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The Bragg peak is a pronounced peak on the Bragg curve which plots the energy loss of ionizing radiation during its travel through matter. For protons , α-rays , and other ion rays , the peak occurs immediately before the particles come to rest.
Bragg curve of 5.49 MeV alpha particles in air. The force usually increases toward the end of range and reaches a maximum, the Bragg peak, shortly before the energy drops to zero. The curve that describes the force as function of the material depth is called the Bragg curve. This is of great practical importance for radiation therapy.
Bragg curve of 5.49 MeV alpha particles in air. This radiation is produced by the decay of radon (222 Rn); its range is 4.14 cm. Stopping power (which is essentially identical to LET) is plotted here versus path length; its peak is the "Bragg peak"
Usually, the energy loss per unit distance increases while the particle slows down. The curve describing this fact is called the Bragg curve. Shortly before the end, the energy loss passes through a maximum, the Bragg Peak, and then drops to zero (see the figures in Bragg Peak and in stopping power).
In order to achieve diffraction conditions, the sample under study must be precisely aligned. The contrast observed strongly depends on the exact position of the angular working point on the rocking curve of the sample, i.e. on the angular distance between the actual sample rotation position and the theoretical position of the Bragg peak.
The dose increases while the particle penetrates the tissue, up to a maximum (the Bragg peak) that occurs near the end of the particle's range, and it then drops to (almost) zero. The advantage of this energy deposition profile is that less energy is deposited into the healthy tissue surrounding the target tissue.
Reflectivities for Laue and Bragg geometries, top and bottom, respectively, as evaluated by the dynamical theory of diffraction for the absorption-less case. The flat top of the peak in Bragg geometry is the so-called Darwin Plateau.
It has been shown [18] that the crystal density can be represented as a complex function where its magnitude is electron density and its phase is the "projection of the local deformations of the crystal lattice onto the reciprocal lattice vector Q of the Bragg peak about which the diffraction is measured". [4]