Determination of uranium contents in different rock samples by neutron activation analysis

Geological samples are irradiated for 7-10 hrs at Apsara reactor with low energy thermal neutron flux of 1012 n cm-2s-1 for the determination of uranium by Neutron Activation Analysis(NAA) using HpGe/Genie-2000 gamma spectroscopy 238U and 235U results in 239Np and 132Te as product nuclides respectively due to (n, a-tilde) reactions. Both instrumental neutron activation analysis (INAA) and radiochemical neutron activation analysis(RNAA) techniques have been followed. 239Np peaks at 228 keV and 278 keV have been measured by INAA and 132Te peak at 228 keV was measured after radiochemical separation from 239Np isotope. NAA results of six samples of granitic composition from Nalagonda area, AP, were closely matching for uranium of two different laboratories. Uranium estimation has also been carried out in another thirty two geological samples of varied nature which were already analysed for eight Rare Earth Elements(REE)-(La, Ce, Nd, Sm, Eu, Tb, Yb and Lu) and six trace elements (Th, Sc, Co, Ta, Hf and Cr). It has been observed that uranium content is more in those rocks which have usually high total REE, Th, Hf and Ta, (Hf or Ta) due to similar ionic radii, geochemical associations and immobilisation of uranium in reducing environments indicated by negative europium anomaly. High uranium contents are found in granitic rocks from Nalagonda area, AP containing high total REE and negative europium anomalies. Areas like Tangripara, Orissa, Karbi-Anglong, Assam need further uranium investigation since anomalous uranium values of 210 ppm (comenditic ignispunite) and 345 ppm (volcanic tuff) respectively are observed. More uranium analysis of rock samples of acidic rock, pegmatite, metamorphic, sedimentary rocks are required for potential uranium occurrences in Indian subcontinent. (author)