# Half life and nuclear decay.

1. Sep 9, 2009

### PrincePhoenix

1-Half life is the time it takes for half of the nuclei in a sample of radioactive material to decay(Am I right?). Why does the first nucleas that decays,decay first and the one that decays in the end, decay in the end? What's the difference between the two nuclei or what causes this the nuclei to decay in different times?
2-Also if radioactivity occurs randomly in time and space (according to my book) then why does every radioactive isotope has its own constant half-life?

2. Sep 9, 2009

### Astronuc

Staff Emeritus
Correct.
It's simply a 'random' process.
There is no discernible difference.

Because each element/radionuclide has a unique set of protons and neutrons. If one looks at the 'chart of nuclides', one will see that there is a band (set) of nuclei that have long half-lives, and either side of the band, the half-lives are shorter, i.e. the radionuclides are less stable.

Last edited: Sep 9, 2009
3. Sep 9, 2009

### PrincePhoenix

But you said it is a random process. Why does it have a specific half-life that is constant?

4. Sep 9, 2009

### Astronuc

Staff Emeritus
The specific half-life is related to the specific compostion, or unique number of protons and neutrons, which is alway the same for a given nuclide.

5. Sep 9, 2009

### hamster143

At the present time, this is postulated rather than explained ... all we can do is "predict" the probability for any nucleus to decay during a given period of time, i.e. if we take 100,000 nuclei of a certain type, we can predict approximately how many of those will decay in one hour or one day. But we can't tell which ones specifically will decay. For all we know, they are all identical, even though some will decay and some will not. That is an example of a perfectly random process. (As opposed to a "pseudorandom" process, where the outcome is theoretically predictable, but we may have enough information to predict it - such as winning numbers in a lottery.)

From there to calculating a half life, it's a fairly straightforward mathematical computation.

6. Sep 10, 2009

### PrincePhoenix

Thank You for explaining.

7. Sep 10, 2009

### Bob S

Collect 100 pennies, or other ubiquitous coin. toss them on the floor.

DO
pickup the "heads" and toss them on the floor
Loop until # "heads" = 0

Could you predict which pennies would be "heads"?
Could you predict how many times (half lives) you had to pick up the pennies before there were zero?

8. Sep 11, 2009

### tyrant91101

With the coin analogy, each time you flip the heads, you are moving in steps. Is the theory for radioactive decay that it goes in steps or is it just such a perfectly random process that happens as a continuous process?

9. Sep 11, 2009

### PrincePhoenix

I think hamster answered that. It is a perfectly random process.