to stop heavy fragments and alpha particles from radioactivity

Telescope used for the ternary particle identification

Energy: E+E+ correction for the energy loss in the sample and the Al-foil

1rst Step: Detection of the Ternary particles



0 5 10 15 20 25 300

1

2

3

4

5

6

E [MeV]

E [

Me

V] 4He

6He

0 5 10 15 20 25 300

2

4

6

8

E [MeV]

E [

Me

V]

4He

6He

3H

Telescope: 49.8 μm E and 1500 μm E

Better separation between ternary particles, but energy threshold higher than with the other telescope

Telescope: 29.8 μm E and 500 μm E;

good separation between LRA and background;



0 4 8 12 16 20 24 28 320

100

200

300

400

500

Counts

LRA Energy [MeV]

0 4 8 12 16 200

50

100

150

200

250

300

Counts

Triton Energy [MeV]

Example of measured spectra: a gaussian fit performed on the experimental data allows the determination:

• average energy • Full width at Half Maximum• Ternary particles counting rate: NLRA, N6He, Nt

LRA-particles 3H-particles



2nd Step: Detection of the heavy fragments: determination of the

binary fission counting Rate: NBF

0 500 1000 1500 2000 25000

1500

3000

4500

6000

Counts

Channel number

249Cf(n,f)N=3 086 669 evtsT=1.32 hB ~ 325 f/s

Collimator:12mm

n-beam

Vacuumchamber

Sample

Empty dummy

E-detector

LRA/B = NLRA / NBF

t/B = Nt / NBF

6He/B = N6He / NBF

Combining both steps allows the determination of

the ternary emission probability:



Institute for Reference Materials and Measurements

238Pu(sf) LRA

240Pu(sf) LRA

242Pu(sf) LRA

244Pu(sf) LRA

244Cm(sf) LRA, 3H, 6He

246Cm(sf) LRA, 3H

248Cm(sf) LRA, 3H

250Cf(sf) LRA, 3H, 6He

252Cf(sf) LRA, 3H, 6He

Institut Laue-Langevin

235U(n,f) LRA

243Cm(n,f) LRA, 3H, 6He

245Cm(n,f) LRA, 3H

247Cm(n,f) LRA, 3H

249Cf(n,f) LRA, 3H, 6He

251Cf(n,f) LRA, 3H, 6He

Spontaneous fission

(n,f) Reactions

For (sf): results on 238Pu up to 256Fm nuclides

For (nth,f): data cover target nuclei between 229Th and 251Cf

Huge enlargement of the available

database:

Ternary Fission / Measurements performed


Ternary Fission / Energy distributions

Energy distribution of the ternary alpha particles presents a low

energy tailing:

Two components: Main component: ‘true’ ternary

4He Smaller component due to

decay of ternary 5He particles: 5He -> 4He+n

(LRA/B)tot-values can be deduced by adding a (6 ± 1)% contribution to a Gaussian fit

Observed for 235U(nth,f) and 252Cf(sf); Assumed to be valid for all fissioning nuclei

Measurement performed here without Al-foil in order to decrease LRA threshold


From S. Vermote, PhD Thesis, Gand (Belgium), 2009

The average energy remains remarkably constant:

consequence of the stability of the heavy fragment peak in the fission fragment mass

distribution

4He 3H

<E>=8.4 ± 0.1 MeV<E>=16.0 ± 0.1 MeV



Linear increase of FWHM with increasing Z2/A of the CN is observed

FWHM is systematically 0.3 MeV smaller for (sf) than for (n,f). Shown for the first time thanks to our systematic study: 9 (sf) nuclides and 13 (n,f) reactions. (already observed for fission fragments).

