Efficiency and directional effects in the detection of low-energy recoil nuclei in a NaI(Tl) single crystal

The pulse shape for low-energy 127I recoils in a NaI(Tl) single crystal was investigated as a function of the crystal orientation, using elastic neutron scattering. For comparison, the pulse shape was also studied for Compton-scattered electrons at small pulse heights. In addition, relative and absolute yields were measured for the elastic scattering of neutrons from 23Na and 127I. Large deficits were observed in the count rates, indicating detection efficiencies for the recoiling nuclei which are much smaller than 100%.