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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 , Alpha particle, and other , the peak occurs immediately before the particles come to rest. It is named after William Henry Bragg, who discovered it in 1903 using alpha particles from radium, (2025). 9781439816073, CRC Press. ISBN 9781439816073 and wrote the first empirical formula for ionization energy loss per distance with Richard Kleeman.
When a fast charged particle moves through matter, it ionisation atoms of the material and deposits a absorbed dose along its path. A peak occurs because the interaction cross section increases as the charged particle's energy decreases. Energy lost by charged particles is inversely proportional to the square of their velocity, which explains the peak occurring just before the particle comes to a complete stop. In the upper figure, it is the peak for alpha particles of 5.49 MeV moving through air. In the lower figure, it is the narrow peak of the "native" proton beam curve which is produced by a particle accelerator of 250 electronvolt. The figure also shows the absorption of a beam of energetic () which is entirely different in nature; the curve is mainly exponential. This characteristic of proton beams was first recommended for use in cancer therapy by Robert R. Wilson in his 1946 article, Radiological Use of Fast Protons. Wilson studied how the depth of proton beam penetration could be controlled by the energy of the protons. This phenomenon is exploited in particle therapy of cancer, specifically in proton therapy, to concentrate the effect of light on the tumor being treated while minimizing the effect on the surrounding healthy tissue.
The blue curve in the figure ("modified proton beam"
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As shown in the plots above, there is a limited range for the particles in a material. The Bragg–Kleeman rule is a way to estimate a Particle range in a medium, serving as a tool in particle detection and dosimetry. The basic form of the rule is:
where R1 and R2 are the ranges of two particles, ρ1 and ρ2 are the Density of the media they traverse, and A1 and A2 are the atomic weights of the particles.
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