Charged particles lose energy in passing through matter almost entirely through collisions with
atomic and/or molecular electrons. The electrons are excited to a higher state or simply ejected (ionization). The interaction is fundamentally different if the incident particle is an electron or positron for a couple of reasons. In the first place, it has exactly the same mass
as the electrons it will interact with. This means that in principle it can give ALL its kinetic
energy to the target electron (try it with billiard balls). Secondly, the electron is identical and indistinguishable from the target electron, and the rules (Pauli principle) says that no two electrons can be in the same state. Not true for positrons, so they lose energy slightly differently than electrons. Both of these complications result in energy loss almost but not quite as described by the Bethe equation; I find it more complicated.
All of this is explained in ICRU Report 37, which also gives tables for several elements and compounds. (International Commission on Radiation Units and Measurements). It's pretty hard to get, but the same thing, by the same people, is on the NIST pages:
http://physics.nist.gov/PhysRefData/Star/Text/ESTAR.html
(You can find this and related links on my
pdg.lbl.gov/2010/AtomicNuclearProperties/index.html ) dE/dx tables for muons are provided, which scale for other heavy particles.
I don't "do" electrons and positrons there for another reason: I am mainly interested in high-energy particles. At energies above a few tens of MeV they lose energy mainly by scattering from nuclei with the production of photons, which in turn scatter from nuclei
with the production of electron-positron pairs. And so on, making for a vast shower of photons, electrons, and positrons. This is discussed lots of places, among them in our Review of Particle Physics, pdg.lbl.gov/2010/reviews/contents_sports.html --> click on
"Passage of particles through matter." (It's fairly technical.)
