  Cold atom ion sources have been developed and commercialized as alternative
sources for focused ion beams (FIB). So far, applications and related research
have not been widely reported. In this paper, a prototype rubidium FIB is used
to study the irradiation damage of 8.5 keV beam energy Rb$^+$ ions on silicon
to examine the suitability of rubidium for nanomachining applications.
Transmission electron microscopy combined with energy dispersive X-ray
spectroscopy is applied to silicon samples irradiated by different doses of
rubidium ions. The experimental results show a duplex damage layer consisting
of an outer layer of oxidation without Rb and an inner layer containing Rb
mostly at the interface to the underlying Si substrate. The steady-state damage
layer is measured to be $23.2(\pm 0.3)$ nm thick with a rubidium staining level
of $7(\pm1)$ atomic percentage.
