system
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human
Why do neutrons shoot through dense material, but get reflected by softer material during Radiography?

I understand that it has to due with the transfer of momentum from the particle colliding with the nucleus of the atoms in the material, but I'm having a difficult time understanding why it shoots through the dense solid. Why doesn't it bounce off of the surface similar to a pool ball hitting the side of the pool table?
gpt
Actually, they don't "shoot through" heavy materials.  They "bounce through" them.

As a matter of fact, heavy nuclei have larger scattering cross-sections than light nuclei.  The difference is what happens when the neutrons hit those nuclei.  If you have a light nucleus (say an H-1 nucleus), then that mass is about the same as a neutron.  So when they hit, it's like 1 billiard ball hitting a stationary one--most of the energy gets transferred over to the hydrogen nucleus and the neutron loses most of its energy.  After only a very small number of these collisions, the neutron is moving as slowly as the protons themselves are moving due to heat (we call this "thermal energy" and it's generally 2200m/s or 0.0253 eV, compared to the energy neutrons emitted from fission have, about 2 MeV, 100 million times more).  At this low energy region, other neutron interactions, such as neutron capture, are much more likely to occur, so the neutron is very likely to be captured.

However, imagine a soccer ball kicked at a much more massive medicine ball.  The soccer ball will bounce off and transfer a *little* bit of momentum to the medicine ball, but will mostly just bounce off, losing only a very little amount of energy.

Now in the case of soccer balls and medicine balls, the mass ratio is about 2-20.  In the case of a neutron and a Pb nucleus, it's a ratio of about 200.  Almost no energy is transferred at all.

So if you have a light nucleus material, then the neutron will hit it, bounce around a bit, and get captured pretty soon (assuming there's something with a decent capture cross-section around).

But if you have a heavy nucleus, then the neutron will hit it, bounce around, bounce around, bounce around, bounce around, bounce around, and just bounce and bounce and bounce.  Now, in actuality, if you have for example a large amount of lead, then most of the neutrons will end up bouncing back towards you, but still, a very large considerable amount will bounce around and go through.