When anthrax-tainted letters killed several people and infected hundreds of others last fall, the U.S. Postal Service immediately sought ways to prevent any repetition. One of them proved to be electron beam irradiation of the mail—the same technique used by food companies to kill E. coli bacteria in ground beef. After initially sending mail to the Titan Corp.'s SureBeam subsidiary sterilization facility in Lima, Ohio, the post office contracted to purchase eight turnkey electron beam systems for $26 million from the firm with the option to buy another 12.
Before any of the electron beam
sterilization systems could be installed, the U.S. Postal Service
collected mail and shipped it to Titan SureBeam's Lima, OH facility for
sterilization. Because electron beams can only penetrate paper to a depth
of two to three inches, one linear accelerator placed above the conveyer
sterilizes the tops of packages, while another, placed below the conveyor,
sterilizes the bottom.
Electron beam (e-beam) irradiation employs linear accelerators to speed electrons derived from an electric supply to the speed of light. Other equipment—usually magnets—make the beam move to where it's needed. The electrons hit the structure of the bacteria and rupture the DNA chain, making it unable to replicate itself, and the pathogen then dies. Because the power of the beam remains constant, the speed of the conveyor determines the dose. The faster the conveyor moves, the lower the dose.
Smaller SureBeam steilization systems,
sized at about 250 sq. ft., can be installed in existing facilities,
saving users th cost of building housing for larger systems.
When working on E. coli, the electron beam can achieve 99.999% eradication of the bacteria in a fraction of a second using less than a two kilogray dose, leaving an almost incalculable chance of any bacteria surviving. The process doesn't change the temperature of the material being irradiated, and thus doesn't affect texture, taste, or nutritional value. E-beam irradiation can be used in the same way to eliminate salmonella—though that bacteria needs longer irradiation. Anthrax is tougher to kill and requires a dose over 40 kilogray to reach the same 99.999% of sterilization, but the process is still the fastest available, and uses the lowest amount of energy.
In the post office operation, bio-hazard specialists put all mail into two layers of bio-hazard bags that have already been sterilized, and put the bags into bio-hazard boxes for shipping to sterilization facilities. The sterilized mail is later returned for delivery. Electron beams are particles, and can go through only a limited amount of mail (calculated at the density of water). To get enough depth to kill all the anthrax bacteria in a typical piece of mail, the sterilization line uses two linear accelerators, set up on opposite sides of the conveyor—one above the line to irradiate the top, and the other underneath, to irradiate the reverse side.
"You don't want to turn the package over to get to the other side with something like anthrax, as the bacteria will just drop down," says Wil Williams, vice president of corporate communications for SureBeam. The two opposing the beams can sterilize six inches of paper. "The beam scans back and forth so that the six-inch density can vary by any practical length." The beam, moving continuously, can perform 350 scans per second.
Other forms of irradiation take much longer, and use a lot more energy. Gamma irradiation, using such radioactive isotopes as cobalt 60, preceded electron beam. "Gamma technology took between 15 minutes to four hours for palletized foods to be irradiated, and left users with the problem of how to dispose of the used isotopes. Unlike e-beam, it also uses heat, which could spoil food," Williams says. Gamma radiation is only about 55% as energy efficient as electron beams. X-rays can also be used, but they are much slower, and tend to be used for applications that are too dense for electron-beams or have odd shapes, "because electron beams like uniform densities," he says.
The post office also performs x-ray sterilization for packages that are too thick for e-beam, and is using equipment from Ion Beam Applications (Oak Park, IL).
For more information about Titan Corp. and SureBeam: Enter 536