Fluctuations of Energy Loss by Heavy Charged Particles in Thin Absorbers

Significant fluctuations of energy loss are expected in certain cases of the passage of fast heavy charged particles through thin absorbers. When the number of particle-electron collisions in the upper collision-loss interval is small, the energy-loss distribution is asymmetric and is characterized by a broad peak around the most probable energy loss (which is significantly less than the mean energy loss) and by a high-energy-loss tail. Several theories predict the energy-loss distribution function, but previous experimental work is incomplete with respect to verification of theory over the whole significant range of the parameters involved. We have passed beams of 730- and 45-MeV protons, 910-MeV helium ions, and 370-MeV ${\pi }^{-}$ mesons through lithium-drifted silicon $p-i-n$ detectors and silicon $p-n$ junction detectors with depletion layers from 0.0085 to 1.094 g/${\mathrm{cm}}^2$, and measured the resulting energy-loss distributions. Within the limits of experimental error, there is very good agreement between the measured energy-loss distributions and those predicted by Vavilov's theory, and good agreement on the value of the most probable energy loss.