  The responds of different common alkali halide crystals to alpha-rays and
gamma-rays are tested in our research. It is found that only CsI(Na) crystals
have significantly different waveforms between alpha and gamma scintillations,
while others have not this phenomena. It is suggested that the fast light of
CsI(Na) crystals arises from the recombination of free electrons with
self-trapped holes of the host crystal CsI. Self-absorption limits the emission
of fast light of CsI(Tl) and NaI(Tl) crystals.
