Core-excited states and core-polarization effects in At-210 and At-211

Excited states in the nuclei At-210 and At-211 have been studied using Pb-2 08(Li-7, xn) reactions. Detailed spectroscopy of levels up to 30 (h) over b ar has been achieved. New isomeric levels arising from core-excited states were observed, with the highest-lying isomers attributed to a coupling of t he pi [h(9/2)(2)i(13/2)] configuration to double neutron-particle-hole exci tations. Clear relationships between the states observed in At-210 and At-2 11 were identified. Semiempirical shell-model calculations reproduce very w ell the yrast states in both nuclei. Uncertainty in the modeling of core-po larization was seen as a limiting factor in the calculation of accurate lev el energies for core-excited states. (C) 2001 Elsevier Science B.V. All rig hts reserved.