# How was the half-life of Uranium 235 determined and by whom? [duplicate]

Wikipedia says that the half-life of Uranium-$$235$$ is $$7.038 \times 10^8$$ ($$\text{703 800 000}$$) years.

This is very long. Therefore, on a human time scale, the decay is very small, posing difficulty for determining the half-life.

So Who and how was the half-life calculated?

• Identical to physics.stackexchange.com/questions/7584/… – user108787 Jun 4 '16 at 14:54
• There is some overlap, but not the same question, I would say. – poisson Jun 4 '16 at 15:06
• As in "Who did It", that's fair enough, (if you mean" Who first measured it" but the very best of luck to you in your Googling, I looked and could not find out. I would guess the technique dates to the time of the Curies, Becquerel and Rutherford. – user108787 Jun 4 '16 at 15:15
• Call the half life a billion years. That is about 3E16 seconds. One mole of atoms is 6E23. So, about 3,000,000 atoms would decay per second. – Jon Custer Jun 4 '16 at 15:22
• For the extreme, Bi-209 has a half-life of 2 x 10E19 years! – DJohnM Jun 4 '16 at 19:04

Nier looked at ratio of 206Pb to 207Pb in 21 samples of radiogenic lead - that is, lead that has been formed by radioactive decay, in this case, from Uranium. 206Pb is a decay product of 238U, while 207Pb is a decay product of 235U. He then determined that the actinium series - the decay chain of 235U to 207Pb - is 4.6$\pm$0.1% as active as the uranium series - the decay chain of 238U to 206Pb. Then, using the ratio of 238U to 235U in natural samples, he was able to calculate the half-life of 235U. It is not mentioned as to how he was able to calculate the half-life of 238U.
Interestingly enough, Nier's value is closer to the accepted value than the half-lives found in many later studies using $\alpha$-counting, which looked at the number of $\alpha$ particles emitted during the decay of 235U.