1. Competition between compound-nucleus formation and quasi-fission in heavy element synthesis B.B.Back Argonne National Laboratory

2. B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015 2 Outline:  Fission history  Dissipation in large amplitude shape changes –Bohr Wheeler vs. Kramers –Evidence in hot fission time scales – various clocks –Evidence in TKE – no or little pre scission KE  Experimental evidence for quasifission (examples) –Mass distribution widths –Fission anisotropy –Mass-angle correlations – short time scales  Quasifission – the mass drift mode  Super-heavy element synthesis  Conclusion

3. Fission history B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015 3

4. Discovery – Chemistry Barium observation Otto Hahn, Lise Meitner, Fritz Strassmann B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015 4 Lise Meitner Element 109, Meitnerium, is named in her honor Z=105 Hahnium → Dubnium Fritz Strassmann Co-discoverer of fission Otto Hahn Nobel Prize for fission discovery

5. Events between Christmas – New year 1938 B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015 5 Meitner & Frisch Dec 24-25, 1938 X-mas in Kungälv Lise Meitner escapes to Stockholm fall 1938 with help from Dutch physicists Bohr to NY End 1938 Frisch returns to Copenhagen Dec 26?

6. Rapid theoretical understanding of fission process  Bohr and Rosenfeld arrives in New York, met by John A. Wheeler, Princeton  Wheeler brings Rosenfeld to Princeton  Rosenfeld talks about fission at a “journal club” at the Palmer Laboratory  Bohr stays New York a couple of days before going to Princeton  Bohr identifies 235 U as responsible for U(n th,f) cross section [Phys. Rev. 55, 418 (1939)]  Bohr and Wheeler starts work on the theory of fission and submit their groundbreaking paper to Physical Review on June 28, [Phys. Rev. 56, 426 (1939)] B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015 6 Niels Bohr John A. Wheeler

7. Phys. Rev. article June 1939  Building on the compound nucleus picture Bohr, Nature 137,344/351 (1936),  Total kinetic energy release from mass formula – (already in Meitner-Frisch letter)  Bohr-Wheeler formula B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015 7 N Z

8. Dependence of fission barriers on fissility B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015 8 Introduction of the concept of a fission barrier Liquid drop model calculation of fission barriers

9. Dissipation in large amplitude shape changes B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015 9

10. Dissipation in fission  H. A. Kramers, Physica 7, 284 (1040) – Brownian motion  This formulation of the fission width was largely ignored, I believe, until fission delays we first observed in hot nuclei in the eighties –  Pre-scission neutron emission (too many) –Gavron et al. PRL 47, 1255 (1981); Err: 48, 835 (1982); PLB 176, 312 (1986) PRC 35, 579(1987) –Hinde et al., PRC 45, 1229 (1992) –P. Grange et al., PRC 27, 2063 (1983)  Pre-scission  emission (too many) –R. Butsch et al., PRC 41, 1530 (1990); PRC 44, 1515 (1991) –I. Dioszegi et al., PRC 46, 627 (1992) –D. J. Hofman et al., PRC 51, 2597 (1995)  Total kinetic energies (not excitation energy dependent) –B. Glagola et al., PRC 29, 486 (1984) B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015 10

11. Heavy-ion reactions: Dissipation → quasifission  Fission cross section (too large) –B. Heusch et al., Z. Phys. A 288, 391 (1978)  Mass distributions (too wide) –B. Borderie et al., Z. Phys. A 299, 263 (1981)  Angular distributions (too anisotropic) –B. B. Back et al., PRL 46, 1068 (1981) –B. B. Back et al., PRL 50, 818 (1983) –B. B. Back et al., PRC 31, 2104 (1985)  Angle – mass correlations (not symmetric) –R. Bock et al., NPA 388, 334 (1982) –J. Toke et al., NPA 440, 327 (1985) –W. Q. Shen et al., PRC 36, 115 (1987) –B. B. Back et al., PRC 53, 1734 (1996) B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015 11 K. Nishio, J. Phys. Conf. Ser. 282, 012011 (2011)  Re-arrangement of many nucleons  Dissipation

