  We developed a general method for evaluating the energy spectrum evolution of
relativistic charged particles that have undergone small quantum losses, such
as the ionization losses when the electrons pass through matter and the
radiation losses in the periodic fields. These processes are characterized by a
small magnitude of the recoil quantum as compared with the particle's initial
energy. The `detector' function for arbitrary recoil spectrum is derived in
addition to the straggling function. These functions are determined by the
average number of scattering events undergone by the particle over the length
of the detector (ionization losses) or the periodic field and the specific
spectrum of the recoil. Moments for both the straggling function and the
detector function are derived. The minimum average number of recoils, above
which both functions depend only upon the mean energy of recoil quantum, is
estimated. The average of this number is about 10 recoil events.
