A gamma spectrometric method fordirect measurement of uranium contents in samples by rege detector

In this thesis, a gamma spectrometric method is described for the direct "absolute" determination of uranium contents in solid samples. To obtain the exact information about the measurement of 238U in samples by gamma-ray spectroscopy, it is essential that any one of the daughters of 238U should exist in radioactive equilibrium with 238U. This condition is mainly fulfilled by one of the first two daughters, namely, 234Th (24.1 d) and 234mPa(1.17 min) because these daughters are very short-lived compared with 238U. Therefore, the present analytical method employs direct determination of 238U via the 63.3 keV gamma emission from 234Th and via the 1001 keV gamma emission from 234mPa. A gamma-ray spectrometer with a reverse-electrode high pure Germanium(REGe) detector which has high efficiencies for low energy gamma rays(<120 keV) has been used for the measurements of uranium concentrations in samples. The absolute photopeak efficiency data in the range of 40 keV to 2000 keV for the detector used is measured using a powder radioactive standard source. Only a small amount, about 8 g of powdered the certified uranium samples filled into the tubes, are counted for the the time predetermined between 75 000 s and 125 000 s. The measured activities of 238U of the samples via the 63.3 keV (234Th ) and via the 1001 keV (234mPa) are corrected for self-absorption effects due to changes in composition of standard reference materials in a given sample-detector geometry. The self-absorption factors applied to the 46.5 keV(210Pb) peak and the 63.3 keV(234Th) peak results ranged from 10.5% to 33.1% and from 4.9% to 18.4%, respectively. For the 1001 keV(234mPa) gamma emission, the necessary self-absorption factors varied between 0.1% and 3.9%. The results obtained from the 46.5 keV of 210Pb being far distant member of 238U and those of the 63.3 keV from 234Th are used to check whether the equilibrium exists in the uranium samples. The results of 238U via the 63.3 keV peak of 234Th for the uranium standards agreed to within 0.1 to 6.9% with the certified values of 238U in the samples with uranium concentration ranging from 0.022±001 to 1.02±0.01 wt%. The results of 238U via the 1001 keV peak of 234mPa agreed well to within 0.01 to 4.7% with the certified activity values of 238U of the same samples. When the measurements of the uranium are carried out in a REGe detector system, the results indicate that the uranium concentrations in the samples can be determined to the average 7% error by using the 63.3 keV(234Th) gamma emission with appropriate corrections for self-absorption effects. The 1001 keV peak of 234mpa gives also accurate results for the determination of 238U in the same samples and the need for self absorption correction for the 1001 keV emission seems to be negligible, except in case of thicker samples than those with thickness of about 1.5 cm. Key Words : Uranium, concentration, measurement, gamma-ray spectrometry, self absorption