I can't really answer your question, but just wanted to clarify your 'radiation is not matter' statement.

3 of the 5 major types of radiation are in fact matter. Alpha, beta and neutron radiation are all particles of matter. Only Gamma and X-Rays are an electromagnetic wave.

As others have said, often it's that the radioactive material itself has 'stuck' to things, although Neutron radiation does have the ability to make make other materials radioactive.

http://hps.org/publicinformation/ate/q12968.html