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 %.
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