I don’t know much about these things but I simply assumed that each alpha particle travels until it hits just ONE obstacle and then stops there. For the “cleared path” to get extended further, a new alpha particle must come along in that same route.
I assumed that it is like the old space invader video, where each projectile fired by an invader makes a little “hole” in the defenses at the bottom—and it takes many such projectiles to pick away at the ground-defenses, pixel by pixel (or small pixel-group by small pixel-group.)
In contrast, I don’t think it is like ballistic gel where a bullet clears a path of some length before stopping. (Yes, goopy ballistic gel will mostly flow back in behind the bullet but I think you get the idea. It is not a great analogy.) Isn’t alpha particle decay in a crystal matrix more like “one alpha particle clears one spot in the crystal and disappears in the process” because it’s energy is gone? So it takes many alpha particles to gradually “hollow out” a shell of sorts—although “hollow out” is certainly going too far and fails as an exact description.
IN ANY CASE: This is an example of a very educational Peaceful Science thread. For a non-specialist, this sort of review of a lot of radioisotopic decay basics is very worthwhile—and quite fun. And I hope that the errors in my description will get corrected by those with a much better grasp of the science. Thanks.