= Radiation length
{title2=$X_0$}
{wiki}
The characteristic thickness over which a high-<energy> <electron> loses all but a fraction $1/e$ of its <energy> through <bremsstrahlung>. At fixed thickness $x$, smaller $X_0$ increases the radiative loss; the relevant dimensionless measure is $x/X_0$. Low-<energy> charged particles can instead be dominated by collisional <stopping power>.<Review of Particle Physics by Particle Data Group (2022)>[Sections 34.4.1–34.4.4: collision losses, radiation length and critical energy]{ref}
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