Question

For photons and neutrons, we tend to talk about probabilities of interacting, yet for charged particles we tend to focus on range and stopping power. Why the difference?

Answer 1

For charged particles, we use terms like range and stopping potential as they define the kind of interactions the particles go through. Every charged particle is bound to undergo some electrostatic interaction. This interaction with its surrounding leads to change in energy of the charged particle. To determine how this energy change occurs, there are certain parameters we evaluate. One such parameter is the stopping potential wherein the charged particle SURELY interacts with matter, resulting in loss of energy.

For uncharged particles, such as photons and neutrons, there is no electrostatic interaction (that is why a photo or a neutron can pass through an atom without deviating from its path). Does parameters such as stopping potential and range cannot be applied as such interactions don't take place. However, uncharged particles do undergo other types of interactions, such as absorption, scattering. For such an interaction to take place, we would have to ascertain at the neutron interacts with the matter, which is usually not controllable. Let us understand this with an example. A neutron collides with the nucleus, however passes through the rest of the atom undeviated. So to make neutron interact with the nucleus, we will have to bombard a set of neutrons on the atom out of which only those neutrons that come within the cross sectional area of the nucleus will interact with the nucleus while others will pass through the atom. This shows that only a certain amount of neutron interacts out of the total and not all. Thus we get the "probability of interaction".

Thus, for charged particles, we have a 100% probability that it will interact, however the same is not true for uncharged particles and thus we talk about the probabilities of interactions.