Thinner makes it get through faster if it doesn't collide. If it is a charged particle dwell time is important as it usually doesn't collide unless its charge

Thinner makes it get through faster if it doesn't collide. If it is a charged particle dwell time is important as it usually doesn't collide unless its charge interacts with other charged particles enough to slow itself down giving it more time to interact with charged particles. This is how I understand ion beam deposition in silicon to work at least, it goes to a predictable depth and if the silicon is thinner than that it goes on through most of the time. Thinner and denser then makes it interact more per unit time but any induced charge imbalances have closer neighboring material to rebalance charge shifts with, maybe you have a better chance of getting through the same number of particles over a shorter distance than at higher distance. You definitely have a better chance faster than slower. its when it slows to a critical speed its non-collision charge based interactions build.