= First-order Fermi acceleration
In <first-order Fermi acceleration>, converging scattering media produce a mean fractional <energy> gain linear in their relative <speed> divided by the particle <speed>. At a nonrelativistic <shock wave>, an almost <isotropic> ultrarelativistic population gains $2(u_1-u_2)/(3c)$ per crossing and $4(u_1-u_2)/(3c)$ per complete return cycle. The distinction between a crossing and a cycle must also be respected when assigning an escape probability.
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