  We present a saddlepoint approximation for ionization energy loss
distributions, valid at arbitrary relativistic velocities of the incident
particle 0<v/c<1, provided that ionizing collisions are still the dominant
energy loss mechanism. We derive a closed form solution closely related to
Moyal's distribution. This distribution is intended for use in simulations with
relatively low computational overhead. The approximation generally reproduces
the Vavilov most probable energy loss and full width at half maximum to better
than 1% and 10%, respectively, with significantly better agreement as Vavilov's
kappa approaches 1.