12. B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015 12 Approximation: Fission angular distribution – theory: I.Halpern and V.M.Strutinski, in Proc. 2 nd UN Conf. on Peaceful uses of atomic energy (UN, Geneva, Switzerland, 1958), p. 408 So: Fission anisotropy gives information about the moments of inertia i.e. the deformation of the saddle point configuration

13. B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015 13 Liquid drop model prediction of saddle shape Reising et al., PR 141, 1161 (1966) Businaro-Galone point Liquid drop model (Cohen & Swiatecki’63) Predicts barrier vanishes above Z 2 /A=50. Angular distribution measurements finds that this happens for Z 2 /A=45. LDM 42.8 MeV  particles

14. B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015 14 Saddle or scission?? This question was essentially settled by the Reising measurements, which clearly follow the saddle shapes predicted by the Liquid Drop Model Q: What happens here?

15. B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015 15 Back et al. PRL 46, 1068 (1981) ReactionCompoundX fis 16 O+ 208 Bi 224 Pa0.774 32 S+ 197 Au 229 Am0.817 32 S+ 232 Th 264 Sg0.899 32 S+ 238 U 270 Ha0.914 32 S+ 248 Cm 280 Cn0.948 Fission by heavy-ions? S.A.Karamyan et al., Yad. Fiz. 6, 494 (1967) [Sov. J. Nucl. Phys. 6, 360 (1968)] 32 S induced fission does not obey LDM prediction of nearly isotropic ang. Distribution If statistical equilibrium is reflected it occurs between saddle and scission Quasifission??

16. B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015 16 B.B.Back et al. PRL 50, 818 (1983) Dependence on entrance channel? Yes: There is a clear dependence on the mass asymmetry in the entrance channel

17. B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015 17 Recent data on 32 S+ 232 Th at lower energies Hinde et al., Fusion08 conf proc, p 281; Phys. Rev. Lett. 101, 092701 (2008) Saddle

18. How does this relate to making SHEs? B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015 18 Cold fusion Hot fusion S+Au,Th,U,Cm There is a large difference In shape between the system at capture and the completely fused system

19. Quasifission – the mass drift mode B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015 19

20. B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015 20 Mass – angle distribution (MAD) J.Toke et al., Nucl. Phys. A440, 327 (1985) Compound nucleus formation:  (A,  )=  (A,  ) and  (A,  )=  (A cn -A,  ) Definite proof of dynamic process This is where fusion-fission should appear Buried under quasi-fission component???

21. B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015 21 Mass drift mode R.Bock et al. NPA 388, 334 (1982)  cm (deg) Counts Fragment Mass (u) Observed early in Pb induced reactions Du Rietz et al, PRC 88, 054618 (2013)

22. B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015 22 Mass-angle distributions for 238 U+ 16 O, 26 Mg, 32 S W. Q. Shen et al., PRC 36, 115 (1987)

23. B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015 23 Mass-angle distributions for 238 U+ 35 Cl, 40 Ca, nat Zn W. Q. Shen et al., PRC 36, 115 (1987)

24. B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015 24 Energy loss caused by one-body dissipation Legend: r = density of nucleons v = mean particle speed R 1,R 2 = radii of nuclei a = window area V= rate of volume change Rate of energy dissipation: E.g. for 48 Ca+ 238 U One finds: Which is in excellent agreement with the values extracted from quasifission experiments

25. B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015 25 Evolution with beam energy W. Q. Shen et al., PRC 36, 115 (1987) Observation: Cross section for symmetric Fusion-fission drops dramatically at near barrier energies used to synthesize S.H.E. 238 U+ 48 Ca 4.6-7.5 MeV/u 7.5 MeV/u 6.7 MeV/u 5.9 MeV/u 4.6 MeV/u 4.8 MeV/u 5.0 MeV/u 5.4 MeV/u S.H.E.

