Electron beam processing

Electron beam processing or electron irradiation is a process which involves using beta radiation, usually of high energy, to treat an object for a variety of purposes. This may take place under elevated temperatures and nitrogen atmosphere. Possible uses for electron irradiation include sterilization and to cross-link polymers.

Electron energies typically varies from the keV to MeV range, depending on the depth of penetration required. The irradiation dose is usually measured in Gray but also in Mrads. Where 1 Gy is equivalent to 100 rad.

The basic components of a typical electron beam processing device are illustrated in the figure. An electron gun (consisting of a cathode, grid, and anode) is used to generate and accelerate the primary beam. A magnetic optical (focusing and deflection) system is used for controlling the way in which the electron beam impinges on the material being processed (the "workpiece"). In operation, the gun cathode is the source of thermally-emitted electrons that are both accelerated and shaped into a collimated beam by the electrostatic field geometry established by the gun electrode (grid and anode) configuration used. The electron beam then emerges from the gun assembly through an exit hole in the ground-plane anode with an energy equal to the value of the negative high voltage (gun operating voltage) being applied to the cathode. This use of a direct high voltage to produce a high energy electron beam allows the conversion of input ac power to beam power at greater than 95% efficiency, making electron beam material processing a highly energy-efficient technique. After exiting the gun, the beam passes through an electromagnetic lens and deflection coil system. The lens is used for producing either a focused or defocused beam spot on the workpiece, while the deflection coil is used to either position the beam spot on a stationary location or provide some form of oscillatory motion.

In polymers, an electron beam may be used on the material to induce effects such as chain scission (which makes the polymer chain shorter) and cross linking. The result is a change in the properties of the polymer which is intended to extend the range of applications for the material. The effects of irradiation may also include changes in crystallinity as well as microstructure. Usually, the irradiation process degrades the polymer. The irradiated polymers may sometimes be characterized using DSC, XRD, FTIR, or SEM.

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