Transmission measurement of porosity with a neutron generator in geophysics applications

Fast neutron transmission measurements, based on a stationary neutron generator, were performed to determine the volume porosity of a sample in a model experiment. The free volume in the mock-up, consisting of a cylindrical holder containing glass rods and sometimes even sand, was determined from measurements of the transmitted fast neutron flux in dry and flooded (with water) condition. In order to extract a quantitative measure of the free volume from the measurement, the neutron attenuation through the sample was modelled by the first flight approximation as well as with more advanced MCNP calculations. The experimental results show good agreement with results of the advanced Monte-Carlo calculations within the estimated error. Porosities of samples that were evaluated from the measurement using MCNP calculations for calibration show an error between 3 and 14% in comparison with the true (known) values.