A benchmark study was carried out to verify whether MCNP is useful in the design stage of a PGNAA facility for large samples
up to 1 m length and 0.15 m diameter, using a 2.54 cm diameter thermal neutron beam. For this facility neutron self-shielding
and gamma-attenuation correction methods have to be developed. The relative spatial neutron-density distributions within three
samples with different macroscopic scattering and absorption cross sections were studied in a comparison between an MCNP simulation
and an irradiation experiment using copper wires as neutron monitors. The neutron density in the sample was within statistical
agreement between experiment and simulation. Typically the relative spatial neutron-density distributions agreed to within
1%. Therefore, MCNP can be used in design studies for the development of a large sample PGNAA facility as specified.