Plutonium etc

[mycatis]

Why, when talking about radioactive material, do people say it has a "half-life" of so many years? Why not just say how long it really is before the material isn't dangerous anymore?
Sorry if that didnt read to well, as you can tell I've not got the most brilliant scientific brain lol

[Loosehead]

The half life is used because the total amount of time for a sample of whatever to decay depends on the size of the sample but the half life is always constant. Lets take substance X and say it has a half life of 1 year. So if we have 10kilos of X after one year we will have 5 kilos of X and 5 kilos of whatever X decays into. Now wait another year and we'll have 2.5 kilos, then 1.25 kilos etc, each year half the remaing X is converted but so the total decay time depends of the size of the substance but the "half life" is always constant so that is the figure that is generally referred to.

[mycatis]

Brilliant! Thank you Loosehead :-)

[jake-the-peg]

There is also no such thing as a safe level of radiation.

You're constantly being bombarded by radiation from space, rocks and various other sources. Mostly they do no harm but there is a small chance that one will.

Double the amount of radiation - double the risk probably still vanishingly small.

Step into the core of a reactor and the radiation becomes billions times higher and the probability of damage to you becomes pretty much a certainty.

[Mac attack]

So if for example you had a piece of metal with a half life of 1 hour, after one hour would it only be half the size it was before? Where would the other half be? And.. why is it dangerous?

[LCBoman]

Okay, "Mac Attack" this is really 2 different questions. To answer your first question;

If the half life was one hour, after one hour you would have half of the orginal substance (lets call this Substance Y) and half of a new subtance that is caused when substance Y breaks down.

[jake-the-peg]

To answer the second question think about the radiation from a decaying particle.

Radiation is generally from when an atom's nucleus breaks up.

When it does this debris is shot out at great speed.

Depending on the type of breakup you can have high speed electrons (beta particles), Helium nucleii (alpha particles) or neutrons.

Now generally speaking the first 2 aren't too dangerous your skin and a few cm of air will stop them. But there are radioactive gasses like Radon and that gets into your lungs and decays there you can get all sorts of damage.

Neutrons are a different story, there are uncharged which means they can breeze right up to another nucleus and break it open.

Now think about what radiation could do to the cells of your body. These high speed particles can rip through and damage or change the cells in your body. There's not much chance of damage from a few but as the level increases the chances of damage increase. Your body can cope with a bit of damage but as more and more damage occurs you're more likely to get overwhelming cellular damage.

You could also get damage that changes cells in such a way that cancers result, again the more exposure the greater the risk.

A bit like someone shooting an arrow in the air, not much chance of it going up, coming down and hitting you. Now what about an army of 10,000 archers? fancy facing that? Now what's a "safe" level?