Correction factors required for quantitative large volume INAA

Extension of current INAA practice to accommodate large volume samples, of the order of 0.1 to 1 m3, requires consideration of the need for correction factors that are usually negligible for most small sample analyses. Direct INAA of large samples can be advantageous for materials that are not homogeneous such as contaminated soil, industrial raw materials or solid wastes. Large sample INAA can be adapted for the screening of such materials in situations when representative small samples would be difficult or tedious to obtain and analyze. However, appropriate corrections are required to take into account neutron flux attenuation and gradients within large samples as well as absorption losses of emerging γ-rays and extended counting geometry. Thermal neutron flux attenuation over several cm within a SLOWPOKE reflector site was measured, using monitors, to amount to 0.84 of incident flux and the quantitative sample self-shielding factors for cylindrical C12H22O11 samples, were modeled within ± 3% through adaptation of published values. Gamma-ray attenuation and variation in counting geometry over sample dimensions were experimentally determined and compared to calculated correction factors. Their computation was based on sample-detector geometry and utilized linear γ-attenuation coefficients of sample matrix and air. Similarly, agreement of large sample SRM's measured concentrations with certified values was within < 5%, thus validating the use of these methods and their future extension to large volume sample INAA. (author)