26. 26 Another way to plot MADs Back et al., PRC 53 1734 (1996) B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015

27. Recent measurements of 48 Ca+ 238 U B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015 27 K. Nishio et al., PRC 86, 034608 (2012) Unified model: Aritomo et al., PRC 85, 044614 (2012) 36 S+ 238 U @ E*=39.5 MeV

28. Data comparison: 48 Ca+ 238 U B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015 28 Shen et al. K. Nishio et al., PRC 86, 034608 (2012); Unified model: Aritomo et al., PRC 85, 044614 (2012) K. Nishio et al., PRC 86, 034608 (2012); Shen et al., PRC 36, 115 (1987)

29. Super-Heavy Element synthesis B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015 29

30. B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015 30 Making Super-heavy elements Fusion-evaporation Fusion-fission Quasi-fission Deep-inelastic scattering This is what we want These reaction channels are only a nuisance

31. Nuclear theory in the Soviet Union, circa 1969 Energy dissipation / nuclear viscosity B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015 31

32. Dissipation in fusion – S.H.E. synthesis Let’s examine the obstacles for W sur :  Fusion barrier high relative to ground state –Actually the Q-values become very negative for heavy beams –Lowers excitation energy of fused system for above barrier fusion  Fission decay because of small barriers –Not such a big problem – fission barriers are high –Fission is hindered relative to simple statistical model estimate  Lifetimes too short –No, lifetimes seem to stabilize in the ms range – not a problem B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015 32 STICK x DIFFUSE x SURVIVE Swiatecki, Siwek-Wilczynska, Wilczynski, PRC 71, 014602 (2005)

33. Fission barriers – no problem B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015 33 Data: Henning et al., PRL 113, 262505 (2014); Itkis, Oganessian, Zagrebaev, PRC 65, 044602 (2002); Theory: Möller et al., At. Data 59, 185 (1995) 254 No Henning 286 112, 292 114, 296 116 Itkis et al.

34. Coulomb barriers and Q-values B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015 34 Q-value for fusion is large and negative Compensated for the increase in interaction barrier Hot fusion reactions lead to high excitation energies => more chances for fission loss Cold fusion suffers entrance channel loses (quasifission) V int Q fus E exc B fis

35. - 35 - Statics: 86 Kr+ 208 Pb → 294 X 118 should be easy  Proximity potential w. Moller-Nix parameters  Bottle-neck for fusion is fission barrier  Why is it so difficult? System is easily captures here But complete fusion requires the system shape to be here B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015

36. Simple model: Fusion by Diffusion B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015 36 Swiatecki, Siwek-Wiczynska, Wiczynski, PRC 71, 014602 (2005) Cold fusion reactions Dynamics: This is the BIG 10 10 problem Fission: This is a smaller problem

37. Heavy-element discoveries B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015 37 Caveat: Some of the heaviest elements made by hot fusion are not yet confirmed Another discovery due soon?

38. B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015 New periodic table (Niels Back ca. 1933) 38 Where did my dad get this from? 38

39. Back to Niels Bohr B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015 39 “The Atom and the Bohr Theory of Its Structure” H. Kramers and H. Holst, Gyldendal, London (1923)

40. Fig. 34 on page 201 B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015 40 Oganessian et al. NPA 734, 109 (2004) Helge Kragh, Physics Today 2013

41. B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015 41 Conclusion  We have come a long way  Remarkable and steady rate of heavy-element discoveries over seven decades – how much longer?  Static properties no obstacle to SHE synthesis  Shell effects ensures relative stability of elements up to Z=120  Life times in milliseconds or longer – fission barriers high  Saddle shapes – big changes in shape required  Dissipation is an important part of the picture  Resists shape changes and hinders fusion, but also slows down fission losses once a compound nucleus has been formed.  Hot fusion wins out over cold fusion in very heavy systems

42. B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015 42 Thank you

43. Stability of heavy elements B.B.Back, ANL "Interfacing Structure and Reaction Dynamics in the Synthesis of the Heaviest Nuclei", Trento, Sept 1-4, 2015 43 Ref: Swiatecki, Siwek-Wilczynska, Wilczynski, PRC 71, 014602 (2005): Shell corrections calculated by Smolancyk, Skalski, and Sobicziewski in Proc. Int. Hirschegg Workshop XXIV, Extremes of Nuclear Structure, eds. H. Feldmeier, J. Knoll, and W. Hornberg (GSI, Darmstadt, Germany, 1996), p. 4.