A muon is similar to an electron, but heavier and unstable. Muons are created in the atmosphere by cosmic rays (high-energy particles from outer space) colliding with air molecules. They arrive at the Earth's surface at a rate of one per square centimetre per minute. Crucially for Dr Morris's and Dr Chartrand's idea, they are good at penetrating metallic objects such as lorries, and yet also tend to be deflected from their paths by heavy atomic nuclei such as those of uranium and plutonium.
Matt said we can use mouns to scan trucks and containers and tell within 20 seconds if there is radioactive stuff inside. Unlike x-rays nothing dangerous about using mouns as they're naturally occurring and surround us (may be more accurate to say they go through us) all the time.