# Per unit of energy, what radiation is the most penetrating?

This question about nuclear reactor shielding got me wondering, what forms of radiation are the hardest to shield? I suppose Neutrinos will have to be excluded since they're the obvious winner.

So assuming an equal kinetic energy, what's the hardest radiation to shield? I think I've read Muons penetrate to much greater depths in accelerators but surely there are other things that are hard to attenuate.

I'm not really looking for a list so much as an explanation for what makes a particular particle (or groups of particles in the case of Alpha particles) difficult to shield.

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neutrinos are the most penetrating, but because of that they are the least dangerous since none of them will stop in your body to cause some harm. –  gigacyan Oct 28 '13 at 8:49
I understand that in order for the radiation to be dangerous it must deposit its energy in our body and so neutrinos aren't dangerous. My question isn't about danger but about particle interaction. –  Brandon Enright Oct 28 '13 at 15:47

As far as I am aware, mass (and subsequently size) is the primary reason such interactions are more or less penetrating. An $\alpha$ particle is one of the higher-mass cases of ionizing radiation and as such is stopped very easily. A photon ($\gamma$ radiation) is comparatively much more difficult to stop because of its physical properties - high energy, 0 mass, lack of coulomb interaction, etc.