Neutron–gamma discrimination of CsI(Na) crystals for dark matter searches

The luminescent properties of CsI(Na) crystals are studied in this report. Using a TDS3054C oscilloscope with a sampling frequency of 5 GS/s, we find out that nuclear recoil signals are dominated by a very fast light pulse with a decay time of ∼20 ns, while γ-ray signals have a decay time of ∼600 ns. The study of n/γ separation shows that the rejection factor can reach an order of 10−7 with a signal efficiency of more than 80% at an equivalent electron recoil energy of 20 keV or more, and the quality factors are ∼10 times smaller than that for CsI(Tl) crystals in the corresponding energy region. It is suggested that the wavelength of nuclear recoil and γ-ray signals is also different. Such a property makes CsI(Na) an ideal candidate for dark matter searches.