4He 3H

From S. Vermote, PhD Thesis, Gand (Belgium), 2009



Rel

ativ

e Y

ield

[%

]

120 130 140 150 1600

2

4

6

8

10

120 130 140 150 1600

2

4

6

8

10

120 130 140 150 1600

2

4

6

8

10

120 130 140 150 1600

2

4

6

8

10

140 160 180 2000

1

2

3

4

5

140 160 180 2000

1

2

3

4

5

140 160 180 2000

1

2

3

4

5

140 160 180 2000

1

2

3

4

5Pre- Neutron Mass [uma]

Total Kinetic Energy [MeV]

238Pu(sf) 240Pu(sf) 242Pu(sf) 244Pu(sf)

Ternary Fission / Influence of the fission modes

From Demattè et al., Nucl.Phys.A617 (1997) 331

Standard I

Standard II

From mass and kinetic

energy distributions: Determination of the St. I

and St. II modes

contributions (Brosa’ s

model)


0

25

50

75

0

10

20

0 4 8 12 16 20 24 28 320

15

30

45

60

ELRA

/ MeV

Nu

mb

er

of

co

un

ts

0 4 8 12 16 20 24 28 320

10

20

30

ELRA

/ MeV

238Pu(sf) 240Pu(sf)

242Pu(sf) 244Pu(sf)



St.I St. II 238Pu 6.3% 93.7% 240Pu 26.4% 73.6% 242Pu 37.3% 62.7% 244Pu 44.5% 55.5%

• Same spin parity• Same excitation energy: Eexc=0• Same charge number: Z=94

Experimental evidence of the influence of the fission modes on LRA/B. Since the Standard II mode corresponds to a more

elongated mode than the Standard I one, this results confirms that LRA/B is strongly influenced by the deformation

energy available at scission.

50 60 70 80 90 100

1.6

2.0

2.4

2.8

LR

A /

B x

103

Contribution Standard II [%]

242Pu

244Pu

240Pu

238Pu



5 10 1000

2

4

Arb

itra

ry u

nit

En / eV

0.01 0.1 1 50

4

8

12

16

100Hz100Hz

800Hz800Hz

Overview of the GELINA Institute for Reference Materials and

Measurements, Geel, (Belgium)

Neutron beam: GELINA (GEel LINear Accelerator)Reaction: 239Pu(n,f)

Investigation of the LRA/B in thermal and resonance regions


Influence of the spin of the resonance (1/3)

Anode

GrilleSBD Grille SBD

AnodeCathode

Télescope 1 Télescope 2

neutrons Al Al

239Pu(651g/cm2)

239Pu(371g/cm2)

Anode

GrilleSBD Grille SBD

AnodeCathode

Télescope 1 Télescope 2

neutrons Al Al

239Pu(651g/cm2)

239Pu(371g/cm2)

222121nf2

212111nf1

NεNε Enσ EnB

LRA Enφ EnY

NεNε Enσ EnB

LRA Enφ EnY

015.0023.1NεNε

NεNε

Y

Y

222121

212111

2

1

0 4 8 12 16 20 24 28 320

50

100

150

200

250

ELRA

/ MeV

Telescope 2:<E>= 15.96 ± 0.15 MeVfwhm= 10.32 ± 0.21 MeVN = 20994 ± 201

Cou

nts

ELRA

/ MeV

0 4 8 12 16 20 24 28 320

50

100

150

200

250

Telescope 1:<E>= 16.03 ± 0.15 MeVfwhm= 10.38 ± 0.21 MeVN = 21506 ± 195

Cou

nts

0.0191.02420120994

19521506

Y

Y

2

1

Double ionisation chamber:• Two telescopes• Two 239Pu samples • Incident neutron energy determined by TOF method


Influence du spin de la résonance (2/3)

Anti-correlation between LRA/B and prompt neutron multiplicity is observed

• What is the impact of the spin of the resonance on LRA emission probability?

Thermal Region (100 Hz)

<

>

Neutron Energy [eV]


Influence du spin de la résonance (3/3)

In the resonance region:

31

3O

100.016 2.101LRA/B

100.014 2.096LRA/B

Impact of the spin still not clear (as for the prompt neutron emission !)

