Collisional Losses (Ionization and Excitation)

The rate of energy loss depends on the electron density of the medium.

The rate of energy loss per gram per centimeter squared, which is called the mass stopping power is greater for low atomic number (Z) material than for high Z materials (Khan pg 298). Compare the water to lead curve in Khan Fig 14.1

There are two reasons for this:

First, high Z materials have fewer electrons than low Z materials
Second, high Z have more tightly bound electrons, which are not as available for this type of interaction

The figure in Khan Ch 14 shows the the energy loss rate first decreases and then increases with increase in electron energy with a miniumum occurring at about 1 MeV. Above 1 MeV, the variation with energy is very gradual.

The energy loss rate of electrons of energy 1 MeV and above in water is roughly 2 MeV/cm.