A swiftly moving charged particle, called an incident particle, moving through a material composed of neutral atoms and molecules, such as gas or biological tissue, loses kinetic energy to the material. For instance, a massive positive particle, such as a bare nucleus, moving at a speed near that of light will ionize the atoms or molecules of the medium it is moving through. In the process, there is a drag force on the incident particle and hence a loss of energy.
The energy loss depends not on the mass of the incident particle, interestingly enough, but it does depend on its charge z, as well as on the average number of electrons Z per atom or molecule in the material. The following equation gives energy loss per distance traveled:
where N is the number of atoms per unit volume of material,
Z is the average number of electrons per atom or molecule in the material,
z is the charge (number of elementary charges) of the incident particle,
eis the electron charge (in Coulombs),
meis the electron mass,
c is the speed of light,
lnB ≈ 10 (approximately constant).
Use this equation to answer the questions. If you have forgotten notation like 4He, then look forward to Chapter 16 and review it.
A hydrogen nucleus (1H) and a helium nucleus (4He) have the same initial kinetic energy, with near light speed. They travel through the air. Which tends to lose more energy in a given distance?