Resonance Region (800 Hz)

Thermal value

5 10 1000

2

4

6

Arb

itra

ry u

nit

En / eV


(nth,f)-data

Ternary Fission / Influence of alpha-cluster

According to the Liquid Drop Model, Z2/A is a measure of the deformation of the nucleus at scission

So, a positive correlation is expected between ternary emission probability and Z2/A

This correlation can be observed, but:

Smooth behavior for tritons,

fluctuations for LRA-particles can be observed

35 36 37 38

0.8

1.2

1.6

2.0

2.4

2.8

t / B

x 1

04

Z2cn / Acn


S=b exp / G (even-even nuclei)

b: branching ratio for the 0+ 0+ transitions

exp : experimental alpha decay constant

G : alpha decay constant calculated from WKB approximation

230 235 240 245 250 255 2602

4

6

8

10

12

14

16

S

x 1

0 3

A

U

Pu

Cm

Cf

Fm

Th

According to the Carjan’s model: LRA/B = S x PLRA

This model suggests the important role played by S in the ternary alpha emission process

S: Alpha cluster pre-formation probability

PLRA: Probability for an alpha cluster to gain enough energy to escape from the scissioning nucleus



Calculated with -nuclear potential derived by Igo

Normalized to 212Po

230 235 240 245 250 255 260

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

S

/ S(

212P

o)

A

U

Pu

Cm

Cf

Fm

Th

Poenaru, Particle emission from nuclei, Vol.II

230 235 240 245 250 255 2602

4

6

8

10

12

14

16

S

x 1

0 3

A

U

Pu

Cm

Cf

Fm

Th

Relative behavior is very similar than the one performed by Poenaru



35 36 37 38

0.8

1.2

1.6

2.0

2.4

2.8

t / B

x 1

04

Z2cn / Acn

Fluctuations of LRA/B less pronounced

Similar behavior between [LRA/B]/S and t/B

Taking into account the spectroscopic factor S:

The strong impact of S seems to confirm the emission mechanism proposed by

Carjan for LRA particles



Ternary Fission / Influence of the excitation energy

What is the impact of the excitation energy of the fissioning nucleus on the ternary emission probability ?

=>

Comparison of this probability for the same fissioning nucleus at Eexc=0 (sf-decay) Eexc=Bn ((nth,f)-reactions)

ExcExc Ea10) E, (A

BLRA

Bn) E, (AB

LRA

Ratio

EXCCN

EXCCN

+

(sf): Eexc=0

(nth,f): Eexc=Sn

Same fissioning nucleus

nA-1 A

A

239Pu(n,f) 240Pu(sf) 4He

241Pu(n,f) 242Pu(sf) 4He

243Cm(n,f) 244Cm(sf) 4He, 3H, 6He

245Cm(n,f) 246Cm(sf) 4He, 3H

247Cm(n,f) 248Cm(sf) 4He, 3H

249Cf(n,f) 250Cf(sf) 4He, 3H, 6He

251Cf(n,f) 252Cf(sf) 4He, 3H, 6He


Ternary Alpha:aEXC = - (0.030 ± 0.003) MeV−1

(7 fissioning nuclei)

Ternary Triton:aEXC = - (0.002 ± 0.012) MeV−1


Ternary 6-He:aEXC = - (0.022 ± 0.010) MeV−1


4He 3H 6He

Differences observed between t and LRA Similarity between LRA and 6-He

Cluster preformation of He-4 and He-6 seems to play a crucial role in the ternary emission process

Ternary Fission / Influence of the excitation energy

From ternary triton: The additional energy due to the capture of a neutron is mainly transformed at scission into intrinsic energy (not into deformation energy)


IAS LRA

JBS LRA | |

Model is valid if: neck << T

already proposed by Halpern (1971), but no quantitative results up to now

What can we learn from the sudden approximation applied to the LRA emission ?

Principe of the sudden approximation:

fast change of the nuclear potential during the neck rupture : lost of the adiabaticity of LRA

JBS=‘Just Before Scission’ ; IAS=‘Immediately After Scission’

neck~ 5.3 10 -23 s

T ~ 10.3 10 -22 s0.05~

Tneck

Investigation of LRA emission using the sudden approximation


Parameterization of the nucleus shape

2 2 22

(z) d z Az

dB

z

d

Mass asymmetryd : Elongation of the nucleus


Immediately after scission: Two spherical fragments with the same

dcm and the same mass asymmetry R=MH /

ML

Just before scission:

