Collimated scanning towards 3D-INAA of inhomogeneous large samples

Kilogram-size samples can be analysed via Large Sample Instrumental Neutron Activation Analysis (LS-INAA), a method developed for samples which are in homogeneous at the scale of a small sampling portion but which are assumed to be macroscopically homogeneous. However, extreme inhomogeneities affecti ng the total gamma-ray detection efficiency, may severely affect the qualit y of the analysis. A method has been developed to monitor large activated s amples for the presence of such extreme inhomogeneities. In addition, a met hod has been developed for the determination of the spatial distribution of the induced radioactivity. These methods have been tested using a cylindri cal phantom, simulating a large activated sample, with a local inhomogeneit y. With a collimated detector, the activity was measured stepwise at 2 cm h eight intervals and 120 degrees rotation intervals, resulting in 150 gamma- ray spectra at 45 seconds per spectrum. The activity distribution is retrie ved from the measured spectra, employing a linear least squares matrix inve rsion of the detection efficiency matrix. This matrix is based on Monte Car lo computed detector responses and experimentally determined transmission c oefficients. It is concluded that the proposed method can be employed in a system for 3D-LNAA of large samples.