  We have measured the scattering strength of charged impurities on a
semiconducting single-walled carbon nanotube with known chirality. The
resistivity of the nanotube is measured as a function of the density of
adsorbed potassium atoms, enabling the determination of the resistance added by
an individual potassium atom. Holes are scattered 37 times more efficiently
than electrons by an adsorbed potassium atom. The determined scattering
strength is used to reveal the spatial extent and depth of the scattering
potential for potassium, a model Coulomb adsorbate. Our result represents an
essential experimental input to understand adsorbate-induced scattering and
provides a crucial step for paving the way to rational design of nanotube-based
sensors.