-16 -8 0 8 16-16

-8

0

8

16

z / fm

/

fm

-16 -8 0 8 16

-16

-8

0

8

16

/

fm

rneck

dcm


Calculation of the potential seen by the -particle

V V 1 expaNUCL 0

V 2d R

R Rcoul

0

3

a aVN

(deformed Woods-Saxon)

-24-16 -8 0 8 16 24

-60

-40

-20

0

20

z / fm

V /

MeV

-24-16 -8 0 8 16 24

-60

-40

-20

0

20

V /

MeV

-16 -8 0 8 16-16

-8

0

8

16

z / fm

/

fm

-16 -8 0 8 16

-16

-8

0

8

16

/

fm

rneck

dcm

Resolution of the stationary Schrödinger equation for both potentials



| | > LRA

JBS

n

JBS

By analogy with alpha decay theory: JBS wave function= eigenstate of the JBS pot.

Calculation of the wave function which is escaping from the nucleus

Part of the WF with higher components than the top of the barrier

out LRA|

dEΨ Ψ Ψ| IASELRA

IASE

IASmLRA

m

IASmLRA

Z / fm Z / fm Z / fm

Probability to escape:

Description of LRA emission using the sudden approximation


Nexp [%

]

L [deg]z [fm]

o

ut |

2

L [deg] From out , the LRA angular distribution can be deduced via the

deflection function L(z) (red curve)

This deflection function was obtained from trajectory calculations

Description of LRA emission using the sudden approximation

Calculation of the angular distribution


0 40 80 120 1600

1

2

3

4

5

L [deg]

0 40 80 120 1600

1

2

3

4

5

dcm=20.5 fmR=1Qa

sc =2 MeV

dcm=20.5 fmR=1.4Qa

sc =2 MeV

-45 -30 -15 0 150.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0 dcm=18.2 fm dcm=19.3 fm dcm=20.5 fm dcm=21.6 fm dcm =22.7 fm

Q-Scission [MeV]

P

Strong enhancement of P with the elongation of the scissioning

nucleus

LRA angular distribution is influenced by the elongation of the scissioning nucleus and the

mass asymmetry

PL=9.6 %EQ=80.8%PH=9.6%

PL=20.5 %EQ=70.0%PH=9.5%

Nexp [%

]



The main properties of the LRA angular distribution can be explained

The strong enhancement of P with the elongation of the scissioning nucleus can be reproduced-20 -10 0 10 20

1E-3

0.01

0.1

z [fm]

Equatorial

Polar

|o

ut |

2

Polar

Strong contraction of

the neck

Small retractionof the extremes

Small retractio

nof the

extremes


From the sudden

approximation


Database for ternary fission yields have been strongly enlarged (energy distribution and emission probability)

From the 238,240,242,244Pu(sf) studies: LRA/B is enhanced when the nucleus follows the Standard II fission mode

Influence of the spin on LRA/B still not clear, but an anti-correlation between prompt neutron multiplicity and LRA emission probability was observed

Our results confirm the different mechanism of ternary emission process between helium and tritium isotopes. In particular, the LRA emission process seems to be governed by the pre-formation of an alpha cluster which is not the case for the triton emission.

The dependence of the ternary fission yields with the fissioning nucleus excitation energy has been investigated:

LRA/B and 6He/B decrease with increasing Eexc low impact on t/B

Conclusion


252Cf(sf)

Fission fragment mass distributions with (dashed line) and without (line) ternary particle emissionFrom Grachev et al., sov. J. Nucl. Phys. 47/3, 1988

Differences between He-Ternary Particles (4-He and 6-He) and 3H can be again observed

Differences between helium and tritium already observed in the litterature

3H

4He

6He

37Workshop Espace de Structure Nucléaire Théorique / 12-16 April, 2010Halpern, 1971

Correlation between relative ternary particle yields and Energy cost Ec= energy needed to eject TP placed in between both fragments.

The observed yields of Hydrogen are much lower than expected

Differences between helium and tritium already observed in the litterature

Recent results on ternary fission Dr. Olivier SEROT CEA-Cadarache, DEN/DER/SPRC/LEPh,

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Transcript of Recent results on ternary fission Dr. Olivier SEROT CEA-Cadarache, DEN/DER/SPRC/LEPh,