"It begins with shining an infrared laser beam near a radioactive source"
Um, sorry, but if your method of finding a radioactive source starts with targeting it then your method doesn't work if you don't know where it is.
Geiger counters work because you just walk around and detect radioactivity. The laser method would mean you have to scan every individual package or crate, and that would only work if every incoming/outgoing crate passes through the same point. I don't think that's going to happen at a large port, or any port for that matter.
Obviously, I know nothing about ports (for ships, that is - for firewalls I know more).