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

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.

ground state isomeric state α β γ τmτm τ g fission p p

ground state isomeric state α β γ τmτm τ g fission p p In what circumstances can an isomer be “more stable” than its ground state?

aspects of proton (and neutron) decay

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

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

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

Peker et al: p and n decay 1971

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

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

212 Po α decay s isomer at 2.9 MeV 0.3 μs ground state 208 Pb + α α 100% (18 + ) 0+0+

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

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

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

data from ANU, Canberra (i.e. ≤1 fission in 10 8 decays). 212 Fr spin-34 isomer at 8.5 MeV

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

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

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

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

K isomers in one of the heaviest deformed nuclei 254 No one broken pair (2qp) fully paired g.s. two broken pairs (4qp) 51 s

K isomers in one of the heaviest deformed nuclei 254 No one broken pair (2qp) fully paired g.s. two broken pairs (4qp) 51 s (0.2% fission) 0.02(1)% fission branch identified by Hessberger et al. [EPJA43 (2010) 55]

fissioning K isomer in 256 Fm

2 delayed- fission events observed => fission half-life ~1 ms (~2 µs expected) 158 m (92% SF) 70 ns

fissioning K isomer in 256 Fm 2 delayed- fission events observed => fission half-life ~1 ms (~2 µs expected) 70 ns g.s. isomer configuration-constrained barrier calculation Xu et al. PRL 92 (2004) including γ and β 4 variations

fissioning K isomer in 250 No

40 µs 4 µs g.s. isomer fission events

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

fissioning K isomer in 250 No 40 µs 4 µs g.s. isomer fission events configuration-constrained barrier calculation Xu et al. to be published γ effect β 3 effect

fission isomers Polikanov et al "super-deformed"

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

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

1969 fission isomers

1969 fission isomers

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

fission isomers (even-even nuclides) 236Pu 238Pu 242Pu 240Cm 242Cm T 1/2 (ns) high K K = 0 data from Bjørnholm and Lynn, Rev. Mod. Phys. 52 (1980) 725

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

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

summary "normal-deformed" K isomers: 256 Fm, K = 7, 70 ns: 2 fission events 250 No, K = (6), 40 µs: but does the isomer fission? 254 No, K = 8, 275 ms: 2±1 in 10 4 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) Isomers can provide extra “stability”, but the fission mode is poorly understood.

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