Thermal neutron scattering and transmission experiments using geologic rock samples

Thermal neutron scattering from geologic formations is one of the underlying techniques used in oil well logging. Thorough knowledge of the scattering and absorption properties of formation rock is essential in the interpretation of neutron logs. Formation rocks often contain trace amounts of highly absorbing elements like boron and gadolinium, and trace-element analysis is very expensive. Hence, direct experimental neutron measurement techniques for macroscopic cross sections are of great value to the oil well logging industry. This paper describes the determination of these parameters using a thermal column beam of neutrons on the PULSTAR reactor at North Carolina State University. A differential scattering measurement for a Maxwellian beam is made. Transmission measurements yield total cross sections. Absorption cross sections computed from these data are compared with the measurements made by the steady neutron source technique. The experiments are also simulated by the Monte Carlo code, MCNP.