NaI detector nonlinearity for PGNAA applications

The nonlinearity of a 6 " x 6 " NaI detector used for PGNAA has been measur ed. The prompt gamma-rays from pure element samples mixed with graphite wer e used for this determination in the range from 1.712 to 10.829 MeV and gam ma-rays from radioisotope sources were used in the range from 0.356 to 1.33 3 MeV. A surprising result is that the pulse height per unit absorbed energ y is slightly higher from single escape as opposed to full energy peaks. On e explanation for this is that the light production efficiency for positron s may be different than that for electrons. By interpolation a value for th e coincidence spectral full energy peak exhibited by NaI detectors that are neutron activated is obtained. The value reported is 6.885 MeV which is sl ightly higher than the cross section weighted value of 6.834 MeV. (The repo rted S-N values for I and Na are 6.826 and 6.959 MeV, respectively.) (C) 19 99 Elsevier Science Ltd. All rights reserved.