Production of electron beams

Production of electron beams necessitates the use of electron accelerators such as betatrons, microtrons, or linear accelerators. All these accelerators are capable of producing clinically useful beams of X-rays or electrons. With an X-ray beam, a narrow electron beam emerging from an accelerator strikes a thick, high-atomic-number target to produce bremsstrahlung photons. An appropriately shaped flattening filter converts the forward-directed bremsstrahlung beam into a clinically useful beam of uniform intensity. In an electron beam, the X-ray target is removed and the emerging electrons are scanned magnetically or scattered in foils to produce a uniform broad beam. Most radiation therapy facilities have high-energy accelerators capable of producing both X-ray and electron beams. Thus, radiotherapists have both treatment modalities at their disposal in the design of an optimal treatment.

Since electrons scatter significantly in air, beam-defining cones or 'trimmer' bars are fitted to the head of the treatment machine in order to collimate the beam near the skin's surface. The beam may be shaped further either by fitting a lead or 'cerrobend' aperture at the end of the cone (often called an electron cut-out), or by using lead sheet laid directly on skin.

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