Development and fabrication of mini gamma spectrometric probe for measurements of U enrichment in fuel clusters

The aim of this project is to develop a technique, based on gamma spectral analysis, for the determination of the U 235/U 238 ratio for the individual rods in a fuel cluster. Among the various compounds of high Z elements with sufficient high energy gap, CdTe was chosen as the most promising for a study. Preliminary tests with two sample detectors from TYCO (7 mm diam. x 2 mm) and one detector from Hughes (kindly lent by IAEA, CdTe detector No.1) gave rise to many questions. First of all the efficiency was much lower than expected and as it was very dependent on amplifier time constants there was evidence of insufficient hole transport at the field strength giving a reasonable leakage current. Noise considerations prevented the use of the very large time constants dictated by the long hole transport time. Although the resolution was quite acceptable for all the detectors (6-10 keV FWHM, depending on amplifier time constants) the background was too high and some irregularities in the continuum could not be explained. Also the time dependence of the leakage current after the application of bias voltage gave rise to suspicions about polarization effects at the contacts. Due to the high γ-photon rate from U 235 at 185 keV and the high photo cross-section for uranium at this energy an X-ray fluorescent spectrum is readily generated in the rods. With the use of this spectrum as representative of the U 235 content the use of the CdTe detector as a photo detector at 185 keV is avoided, to the benefit of the efficiency which is quite reasonable at the X-ray energies (main peak about 100 keV), even with normal amplifier time constants. Incomplete Compton absorption in the CdTe detector of the high energy Pa 234m (0,765 and 1.001 MeV) γ photons representing U238 is used as a main geometry size reference. Very encouraging results for the determination of the U 235/U 238 ratio in fuel element clusters have been obtained with this measurement procedure