These cosmic ray muon particles strike Earth at the rate of 10,000 per square metre every minute. By tracking the muons, the scientists can see through lead, steel and other heavy shielding that might be used to mask a radioactive source.
When cosmic rays hit the upper atmosphere, they produce muons, which are charged particles similar to electrons. Their electric charges make them very easy to detect and they can penetrate heavy metal and thick rock. Indeed, with an average energy of three billion electron volts, most muons can penetrate about 1.8m of lead.
The researchers say the X-ray and gamma-ray detectors currently used at US borders are inefficient for detecting nuclear materials shielded with lead or steel.
Muons are also harmless, unlike X-rays or gamma-rays…
(Which, of course, turn you into the Hulk. Have people learned nothing from the plight of Bill Bixby?)