1Workshop Espace de Structure Nucléaire Théorique / April, 2010 Recent results on ternary fission Dr. Olivier SEROT CEA-Cadarache, DEN/DER/SPRC/LEPh, F 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

2Workshop Espace de Structure Nucléaire Théorique / April, 2010 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

3Workshop Espace de Structure Nucléaire Théorique / April, 2010 Binary FissionTernary Fission Long Range Alpha (LRA) FF L FF H LRA Phys. Rev. 71, (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)

4Workshop Espace de Structure Nucléaire Théorique / April, 2010 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 Ternary Fission / Introduction (2/4) Tritium Formation in UOx fuel elementContribution Ternary Fission : 3 H % Ternary Fission : 6 He 6 He  6 Li +  - 6 Li + n  4 He + 3 H (941.3b) % 3 H  3 He +  - 3 He + n  p + 3 H (5317b) 4.82 % n + 16 O  14 N + 3 H 0.96 % n + 16 O  14 C + 3 He 3 He + n  p + 3 H 0.05 % Calculations performed for UOx (3.25%) at 33GWd/t From J. Pavageau, NT-SPRC-Lecy 00329

5Workshop Espace de Structure Nucléaire Théorique / April, 2010 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., E LRA / MeV Ternary Fission / Introduction (4/4)

6Workshop Espace de Structure Nucléaire Théorique / April, 2010 Ternary Fission / Experimental Procedure Telescopes used for the ternary particles detection Sample placed in between the two telescopes Vacuum chamber

7Workshop Espace de Structure Nucléaire Théorique / April, 2010 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 1 rst Step: Detection of the Ternary particles Ternary Fission / Experimental Procedure

8Workshop Espace de Structure Nucléaire Théorique / April, He 6 He 4 He 6 He 3H3H 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; Ternary Fission / Experimental Procedure

9Workshop Espace de Structure Nucléaire Théorique / April, 2010 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: N LRA, N 6He, N t LRA-particles3H-particles Ternary Fission / Experimental Procedure

10Workshop Espace de Structure Nucléaire Théorique / April, nd Step: Detection of the heavy fragments: determination of the binary fission counting Rate: N BF Collimator: 12mm n-beam Vacuum chamber Sample Empty dummy E-detector LRA/B = N LRA / N BF t/B = N t / N BF 6 He/B = N 6He / N BF Combining both steps allows the determination of the ternary emission probability: Ternary Fission / Experimental Procedure

11Workshop Espace de Structure Nucléaire Théorique / April, 2010 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 (n th,f): data cover target nuclei between 229Th and 251Cf Huge enlargement of the available database: Ternary Fission / Measurements performed

12Workshop Espace de Structure Nucléaire Théorique / April, 2010 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

13Workshop Espace de Structure Nucléaire Théorique / April, 2010 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 4 He 3H3H =8.4 ± 0.1 MeV =16.0 ± 0.1 MeV Ternary Fission / Energy distributions

14Workshop Espace de Structure Nucléaire Théorique / April, 2010 Linear increase of FWHM with increasing Z 2 /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). 4 He 3H3H From S. Vermote, PhD Thesis, Gand (Belgium), 2009 Ternary Fission / Energy distributions

15Workshop Espace de Structure Nucléaire Théorique / April, 2010 Relative Yield [%] Pre- Neutron Mass [uma] Total Kinetic Energy [MeV] 238 Pu(sf) 240 Pu(sf) 242 Pu(sf) 244 Pu(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)

16Workshop Espace de Structure Nucléaire Théorique / April, Pu(sf) 240 Pu(sf) 242 Pu(sf) 244 Pu(sf) Ternary Fission / Influence of the fission modes

17Workshop Espace de Structure Nucléaire Théorique / April, 2010 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. 242 Pu 244 Pu 240 Pu 238 Pu Ternary Fission / Influence of the fission modes

18Workshop Espace de Structure Nucléaire Théorique / April, Hz 800Hz 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

19Workshop Espace de Structure Nucléaire Théorique / April, 2010 Influence of the spin of the resonance (1/3) Double ionisation chamber: Two telescopes Two 239Pu samples Incident neutron energy determined by TOF method

