Eur. Phys. J. Special Topics 150, 121–122 (2007) c  EDP Sciences, Springer-Verlag 2007 DOI: 10.1140/epjst/e2007-00282-8 T HE EUROPEAN P HYSICAL JOURNAL SPECIAL TOPICS Collectivity in neutron-deficient Pb and Po nuclei T. Grahn 1,3 , A. Dewald 2 , O. M¨ oller 2 , R. Julin 1 , C.W. Beausang 5, a , S. Christen 2 , I.G. Darby 1,3 , S. Eeckhaudt 1 , P.T. Greenlees 1 , A. G¨ orgen 8 , K. Helariutta 4 , J. Jolie 2 , H. Kettunen 1 , T. Kr¨ oll 6 , R. Kr¨ ucken 6 , P. Jones 1 , S. Juutinen 1 , Y. Le Coz 8 , M. Leino 1 , A.-P. Lepp¨ anen 1, b , P. Maierbeck 6 , B. Melon 2 , D.A. Meyer 5, c , P. Nieminen 1, d , M. Nyman 1 , R. Page 3 , J. Pakarinen 1,3 , P. Petkov 7 , P. Rahkila 1 , B. Saha 2 , M. Sandzelius 1, e , J. Sar´ en 1 , C. Scholey 1 , and J. Uusitalo 1 1 Department of Physics, University of Jyv¨askyl¨a, PL 35, 40014 University of Jyv¨ askyl¨a,Finland 2 Institut f¨ ur Kerphysik, Universit¨ at zu K¨ oln, Z¨ ulpicher Str. 77, 50937 K¨ oln, Germany 3 Department of Physics, Oliver Lodge Laboratory, University of Liverpool, Liverpool L69 7ZE, UK 4 Laboratory of Radiochemistry, Department of Chemistry, PL 55, 00014 University of Helsinki, Finland 5 Wright Nuclear Structure Laboratory, Yale University, New Haven, CT 06520, USA 6 Physik-Department E12, TU M¨ unchen, 85748 Garching, Germany 7 Institute for Nuclear Research and Nuclear Energy, Sofia, Bulgaria 8 CEA Saclay DSM/DAPNIA/SPhN, 91191 Gif-sur-Yvette Cedex, France Received: January 31, 2007 Abstract. Lifetimes of low-lying yrast states in 186 Pb and 194 Po have been determined by employing, for the first time, the recoil-decay tagging technique in recoil distance Doppler-shift lifetime measurements. In addition, lifetime measurements of prolate states in 188 Pb up to the 8 + state were carried out using the recoil gating method. The deformation parameters |β2| =0.29(5) and |β2| =0.17(3) for the prolate ( 186,188 Pb) and the oblate ( 194 Po) bands, respectively, have been extracted from the measured lifetimes. PACS. 21.10.Tg - 27.70.+q – 27.80.+w The origin of shape coexistence observed in nuclei around Z = 82 close to the proton drip line is a topical chal- lenge in both theoretical and experimental nuclear struc- ture research. In the neutron mid-shell nucleus 186 Pb, the first two excited states above the spherical ground state are assigned as I π =0 + states [1]. In calculations based on the deformed mean field they appear as oblate and prolate minima [2,3]. The yrast line above the spheri- cal ground states in 186,188 Pb is formed by a collective band associated with the prolate minimum. Recently, can- didates for collective non-yrast bands built on the coex- isting oblate minimum have been observed in 186,188 Pb [4,5]. In neutron-deficient Po isotopes similar intruder structures associated with oblate shape cross the yrast line of nearly spherical states reaching the ground state in 192 Po [6]. Calculations predict that the deformation associated with the prolate shape of mid-shell Pb and Po nuclei is a Present address: University of Richmond, VA, USA b Present address: STUK, Rovaniemi, Finland c Present address: Rhodes College, Memphis, TN, USA d Present address: Department of Nuclear Physics, ANU, Canberra, ACT 0200, Australia e Present address: Department of Physics, KTH, 10691 Stockholm, Sweden larger than that for the oblate one. Experimentally, the degree of deformation is in many cases based on mea- sured moments of inertia, which is in general larger for prolate than oblate bands. However, when level lifetimes are known, the absolute transition probabilities provide a more direct measure of collectivity and allow fundamen- tal questions such as the degree of mixing of shapes as a function of spin to be addressed. To verify these questions, absolute transition probabilities are needed. Recoil Distance Doppler-Shift (RDDS) lifetime mea- surements have been carried out for the prolate yrast states up to I π =8 + in 186,188 Pb and for the oblate yrast states up to I π =4 + in 194 Po at the Accelerator Labora- tory of the University of Jyv¨ askyl¨ a. The K¨ oln plunger de- vice has been combined with the JUROGAM Ge-detector array and the RITU gas-filled separator [8]. The stan- dard stopper foil of the plunger device was replaced by a degrader foil in order to allow the fusion-evaporation residues to recoil into RITU [9]. Experimental details and results are presented in [7]. Assuming a quadrupole de- formed rotating nucleus with uniform charge distribution, a quadrupole deformation parameter |β 2 | for the given transition can be extracted from the measured lifetimes. For the prolate bands in 186,188 Pb and oblate band in 194 Po an average experimental values of |β 2 | =0.29(5) and |β 2 | =0.17(3), respectively, are obtained.