The excitation and decay of nuclear isomers

1. The excitation and decay of nuclear isomers 3. Isomers at the limits of stability Phil Walker CERN and University of Surrey, UK ● p decay ● n decay ● α decay ● fission

2. isomers defined “The existence of isotopic isobars (same-Z, same-A), with clearly distinguishable properties such as different radioactive half-periods, was anticipated in 1917 when Soddy proposed that such nuclei be called isomers if and when found.” Evans, 1955 “An excited nuclear state which endures long enough to have a directly measurable lifetime is called an isomeric state.” Bethe, 1956 rule of thumb: τ > 1 ns The possibility to separate them in time and/or space, from the other products of nuclear reactions, gives isomers an experimental status akin to ground states.

3. isomeric state τm α β fission γ p ground state τg α β fission p

4. In what circumstances can an isomer be “more stable” than its ground state? isomeric state τm α β fission γ p ground state τg α β fission p

5. aspects of proton (and neutron) decay

6. 53Co proton decay (1.56 MeV protons) first example of proton radioactivity 247 ms p 1.5% β+ 240 ms Jackson et al., Phys. Lett. B33 (1970) 281

7. 94Ag (21+) p-decay 300 ms p 2% p 2% 37 ms Mukha et al., Phys. Rev. Lett. 95 (2005) 022501

8. 159Re pdecay 75 21 μs isomer (11/2-) importance of centrifugal barrier ground state unknown (1/2+) Joss et al., Phys. Lett. B641 (2006) 34 Liu et al., Phys. Rev. C76 (2007) 034313

9. Peker et al: p and n decay 1971

10. neutron radioactivity unique to isomers? high-spin isomer n threshold A – 1 + n gs A β NB: mono-energetic neutrons ~1 MeV

11. aspects of α decay (restricted to broken-pair excitations)

12. 208Pb + α 212Po αdecay 84 45 s isomer at 2.9 MeV (18+) α 100% 0.3 μs ground state 0+ α 100%

13. superheavy 270Ds αdecay 110 6 ms isomer at 1 MeV (10-) α 100%? 0.1 ms ground state 0+ α 100% Hofmann et al., Eur. Phys. J. 10 (2001) 5 Xu et al., Phys. Rev. Lett. 92 (2004) 252501

14. superheavy 270Ds αdecay 110 isomers can provide extra stability for superheavy nuclei 6 ms isomer at 1 MeV (10-) α 100%? 0.1 ms ground state 0+ α 100% Hofmann et al., Eur. Phys. J. 10 (2001) 5 Xu et al., Phys. Rev. Lett. 92 (2004) 252501

15. Focus on fission Broken-pair isomers with enhanced stability 212Fr, 254No, 256Fm, 250No and “fission isomers” (still restricted to broken-pair excitations)

16. data from ANU, Canberra 212Fr spin-34 isomer at 8.5 MeV (i.e. ≤1 fission in 108 decays).

17. angular momentum orientation and fission fission I non-collective isomer: anti-fission!? collective rotation I “centrifugal force” helps fission

18. Moller et al. Möller et al., Phys. Rev. C79 (2009) 064304

19. Moller et al. Möller et al., Phys. Rev. C79 (2009) 064304

20. Moller et al. These are for ground states. What about isomers? Möller et al., Phys. Rev. C79 (2009) 064304

21. K isomers in one of the heaviest deformed nuclei two broken pairs (4qp) 254No152 102 one broken pair (2qp) fully paired g.s. 51 s

22. K isomers in one of the heaviest deformed nuclei two broken pairs (4qp) 254No152 102 one broken pair (2qp) 0.02(1)% fission branch identified by Hessberger et al. [EPJA43 (2010) 55] fully paired g.s. 51 s (0.2% fission)

23. fissioning K isomer in 256Fm

24. fissioning K isomer in 256Fm 2 delayed- fission events observed => fission half-life ~1 ms (~2 µs expected) 70 ns 158 m (92% SF)

25. fissioning K isomer in 256Fm 2 delayed- fission events observed => fission half-life ~1 ms (~2 µs expected) configuration-constrained barrier calculation Xu et al. PRL 92 (2004)252501 70 ns isomer including γ and β4 variations g.s.

26. fissioning K isomer in 250No

27. fissioning K isomer in 250No fission events 40 µs isomer 4 µs g.s.

28. fissioning K isomer in 250No fission events 40 µs isomer 4 µs g.s. But is there direct fission from the isomer?

29. fissioning K isomer in 250No configuration-constrained barrier calculation Xu et al. to be published fission events γeffect β3effect 40 µs isomer 4 µs g.s.

30. fission isomers "super-deformed" Polikanov et al. 1962

31. fission isomers "super-deformed" Bjørnholm and Lynn, Rev. Mod. Phys. 52 (1980) 725 K = 0

32. fission isomers "super-deformed" Bjørnholm and Lynn, Rev. Mod. Phys. 52 (1980) 725 K ~ 8 K = 0

33. fission isomers 1969

34. fission isomers 1969

35. fission isomers 238Pu 2nd isomer at ~1 MeV Limkilde and Sletten, NPA199 (1973) 504

36. fission isomers (even-even nuclides) high K T1/2 (ns) K = 0 236Pu 238Pu 242Pu 240Cm 242Cm data from Bjørnholm and Lynn, Rev. Mod. Phys. 52 (1980) 725

37. configuration-constrainedpotential-energy-surface calculationsin the second well Liu, Xu, Sun, Walker and Wyss, Eur Phys J A47 (2011) 135 K = 0 (40 ps) 236Pu K = 8 (34 ns) half-lives are experimental values [Bjørnholm and Lynn, Rev. Mod. Phys. 52 (1980) 725]

38. summary Isomers can provide extra “stability”, but the fission mode is poorly understood.

39. summary Isomers can provide extra “stability”, but the fission mode is poorly understood. "normal-deformed" K isomers: 256Fm, K = 7, 70 ns: 2 fission events 250No, K = (6), 40 µs: but does the isomer fission? 254No, K = 8, 275 ms: 2±1 in 104 fission branch "super-deformed" K isomers: several examples, but no spectroscopic information provisional conclusion: high-K isomers => large fission inhibition special thanks to Furong Xu and Hongliang Liu (Peking University)

41. Broken-pair isomers with enhanced stability odd-A nuclei involving broken-pair excitations: 211Po (516 ms g.s., 25 s isomer) α decay 217Ac (70 ns g.s., 700 ns isomer) α decay 177Lu (7 d g.s., 160 d isomer) β decay 187Ta (2 m g.s., >5 m isomer) β decay odd-odd nuclei involving broken-pair excitations: 94Ag (37 ms g.s., 300 ms isomer) β decay even-even nuclei involvingbroken pair excitations: 212Po (300 ns g.s., 45 s isomer) α decay 270Ds (0.1 ms g.s., 6 ms isomer) α decay 250No (4 µs g.s., 40 µs isomer) fission