20Workshop Espace de Structure Nucléaire Théorique / April, 2010 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]

21Workshop Espace de Structure Nucléaire Théorique / April, 2010 Influence du spin de la résonance (3/3) In the resonance region: Impact of the spin still not clear (as for the prompt neutron emission !) Resonance Region (800 Hz) Thermal value

22Workshop Espace de Structure Nucléaire Théorique / April, 2010 (n th,f)-data Ternary Fission / Influence of alpha-cluster According to the Liquid Drop Model, Z 2 /A is a measure of the deformation of the nucleus at scission So, a positive correlation is expected between ternary emission probability and Z 2 /A This correlation can be observed, but: Smooth behavior for tritons, fluctuations for LRA-particles can be observed

23Workshop Espace de Structure Nucléaire Théorique / April, 2010 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 U Pu Cm Cf Fm Th According to the Carjan’s model: LRA/B = S  x P LRA This model suggests the important role played by S  in the ternary alpha emission process S  : Alpha cluster pre-formation probability P LRA : Probability for an alpha cluster to gain enough energy to escape from the scissioning nucleus Ternary Fission / Influence of alpha-cluster

24Workshop Espace de Structure Nucléaire Théorique / April, 2010 Calculated with  -nuclear potential derived by Igo Normalized to 212 Po U Pu Cm Cf Fm Th Poenaru, Particle emission from nuclei, Vol.II U Pu Cm Cf Fm Th Relative behavior is very similar than the one performed by Poenaru Ternary Fission / Influence of alpha-cluster

25Workshop Espace de Structure Nucléaire Théorique / April, 2010 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 alpha-cluster

26Workshop Espace de Structure Nucléaire Théorique / April, 2010 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) + (sf): Eexc=0 (n th,f): Eexc=Sn Same fissioning nucleus n A-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

27Workshop Espace de Structure Nucléaire Théorique / April, 2010 Ternary Alpha: a EXC = - (0.030 ± 0.003) MeV −1 (7 fissioning nuclei) Ternary Triton: a EXC = - (0.002 ± 0.012) MeV −1 (5 fissioning nuclei) Ternary 6-He: a EXC = - (0.022 ± 0.010) MeV −1 (3 fissioning nuclei) 4He3H6He 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)

28Workshop Espace de Structure Nucléaire Théorique / April, 2010 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  s T  ~ 10.3  s Investigation of LRA emission using the sudden approximation

29Workshop Espace de Structure Nucléaire Théorique / April, 2010 Parameterization of the nucleus shape  Mass asymmetry d : Elongation of the nucleus Investigation of LRA emission using the sudden approximation Immediately after scission: Two spherical fragments with the same d cm and the same mass asymmetry R=M H / M L Just before scission: r neck d cm 

30Workshop Espace de Structure Nucléaire Théorique / April, 2010 Calculation of the potential seen by the  -particle (deformed Woods-Saxon) r neck d cm  Resolution of the stationary Schrödinger equation for both potentials  Investigation of LRA emission using the sudden approximation

31Workshop Espace de Structure Nucléaire Théorique / April, 2010 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 Z / fm Probability to escape: Description of LRA emission using the sudden approximation

32Workshop Espace de Structure Nucléaire Théorique / April, 2010 N exp [%]  L [deg] z [fm]  out | 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 

33Workshop Espace de Structure Nucléaire Théorique / April, 2010 d cm =20.5 fm R=1 Q a sc =2 MeV d cm =20.5 fm R=1.4 Q a sc =2 MeV 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% N exp [%] Investigation of LRA emission using the sudden approximation

34Workshop Espace de Structure Nucléaire Théorique / April, 2010 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 z [fm] Equatorial Polar |  out | 2 Polar Strong contraction of the neck Small retraction of the extremes Small retraction of the extremes Investigation of LRA emission using the sudden approximation From the sudden approximation

35Workshop Espace de Structure Nucléaire Théorique / April, 2010 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

36Workshop Espace de Structure Nucléaire Théorique / April, Cf(sf) Fission fragment mass distributions with (dashed line) and without (line) ternary particle emission From 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 / April, 2010 Halpern, 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