NaI detector nonlinearity for PGNAA applications

The nonlinearity of a 6″×6″ NaI detector used for PGNAA has been measured. The prompt γ-rays from pure element samples mixed with graphite were used for this determination in the range from 1.712 to 10.829 MeV and γ-rays from radioisotope sources were used in the range from 0.356 to 1.333 MeV. A surprising result is that the pulse height per unit absorbed energy is slightly higher from single escape as opposed to full energy peaks. One explanation for this is that the light production efficiency for positrons may be different than that for electrons. By interpolation a value for the coincidence spectral full energy peak exhibited by NaI detectors that are neutron activated is obtained. The value reported is 6.885 MeV which is slightly higher than the cross section weighted value of 6.834 MeV. (The reported SN values for I and Na are 6.826 and 6.959 MeV, respectively.)