  Relativistic millicharged particles ($\chi_q$) have been proposed in various
extensions to the Standard Model of particle physics. We consider the scenarios
where they are produced at nuclear reactor core and via interactions of
cosmic-rays with the earth's atmosphere. Millicharged particles could also be
candidates for dark matter, and become relativistic through acceleration by
supernova explosion shock waves. The atomic ionization cross section of
$\chi_q$ with matter are derived with the equivalent photon approximation.
Smoking-gun signatures with significant enhancement in the differential cross
section are identified. New limits on the mass and charge of $\chi_q$ are
derived, using data taken with a point-contact germanium detector with 500g
mass functioning at an energy threshold of 300~eV at the Kuo-Sheng Reactor
Neutrino Laboratory.
