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

1. Recent results on ternary fission Dr. Olivier SEROT CEA-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

2. Content • 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

3. FFL LRA FFH Ternary Fission / Introduction (1/4) Binary Fission Ternary Fission • 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) Observed for the first time in 1946 Long Range Alpha (LRA) Phys. Rev. 71, 382 - 383 (1947)

4. Ternary Fission / Introduction (2/4) AppliedResearch 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 Calculations performed for UOx (3.25%) at 33GWd/t From J. Pavageau, NT-SPRC-Lecy 00329

5. 30 25 20 ELRA / MeV 15 10 From Theobald et al., 1985 Ternary Fission / Introduction (4/4) 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 Ternary Particle = unique probe of the nucleus at the scission point

6. Ternary Fission / Experimental Procedure Vacuum chamber Sample placed in between the two telescopes Telescopes used for the ternary particles detection

7. Ternary Fission / Experimental Procedure Vacuum chamber 1rst Step: Detection of the Ternary particles 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: DE+E+ correction for the energy loss in the sample and the Al-foil

8. 6He 6He 4He 4He 3H Ternary Fission / Experimental Procedure • Telescope: • 49.8 μm DE and 1500 μm E • Better separation between ternary particles, but energy threshold higher than with the other telescope • Telescope: • 29.8 μm DE and 500 μm E; • good separation between LRA and background;

9. Ternary Fission / Experimental Procedure • 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

10. Ternary Fission / Experimental Procedure Collimator: 12mm 2nd Step: Detection of the heavy fragments: determination of the binary fission counting Rate: NBF E-detector Empty dummy Vacuum chamber Sample n-beam Combining both steps allows the determination of the ternary emission probability: LRA/B = NLRA / NBF t/B = Nt / NBF 6He/B = N6He / NBF

11. Ternary Fission / Measurements performed Spontaneous fission Huge enlargement of the available database: Institute for Reference Materials and Measurements • For (sf): results on 238Pu up to 256Fm nuclides • For (nth,f): data cover target nuclei between 229Th and 251Cf (n,f) Reactions Institut Laue-Langevin

12. Ternary Fission / Energy distributions Measurement performed here without Al-foil in order to decrease LRA threshold • 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

13. Ternary Fission / Energy distributions From S. Vermote, PhD Thesis, Gand (Belgium), 2009 4He 3H <E>=16.0 ± 0.1 MeV <E>=8.4 ± 0.1 MeV The average energy remains remarkably constant: consequence of the stability of the heavy fragment peak in the fission fragment mass distribution

14. Ternary Fission / Energy distributions From S. Vermote, PhD Thesis, Gand (Belgium), 2009 4He 3H • 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).

15. Standard I Standard II Ternary Fission / Influence of the fission modes 238Pu(sf) 240Pu(sf) 242Pu(sf) 244Pu(sf) From mass and kinetic energy distributions: Determination of the St. I and St. II modes contributions (Brosa’ s model) Pre- Neutron Mass [uma] Relative Yield [%] Total Kinetic Energy [MeV] From Demattè et al., Nucl.Phys.A617 (1997) 331

16. Ternary Fission / Influence of the fission modes 238Pu(sf) 240Pu(sf) 242Pu(sf) 244Pu(sf)

17. Ternary Fission / Influence of the fission modes • Same spin parity • Same excitation energy: Eexc=0 • Same charge number: Z=94 238Pu 240Pu 242Pu 244Pu 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.

18. 100Hz 800Hz Investigation of the LRA/B in thermal and resonance regions Neutron beam: GELINA (GEel LINear Accelerator) Reaction: 239Pu(n,f) Overview of the GELINA Institute for Reference Materials and Measurements, Geel, (Belgium)

19. Influence of the spin of the resonance (1/3) • Double ionisation chamber: • Two telescopes • Two 239Pu samples • Incident neutron energy determined by TOF method

20. < n > Neutron Energy [eV] Influence du spin de la résonance (2/3) Thermal Region (100 Hz) • 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?

21. Influence du spin de la résonance (3/3) Resonance Region (800 Hz) • In the resonance region: Thermal value • Impact of the spin still not clear (as for the prompt neutron emission !)

