PHYSICAL REVIEW C 73, 044324 (2006) α-decay of the new isotope 187 Po: Probing prolate structures beyond the neutron mid-shell at N = 104 A. N. Andreyev, 1,7 S. Antalic, 2 D. Ackermann, 3,8 S. Franchoo, 4 F. P. Heßberger, 3 S. Hofmann, 3,9 M. Huyse, 5 I. Kojouharov, 3 B. Kindler, 3 P. Kuusiniemi, 3 S. R. Lesher, 5 B. Lommel, 3 R. Mann, 3 G. M¨ unzenberg, 3,8 K. Nishio, 3,10 R. D. Page, 6 J. J. Ressler, 7 B. Streicher, 2 S. Saro, 2 B. Sulignano, 3 P. Van Duppen, 5 D. Wiseman, 6 and R. Wyss 11 1 TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia, Canada V6T 2A3 2 Department of Nuclear Physics and Biophysics, Comenius University, Bratislava SK-84248, Slovakia 3 Gesellschaft f¨ ur Schwerionenforschung, Planckstrasse 1, D-64291 Darmstadt, Germany 4 IPN Orsay, F-91406 Orsay Cedex, France 5 Instituut voor Kern- en Stralingsfysica, K.U. Leuven, University of Leuven, B-3001 Leuven, Belgium 6 Department of Physics, Oliver Lodge Laboratory, University of Liverpool, Liverpool L69 7ZE, United Kingdom 7 Department of Chemistry, Simon Fraser University, Burnaby, British Columbia, Canada V5A-1S6 8 Institut fur Physik, Johannes Gutenberg-University, D-55099 Mainz, Germany 9 Physikalisches Institut, J.W. Goethe-Universit¨ at, D-60054 Frankfurt, Germany 10 Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Ibaraki 319-1195, Japan 11 Department of Physics, Royal Institute of Technology, 104 05 Stockholm, Sweden (Received 7 February 2006; published 25 April 2006) The new neutron-deficient isotope 187 Po has been identified in the complete fusion reaction 46 Ti+ 144 Sm → 187 Po+3n at the velocity filter SHIP. Striking features of the 187 Po α decay are the strongly- hindered decay to the spherical ground state and unhindered decay to a surprisingly low-lying deformed excited state at 286 keV in the daughter nucleus 183 Pb. Based on the potential energy surface calculations, the 187 Po ground state and the 286 keV excited state in 183 Pb were interpreted as being of prolate origin. The systematic deviation of the α-decay properties in the lightest odd-A Po isotopes relative to the smooth behavior in the even-A neighbors is discussed. Improved data for the decay of 187 Bi m,g were also obtained. DOI: 10.1103/PhysRevC.73.044324 PACS number(s): 23.60.+e, 27.70.+q I. INTRODUCTION In the neutron-deficient Po isotopes the richest examples of shape coexistence at low excitation energy can be found. Historically, the first work in which shape coexistence in these nuclei was discussed was the Nilsson-Strutinsky calculations by May et al. [1]. The authors suggested a gradual change of the Po ground state (g.s.) from a nearly spherical configuration around the neutron shell closure at N = 126 ( 210 Po), to an oblate configuration in the vicinity of 192 Po, with a prolate ground state expected close to and beyond the neutron mid-shell at N = 104 ( 188 Po). It is important to stress that practically all modern approaches, based both on improved Nilsson-Strutinsky methods or on self-consistent Hartree- Fock-Bogoliubov calculations, are in agreement with the earlier study by May et al., see discussion, e.g., in Refs. [2,3]. These theoretical findings are strongly supported by complementary data both from in-beam studies, see, e.g., Refs. [4,5] and particle (β and α) decay (e.g., Refs. [6–12]), which provided extensive systematics on the evolution of shape coexistence in the long sequence of 188–210 Po isotopes. However, due to low production cross sections and high background from fission, the most neutron-deficient Po nuclei cannot presently be investigated with in-beam techniques, 190 Po being the lightest Po isotope studied by this method so far [5,13] (the current cross section limit for this technique is σ ∼ 50 nb). On the other hand, α decay has proven to be a sensitive tool to study shape coexistence in nuclei, providing information on both parent and daughter states involved in the decay, see, e.g., Refs. [14,15]. Furthermore, nuclei with production cross sections in the subnanobarn region become accessible. A recent example of such work is our α-decay study of the neutron-deficient isotopes 188,189 Po (see Ref. [12] and references therein), which are presently not accessible by any other methods. A striking observation in the 189 Po α decay was that the 7532 keV g.s.→g.s. α decay to the spherical ground state in the daughter isotope 185 Pb was hindered by a factor of ∼77 (in terms of reduced α widths as defined by Rasmussen [16]) relative to the 7259 keV fine structure (f.s.) α decay to an excited state at 278 keV in 185 Pb [12]. A similar pattern, though with a lower hindrance factor (HF) of ∼12 for the 7910 keV g.s.→g.s. decay relative to the 7355 keV fine structure α decay was also observed in the neighboring isotope 188 Po [11]. Combined with the potential energy surface cal- culations, these data provided the first experimental evidence that the ground states of 188,189 Po and the excited states in their respective daughters 184,185 Pb, fed by unhindered fine structure decays, are of prolate origin, see details in Refs. [11,12]. The sphericity of the ground states in the isotopes 184,185 Pb was recently proved by the charge radii measurements [17]. The present study extends our previous work in this region, performed at the velocity filter SHIP of the GSI in Darmstadt [18,19] and reports on an α-decay study of the new isotope 187 Po. The data for the new isotopes 186 Po and 192 At, identified in the same experiment, will be discussed elsewhere [20,21]. 0556-2813/2006/73(4)/044324(8)/$23.00 044324-1 ©2006 The American Physical Society