Bragg Peak
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When a fast charged particle moves through matter, it ionizes particles and deposits a dose along its path. A peak occurs because the interaction cross section increases as the charged particle's energy decreases. The maximum of this dose is called the Bragg Peak; it occurs shortly before the particle has lost all its energy and stops. In the figure to the right, it is the narrow peak of the native proton beam curve which is produced by a particle accelerator of 250 MeV. The figure also shows the absorption of a beam of energetic photons which is entirely different in nature; the curve is mainly exponential.
The Bragg Peak was discovered by William Henry Bragg in 1903.
The phenomenon is exploited in the radiation therapy of cancer (eg proton therapy, carbon ion therapy), to concentrate the effect of light ion beams on the tumor being treated while minimizing the effect on the surrounding healthy tissue. The blue curve in the figure ("modified proton beam") shows how the originally monoenergetic proton beam with the sharp peak is widened (e.g., by varying the accelerator energy), so that a larger tumor volume can be irradiated. [1]
