Electron Beams

Beams of electrons or "e-beams" are streams of high energy electrons propelled out of an electron gun, the same—but larger—version of the device at the back of the household TV tube (before flat screens came on the scene) that propels electrons into the TV screen at the front of the tube, making it light up. The e-beam generator works on an on/off switch, and no radioactive substances are involved. Massive concrete shielding is unnecessary as gamma rays are not involved. Electrons, however, with their far less penetrating energy, can go no more than an inch into a food so that irradiation by electron beam requires thin sections—or the process must use two beams, one above, and one below to irradiate thicker portions. The electrically charged particles emerge from a linear accelerator at tremendously high speeds, with energies ranging from 3-10 million electronvolts (MeV). Figures 4.1a and 4.1b display a 10-MeV Rhodotron, a recirculating accelerator that operates on the principle that electrons gain energy when crossing an existing electric field. This makes it possible for the unit to be operated continuously, which has the advantage of high electrical efficiency and low maintenance costs. Its electron beam travels a rose-shaped path; hence its name Rhodotron, from Rhodos, Greek, meaning rose. Figure 4.1b displays the process, showing the accelerator in a shielded section above the the food irradiation area. The arrow points to a scanning horn that delivers the deeply penetrating beam to food packages on a conveyor belt. As the electrons flash through the foods, they create secondary particles, ions, and free radicals that disrupt microbial and insect DNA chains, destroying most but not all of them. This type of radiation is called ionizing radiation because the energy emitted is high enough to dislodge electrons from atoms and molecules and convert them to electrically charged particles, called ions.

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