22. Ternary Fission / Influence of alpha-cluster (nth,f)-data • 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

23. Th U Pu Fm Cm Cf Ternary Fission / Influence of alpha-cluster • Sa: Alpha cluster pre-formation probability • PLRA: Probability for an alpha cluster to gain enough energy to escape from the scissioning nucleus According to the Carjan’s model: LRA/B = Sa x PLRA This model suggests the important role played by Sa in the ternary alpha emission process • Sa=balexp / lG • (even-even nuclei) • ba: branching ratio for the 0+ 0+ transitions • lexp : experimental alpha decay constant • lG : alpha decay constant calculated from WKB approximation

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

25. Ternary Fission / Influence of alpha-cluster Taking into account the spectroscopic factor Sa: • Fluctuations of LRA/B less pronounced • Similar behavior between [LRA/B]/Sa and t/B The strong impact of Sa seems to confirm the emission mechanism proposed by Carjan for LRA particles

26. Ternary Fission /Influence of the excitation energy Same fissioning nucleus 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 A (nth,f): Eexc=Sn + n A A-1

27. Ternary Fission /Influence of the excitation energy • Ternary Alpha: • aEXC = - (0.030 ± 0.003) MeV−1 • (7 fissioning nuclei) • Ternary Triton: • aEXC = - (0.002 ± 0.012) MeV−1 • (5 fissioning nuclei) • Ternary 6-He: • aEXC = - (0.022 ± 0.010) MeV−1 • (3 fissioning nuclei) 4He 3H 6He • Differences observed between t and LRA • Similarity between LRA and 6-He • 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) • Cluster preformation of He-4 and He-6 seems to play a crucial role in the ternary emission process

28. Investigation of LRA emission using the sudden approximation 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’ • Model is valid if: tneck << Ta • already proposed by Halpern (1971), but no quantitative results up to now tneck~ 5.3  10 -23 s Ta ~ 10.3  10 -22 s

29. Investigation of LRA emission using the sudden approximation  Parameterization of the nucleus shape rneck • Just before scission: a : Mass asymmetry d : Elongation of the nucleus • Immediately after scission: • Two spherical fragments with the samedcm and the same mass asymmetry R=MH / ML dcm

30. rneck dcm Investigation of LRA emission using the sudden approximation  Calculation of the potential seen by the a-particle (deformed Woods-Saxon)  Resolution of the stationary Schrödinger equation for both potentials

31. Description of LRA emission using the sudden approximation  Calculation of the wave function which is escaping from the nucleus By analogy with alpha decay theory: JBS wave function= eigenstate of the JBS pot. Z / fm Z / fm Z / fm Part of the WF with higher components than the top of the barrier Probability to escape:

32. Description of LRA emission using the sudden approximation  Calculation of the angular distribution Nexp [%] |y out |2 JaL [deg] z [fm] JaL [deg] • From Yout , the LRA angular distribution can be deduced via the deflection function qaL(z) (red curve) • This deflection function was obtained from trajectory calculations

33. 1.0 d =18.2 fm 0.9 cm d =19.3 fm cm 0.8 d =20.5 fm cm d =21.6 fm 0.7 cm d =22.7 fm 0.6 cm P 0.5 0.4 0.3 0.2 0.1 0.0 -45 -30 -15 0 15 Qa-Scission [MeV] Investigation of LRA emission using the sudden approximation PL=20.5 % EQ=70.0% PH=9.5% dcm=20.5 fm R=1.4 Qasc =2 MeV Nexp [%] PL=9.6 % EQ=80.8% PH=9.6% dcm=20.5 fm R=1 Qasc =2 MeV LRA angular distribution is influenced by the elongation of the scissioning nucleus and the mass asymmetry Strong enhancement of P with the elongation of the scissioning nucleus

34. Small retraction of the extremes Small retraction of the extremes Strong contraction of the neck Equatorial |y out| 2 Polar Polar z [fm] Investigation of LRA emission using the sudden approximation From the sudden approximation • 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

35. Conclusion • 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

36. Differences between helium and tritium already observed in the litterature 3H 252Cf(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 4He 6He Differences between He-Ternary Particles (4-He and 6-He) and 3H can be again observed

37. Differences between helium and tritium already observed in the litterature • 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 Halpern, 1971