  Scintillation crystals are commonly used for direct detection of weakly
interacting massive particles (WIMPs), which are suitable candidates for a
particle dark matter. It is well known that the scintillation light yields are
different for electron recoil and nuclear recoil. To calibrate the energies of
WIMP-induced nuclear recoil signals, the quenching factor (QF) needs to be
measured, which is the light yield ratio of the nuclear recoil to electron
recoil. Measurements of the QFs for Na and I recoils in a small (2 cm x 2 cm x
1.5 cm) NaI(Tl) crystal are performed with 2.43-MeV mono-energetic neutrons
generated by deuteron-deuteron fusion. Depending on the scattering angle of the
neutrons, the energies of the recoiled ions vary in the range of 9 - 152 keV
for Na and 19 - 75 keV for I. The QFs of Na are measured at 9 points with
values in the range of 10 - 23 % while those of I are measured at 4 points with
values in the range of 4 - 6 %.
