2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/5992The probability that a particle, crossing the shock along a given direction, be reflected backwards along another direction, was shown to be the key element in determining the spectrum of non--thermal particles accelerated via Fermi mechanism around a shock. Here an explicit equation for this probability distribution is given, for both the upstream and downstream sections. Though analytically intractable, this equation can be easily solved numerically.This paper is withdrawn. A revised version, already accepted for publication in The Astrophysical Journal, will be submitted tomorrowAstrophysicstext