# What exactly is a half-life?

1. Feb 26, 2014

### marc32123

I don't understand what this means???

Including artificially produced nuclides, more than 3300 nuclides are known (including ~3000 radionuclides), many of which (> ~2400) with decay half-lives shorter than 60 minutes. This list expands as new radionuclides with very short half-lives are characterized.

What does a nuclide having a decay half-life shorter than 60 minutes mean exactly???
( I understand half-lives kind of. From what I have learned, a half-life is the amount of time it takes for something to reduce to half it's value. I just don't understand how that would relate to the above question -
What does a nuclide having a decay half-life shorter than 60 minutes mean exactly???

2. Feb 26, 2014

### Simon Bridge

Welcome to PF;
the half life is exactly the amount of time it takes for a sample to decrease to half it's original amount.
if you started out with 1kg of pure Am241, then, come back in 432-odd years and you have 0.5kg left (mixed in with some other stuff).

exactly?
having a half life less than 60mins means that half of the sample is gone (turned into some other material + energy) inside an hour. in less than two hours there's a quarter left and so on.

They are telling you that the material is very radioactive but it does not last long.

3. Feb 26, 2014

### marc32123

I don't quite understand though. I do understand it if it was talking about elements with many radionuclides. According to what you are saying, if there was a newly formed element with many radionuclides within it and a half life of 60 minutes, it would take 60 minutes for half of the radionuclides within that sample of material to have changed into some other element+energy. This is correct right? What I don't understand though is if you were to relate the whole thing to just one radionuclide, not an element with many of them. How would it work then?

4. Feb 26, 2014

### Simon Bridge

... that's not what I said. I said that half the amount of the sample would have changed to something else.
eg. if you started out with 1kg of pure Americium, composed of equal parts each of it's radioactive isotopes, you can look them up, then the half-life of the sample is the time it takes to end up with 0.5kg of Americium (and some other stuff). You should be able to figure out when that is.

But the original statement in post #1 was:
... this is talking about lots of radionucitides which each have half-lives less that 1hour. Some may have half-lives in seconds, some minutes etc. This means that after 1 hour the amount of each individual substance in the sample is reduced by at least half.

5. Feb 27, 2014

### ehild

It means that the nuclide decays during the next one hour with 50% probability.

ehild

6. Feb 27, 2014

### ModusPwnd

But if that nuclide was not in an atom with other nuclides, would it still be radioactive at all?

At first I answered similar to yours, then my question above compelled me to erase my answer.

7. Feb 27, 2014

### Simon Bridge

Yes. Kind-of.
The consequence of the 50% probability is that, if there are lots of them in one place, about half of them will decay in their half-life - so the two answers are related.
On the scale of an individual atom, being radioactive means having a probability of radioactive decay.

The probability version is more fundamental.

Last edited: Feb 27, 2014
8. Feb 27, 2014

### Varun Bhardwaj

It is simply like "A Ancient Sword turns into dust after thousand years" ?

9. Feb 27, 2014

### sophiecentaur

Have we understood your question correctly? I suspect not.
Are you referring to a single nucleus and you want to know it's probable lifetime? Half life is a quantity, used to describe what happens to a statistically large number of nuclei.
Look at this link for details of half life, decay constant and average lifetime of a particular nucleus.
Near the end of the page it says:

10. Feb 27, 2014

### marc32123

Thanks everyone for your insight, I found the answers I was looking for....

11. Feb 27, 2014

### dauto

No. Swords age over time, Nuclei don't. An old radioactive nucleus is identical to a young one. They both have the same probability of decaying at any moment.

12. Feb 27, 2014

### Simon Bridge

May be more like "ancient glowing sword stops glowing after thousands of years and is discovered to have turned to lead."

Last edited: Feb 27, 2014