# Question #d496b

##### 1 Answer

1/2.

#### Explanation:

Radioactive decay of a nucleus is a **poisson process**. The probability of decay per unit time is always the same. It does not change with time, no matter have many half-lives it has survived.

If I have a very large number of radioactive nuclei, I can predict how many will survive after a certain time but I cannot predict which ones will survive. The survival of surviving ones in the next half-life is independent of how long it has been existing. It is as though they were born just at that instant.

Tritium has a half-life of 12.5 years. If I start with 100g of tritium nuclei today and see how many survive after 1000 years (8 half-lives), there **may** still be 16 tritium nuclei surviving. At that time if I ask myself what is the half-life of those surviving nuclei. The answer is it is still 12.5 years. And 12.5 years from that point there **may** still be 6 nuclei surviving. They will all have a half-life of 12.5 years again.