THE CALIBRATION OF A LOW-ENERGY PHOTON GERMANIUM DETECTOR FOR USE IN THE NAA K0 STANDARDIZATION METHOD

To extend the NAA k0 method to LEPD counting, procedures were elaborated for determining the full-energy peak detection efficiency epsilon-p and the peak-to-total ratio P/T (yielding the total efficiency epsilon-t = epsilon-p/(P/T). epsilon-p is needed for concentration calculation. epsilon-p and epsilon-t are needed to correct for true coincidences (including KX and LX lines) in case of small source-detector separation. First the "reference" efficiency curve (log-epsilon-p,ref vs log E) was determined with calibrated point sources (from 4.65 keV (Ce-139) to 383.9 keV (Ba-133)) measured at approximately 16 cm distance from the LEPD. The accuracy is approximately 1-2% for E > 11 keV, and approximately 5% for E < 11 keV. In view of actual analyses, epsilon-p,ref was converted to epsilon-p,geo (i.e. at a small distance from the LEPD) based on the calculation of the effective solid angles OMEGABAR with the program SOLANG. The accuracy of this, checked with practically coincidence-free lines of point sources measured close to the LEPD, was approximately 5% on the average. After identifying the origin of all the pulses in the spectra, a P/T curve (log P/T vs log E) was constructed from results obtained for the most prominent lines of some nine sources (from 5.9 keV (55-Fe) to 320.1 keV (Cr-51)). P/T was found to be practically independent of the source-detector separation. Its accuracy of approximately 5% is satisfactory in view of the large error reduction. The results and conclusions reached here were obtained from experiments with two different LEPDs at the INW, Gent and the LNETI, Sacavem.