  We have studied the response of Tl-doped and Na-doped CsI crystals to nuclear
recoils and $\gamma$'s below 10 keV. The response of CsI crystals to nuclear
recoil was studied with mono-energetic neutrons produced by the
$^3$H(p,n)$^3$He reaction. This was compared to the response to Compton
electrons scattered by 662 keV $\gamma$-ray. Pulse shape discrimination between
the response to these $\gamma$'s and nuclear recoils was studied, and quality
factors were estimated. The quenching factors for nuclear recoils were derived
for both CsI(Na) and CsI(Tl) crystals.
