  We present measurements of the neutron-capture elements Rb and Pb in five
giant stars of the globular cluster NGC 6752 and Pb measurements in four giants
of the globular cluster M 13. The abundances were derived by comparing
synthetic spectra with high resolution, high signal-to-noise ratio spectra
obtained using HDS on the Subaru telescope and MIKE on the Magellan telescope.
The program stars span the range of the O-Al abundance variation. In NGC 6752,
the mean abundances are [Rb/Fe] = -0.17 +/- 0.06 (sigma = 0.14), [Rb/Zr] =
-0.12 +/- 0.06 (sigma = 0.13), and [Pb/Fe] = -0.17 +/- 0.04 (sigma = 0.08). In
M 13 the mean abundance is [Pb/Fe] = -0.28 +/- 0.03 (sigma = 0.06). Within the
measurement uncertainties, we find no evidence for a star-to-star variation for
either Rb or Pb within these clusters. None of the abundance ratios [Rb/Fe],
[Rb/Zr], or [Pb/Fe] are correlated with the Al abundance. NGC 6752 may have
slightly lower abundances of [Rb/Fe] and [Rb/Zr] compared to the small sample
of field stars at the same metallicity. For M 13 and NGC 6752 the Pb abundances
are in accord with predictions from a Galactic chemical evolution model. If
metal-poor intermediate-mass asymptotic giant branch stars did produce the
globular cluster abundance anomalies, then such stars do not synthesize
significant quantities of Rb or Pb. Alternatively, if such stars do synthesize
large amounts of Rb or Pb, then they are not responsible for the abundance
anomalies seen in globular clusters.
