The aggressive doping of helium droplets (similar to10(5) atoms) with sodium vapor results in the growth of sodium clusters having up to at least 13 atoms, as determined by a Penning ionization technique. Signatures of electronic shell effects are observed in Na-8(+) cluster ions, including an odd-even intensity oscillation and an enhanced Na-9(+) peak, which is a magic number effect. The size distributions are consistent with cluster ion fragmentation rather than simply sequential pickup statistics. The dependence of ion yield on mean ionizing electron energy suggests that neutral alkali clusters are located preferentially on the droplet surfaces. (C) 2002 Elsevier Science B.V. All rights reserved.