Shape coexistence in the neutron- deficient Pb region: Coulomb excitation at REX-ISOLDE Liam Gaffney 1,2 Nele Kesteloot 2,3 1 University of the West of Scotland, Paisley, UK 2 KU Leuven, Belgium 3 SCKCEN, Mol, Belgium

Shape coexistence Different types of deformation at low excitation energy Interplay between two opposing tendencies o Stabilizing effect of closed shells o Residual proton-neutron interaction Heyde and Wood, Review of Modern Physics (2011) Andreyev et al Nature 405:430 (2000) Evidence across the light lead region Lack of experimental information o Nature of deformation o Degree of mixing o Also appears in other regions of the nuclear chart... o Campaign to characterise properties of shape coexistence using complementary experimental probes T.E. Cocolios et al, Phys. Rev. Lett. (2011) Pb

Pb region: Coulex REX-ISOLDE Pb Rn Po Hg Hg – Leuven/Liverpool N. Bree et al. PRL 112 (2014) Pb Pb – Jyväskylä J. Pakarinen (in analysis) 208,210 Rn, 206 Po – Jyväskylä T. Grahn (in preparation) 202,204 Rn – York/Leuven Po – Leuven

REX-ISOLDE 1.4 GeV p + primary target RILIS (Po) HRS REXTRAP + EBIS Post acceleration up to 3 MeV/u Miniball GPS VADIS (Rn) 4

REX-ISOLDE A/Q ~< MeV/u Coulex target ~2 mg/cm 2

Miniball: Coulex set-up Particle ID in a Double- Sided Si Strip Detector. Event-by-event Doppler correction. 17˚ < θ lab < 54˚ Array of HPGe of 8 triple clusters 6-fold segmentation for positioning ε > 7% for 1.3MeV γ-rays N. Warr et al., EPJ 49 (2013) 6

7 Target eg: 109 Ag Projectile eg: 202 Rn Coulomb excitation : Particle-γ 2-particle coincidences considered Clean kinematics Particle – γ time used for random subtraction

8 γ-ray spectra Doppler corrected for projectile/target 677 keV 2 +  Po 556 keV 2 +  Pd 202 Po on 104 Pd 556 keV 2 +  Pd 196 Po on 104 Pd 463 keV 2 +  Po 202 Rn on 109 Ag 204 Rn on 109 Ag

Gosia results: Radon (Z=84) Rn on 109 Ag: 5 angular ranges 9

QtQt Comparison to theory 202 Rn [BMF] J.M. Yao, M. Bender, and P.-H. Heenen, Phys. Rev. C 87, (2013). BMF = Beyond-mean-field with Skyrme force Q s (2 + )

Comparison to theory 202 Rn IBM = Interacting boson model + configuration mixing GBH = General Bohr Hamiltonian collective model [BMF] J.M. Yao, M. Bender, and P.-H. Heenen, Phys. Rev. C 87, (2013). [IBM] J.E. García-Ramos and K. Heyde, (in preparation)[GBH] L. Próchniak and S.G. Rohoziński, J. Phys. G 36, (2009). BMF = Beyond-mean-field with Skyrme force QtQt QtQt

M.D. Seliverstov et al, Phys. Lett. B 719, 362 (2013). T.E. Cocolios et al, Phys. Rev. Lett. 106, (2011). Extracting deformation of 0 + states 12 Po Validity of deformation parameters extracted with complementary techniques Rn T. Grahn et al, Phys. Rev. Lett. 97, (2006).

Two-state mixing calculations on Po Po Use VMI to fit intruder band in Po Extrapolate to 2+ and 0+ energies Aα0²α0²α2²α2² 19412%29% 19685%50% 19894%69%/31% 20097%92%/8% 20299%88% V = 200keV unperturbed ME2’s - Po: 1.1eb 1.5eb -0.4eb 1.8eb 0+I0+I 2+I2+I 0 + II 2 + II 1.2 eb 3.3 eb 1.8 eb -4.0 eb 0+I0+I 2+I2+I 0 + II 2 + II unperturbed ME2’s - Hg: Po 198 Po 200 Po 202 Po

Summary Coulomb excitation performed on Po and Rn beams. Large number of electromagnetic matrix elements extracted. Systematic trends of B(E2; → ) established. Onset of deformation towards mid-shell consistent with measurements. Comparison to BMF calculations (also IBM and GBH) Sensitive to Q s in 200 Po, but not so much in other isotopes. Po data interpreted in terms of two-level mixing calculations o Similar oblate structure to that deduced in Hg is found Are we observing the same prolate/oblate structures throughout this region? Thank you! 14

Interpretation with two-level mixing model α 0+ 2 α 2+ 2 α Hg92%29%3% 184 Hg95%51%4% 186 Hg98%90%7% 188 Hg99%98%20% “concealed” configuration mixing of the states of Hg un-mixed ME2’s: 1.2 eb 3.3 eb 1.8 eb -4.0 eb 0+I0+I 2+I2+I 0 + II 2 + II 182 Hg 184 Hg 186 Hg 188 Hg 15 L.P. Gaffney et al, PRC 89, (2014) N. Bree et al, PRL (2014)

Polonium and Radon Hints of deformation when approaching mid- shell - Energy level systematics - Mean-square charge radii

Z = 84: Po: γ -ray spectra Po on 104 Pd p γ + pp γ events, Doppler corrected for projectile/target 605 keV 2 +  Po 666 keV 2 +  Po 677 keV 2 +  Po 556 keV 2 +  Pd 556 keV 2 +  Pd 871 keV 2 +  Mo 556 keV 2 +  Pd 196 Po on 104 Pd 198 Po on 94 Mo 202 Po on 104 Pd 463 keV 2 +  Po

Po (Z = 84) isotopes 18 Transitional quadrupole momentsDiagonal quadrupole moments

Hg (Z = 80) isotopes Hg

Hg (Z = 80) isotopes Hg 184 Hg 186 Hg 188 Hg un-mixed ME2’s: 1.2 eb 3.3 eb 1.8 eb -4.0 eb 0+I0+I 2+I2+I 0 + II 2 + II

Hg (Z = 80) isotopes 21 Quadrupole sum rules Comparison to isotope shift