Warp wrote:
>   So this is the reason why when you shoot a lone neutron towards an object,
> it will (usually) go right through as if there was nothing there (except in
> the odd cases where it just happens to collide with a nucleus, as there are
> quite many of them there, in which case it's deviated or even reflected),
> because neutrons are electromagnetically neutral and thus don't interact
> with the electrons and protons? 

That's my understanding of the basics of it, yes. "Charge" is basically the 
probability that any given photon will be emitted or absorbed by another 
particle. A charge of "1" (plus or minus) means you have about one chance in 
137 of a particular photon interacting with a particular electron at any 
particular instant they meet.  Nobody knows where that number comes from.

> Moreso with neutrinos, which are like
> neutrons but a lot smaller (so a lot less chances of colliding with
> something in there).

That I don't know about.

>   Btw, I don't remember now what happens if you shoot a lone proton towards
> an object. As it's electrically charged, it ought to interact with the
> object immediately, if I understand correctly. But what happens?

Yes, but it's so much bigger than an electron that it can knock an electron 
out of orbit (i.e., it can affect an electron with so many photons that the 
electron's attraction to the electron's associated nucleus is basically 
overwhelmed) without losing much of its own momentum. So it tends to go 
quite some way if it's moving quickly, but with the charge it tends to stop 
quickly once it slows down (compared to neutrons), in part because it'll 
pick up an electron of its own, turn into a hydrogen atom, and now be much 
closer to the size of other atoms instead of the size of a nucleus.

In other words, yes, it interacts with (mostly) the electrons and possibly 
the protons if it happens to get close, but mostly by smashing through the 
substance you're shooting them at if they're going fast.  Electrons, on the 
other hand, will run into other electrons and stop quickly, no matter how 
fast they're going, because each interaction will on average result in two 
electrons with half the momentum (and shooting off a bunch of photons in the 
process, which is where you get x-ray machines and CRTs and such). A proton 
is some 2000x as heavy, so it can go a lot farther before it stops.

That's why gamma radiation (high-energy photons) takes thick lead to stop, 
x-rays (slightly less energetic) takes thin lead to stop, beta radiation 
(high-energy electrons) takes a few millimeters of lighter metal to stop, 
and alpha radiation (basically helium nuclei) takes like a piece of paper to 
stop.

-- 
Darren New, San Diego CA, USA (PST)
   Serving Suggestion:
     "Don't serve this any more. It's awful."

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