A dual modality gas ionization detector for differentiating neutron attenuating material from high x-ray attenuating material in an object. The dual modality gas ionization detector uses .sup.3 He and Xe gases pressurized in an ionization chamber to detect neutrons x-rays or gamma rays. The dual modality gas ionization detector is used in an inspection system for detecting, characterizing, or differentiating small amounts of neutron attenuating materials in objects composed primarily of materials of large attenuating x-ray material.