# Radioactivity and density

1. Sep 8, 2004

### mee

What is it about the density of a substance that makes it more resistant to allowing radioactivity to pass through it? Since most of the atom is empty space?

2. Sep 8, 2004

### humanino

No. The denser a material, the bigger its atoms, or more precisely the bigger the nuclei This is what is relevant to protect humans. I found a good explanation on this DOE site
There are also good illustrations and explanations on this english Atomic Weapons Establishment

Last edited: Sep 8, 2004
3. Sep 8, 2004

### mee

yes

I realise dense materials stop or reduce radiation, I was just wondering why 70 grains of salt in the center of saint peters dome would stop much better than 1 grain of salt in the middle of saint peters dome. Atoms are seemingly considered mostly space. Is it a charge? It seems the chance of a particle hitting a nucleus instead of the rest of the atom woud be small. Thanks for your help.

4. Sep 8, 2004

### humanino

It is very easy to protect oneself of alpha or beta rays. Only gamma rays are a problem. Gamma rays can only be stopped by the nucleus, the rest of the atom, i.e. the electrons, are unlikely to do anything. So you need to rely on the nucleus. The nuclei will have a more efficient stopping power if they are bigger. It is not due to the charge, because gamma rays, which are photons, do not carry charge (although the photons are the mediator of the EM interaction : photons won't be deviated by a charge. Photon cannot be deviated ot slowed down at all. They can only be absorbed.) I understand your idea : how on earth could this tiny nuclei have any effect at all ? It is so small !" This is true, if the nuclei would be even bigger, the absorption would be even greater. Indeed, gamma rays are very penetrating ! Way more than other form of radiations as said above.

The radiation yield $$y$$ remaining after a thickness $$t$$ is given by :
$$y(t)=y_0 e^{-t/t_0} \rightarrow_{t \rightarrow \infty}0$$
with $$t_0$$ a stopping length. There are so many nuclei in a thickness $$t_0$$ ! The photon will escape most of them, but eventually be more and more likely to meet one.

It is the contrary for neutron protection : in order to slow down the neutrons, you need a material that is able to absorb as much quantity of movement as possible, and this is more efficient with light nuclei. This is why we use water in nuclear plants to cool the neutrons down. However, neutron radiation is usually not a problem, the problem is more to create the neutrons. We also use carbon in order to absorb the neutrons.

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