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The characteristic thickness over which a high-energy electron loses all but a fraction of its energy through bremsstrahlung. At fixed thickness , smaller increases the radiative loss; the relevant dimensionless measure is . Low-energy charged particles can instead be dominated by collisional stopping power.[ref]
Review of Particle Physics by Particle Data Group (2022). Sections 34.4.1–34.4.4: collision losses, radiation length and critical energy