Any irradiation process used (X-rays, gamma-rays, electron beams) for irradiating mail, food, aerospace components, bone fragments (for bone transplants), etc. do not use particles or rays with anywhere near sufficient energies to induce radiation (i.e. make something radioactive). Thus any irradiation of mail, food, etc., cannot make the irradiated material radioactive.
A neutron field (i.e. created in a nuclear reactor) is a common way to induce radiation, a technique used for all sorts of material studies (including medical, chemical analysis, archaeological and forensic work). This is due to atomic nuclei absorbing neutrons, and some of the nuclei become unstable (i.e. there are now too many neutrons in the nuclei of some atoms). In order to regain nucleic stability, the (now unstable) nucleus must spit out something - electrons (also known as beta particles when they originate in the nucleus), an alpha particle (two neutrons and two protons together), or neutron(s). Thus the unstable neutron reaches stability again (but usually as a different nucleus) by becoming radioactive. Often the particle emissions are accompanied by energy emissions (gamma rays).
You have a good question, though - would some materials need screening out before irradiation? In particular I would be concerned about sending undeveloped film through an irradiator. Especially X-ray film (I'm sure new X-ray film is not shipped by mail). I've asked about this when passing through airports - often they x-ray all the baggage. I've never had my undeveloped rolls of holiday snaps damaged by x-rays. But I have heard that fast film (high ASA) is more susceptible to x-ray damage. If they now start x-raying mail to kill bacteria, I'd think they'd have to use higher exposures that what they use to simply take pictures of your baggage. Higher exposures would have more chance of damaging film.
Comments anyone? Any other possible hazards in irradiating mail?
From: Brian Alan Mason[SMTP:firstname.lastname@example.org]
Sent: Friday, November 02, 2001 7:31 PM
To: Adam McLean; email@example.com.McMaster.CA
Subject: Re: [cdn-nucl-l] Mail Irradiation - AECL sale of IMPELA - connection?
Would there be some danger of radiation from some of the metallic films on
cards or tinfoil used in packages?
Would the process require some pre-filtering of the mail to separate out
potential X-ray hazards, or has this already been considered?
Brian Alan Mason