Pergamon Appl. Radiat. Isot. Vol. 48, No. 7, pp. 991-996, 1997 1997 Elsevier Soence Ltd. All rights reserved Printed m Great Britain PII: S0969-8043(9"/)00341-2 0969-8043/97 $17.00 + 0.00 Advantages of Combined IENAA and K0-factor Technique in the Determination of U and Th Concentrations in Exploration Rock Samples C. A. ADESANMI, I. A. TUBOSUN, F. A. BALOGUN and A. A. OLADIPO Centre for Energy Research and Development, Obafemi Awolowo University, lle-lfe, Nigeria (Received 25 February 1996; in rerised form 2 December 1996) The improved accuracy and precision of instrumental epithermal neutron activation analysis (IENAA), using the K0-factor technique over the conventional one-shot instrumental neutron activation analysis (|NAA), in the measurement of uranium and thorium in geological samples is demonstrated. Using the geological reference material, GS-N, detection limits of 0.4 and 0.5 ppm were obtained for U and Th, respectively, by means of the INAA method, while superior 0.1 ppm and 0.25 ppm detection limits were obtained for these elements when the IENAA/K0-factor technique was employed. The concentrations of U in three samples of this reference material were measured as 8.05 +_0.12, 8.12_+0.13 and 7.88 _+0.19 ppm by the IENAA/K0-factor method showing an improved data disperson and accuracy over the values 6.17 _+0.61, 6.39 _+0.36 and 7.31 _+0.39 ppm obtained by the INAA technique. These compare with the 8 ppm certified value for this element in this material. However, the improvement in precision and accuracy obtained for the thorium measurement is marginal. Analysis of a geological sample from Nigeria showed thorium to be more abundant than uranium in these rocks. (~ 1997 Elsevier Science Ltd Introduction There exists a wide distribution of U and Th in various rock formations of the Earth's crust. Average crustal abundance is about 2 ppm (Roger and Adams, 1967). Exploration for economical U deposits span many geological environments and varied geological ages. Previous studies, e.g. Oshin and Rahaman (1986) have shown that certain areas of Nigeria are favourable for hosting uranium deposits. Among potentially favourable areas are sedimentary formations, especially sandstones (Finch and Davis, 1985). Sandstones are also the dominant domestic source of U in the neighbouring Republic of Niger, an extension of which has been widely speculated to exist in Nigeria. It is worth noting that the success of any exploration programme depends on high quality exploration data. Drilling is an important exploration tool as it provides a third dimension to the geologic picture. In the present work, core samples collected during a U exploration drilling programme have been analysed as part of the data gathering exercise. It has been well established that U and Th are present in nearly all geological materials, but their concentrations are usually so low that a quantitative determination requires trace level analysis. A number of techniques have been tried, each with its own merits (Govindaraju and MeveUe, 1983). Mass spectrometry, one of the methods used to determine U and Th in geological materials, is rather complicated and time consuming (Morrison and Kashuba. 1969), and also needs highly skilled personnel. Delayed neutron emission is rather difficult and not very sensitive when a large amount of thorium is present with uranium (Amiel. 1962). Thermal neutron activation followed by radiochemi- cal separation is time-consuming and may have a large error due to matrix effects. The technique employed in this work involves the use of a simple, non-destructive and highly selective combined instru- mental epithermal neutron activation analysis (IENAA) and a normalization process called the K0-factor, which eliminates the rigorous standard sample preparation necessary in the conventional comparative mode of INAA. Theory The instrumental epithermal neutron activation analysis (IENAA) involves selective irradation of samples with neutrons from which the thermal component has been excluded (Chen et al., 1981), Use is made of cadmium, which has a neutron absorption cross-section such that it is effectively opaque to thermal neutrons, but transparent to 991