Nij writes: Mass, radioactivity count, whatever. Just substitute the quantity, the formula still works exactly the same because the pattern is still exactly the same: an exponential decay. |

Yes the formulas do work, but some methods for measuring half life are are impractical regardless of what the algebra suggests.

It's very difficult to measure the half life of U_{238} using the method you describe. After one year of decay 100 grams of U_{238} would become 99.9999999845 grams of U_{238} with the decay nuclei still mixed in with the sample. You cannot measure the U_{238} accurately enough to determine the half-life of U_{238} even after 1 year.

On the other hand, U_{238} does emit enough alpha particles (tens of thousands per second per gram) to count over a reasonably short period of time in which the decay rate is constant. For that reason, making an activity measurement is the better approach for determining long half lives.

quote:

I'm fairly sure RAZD has slapped this down before too. We **don't need to know** the original absolute amount; we only need relative amounts i.e. the ratio of uranium to lead, and these can be calculated from the current relative amounts by the simple fact that x uranium atoms turn into exactly y lead atoms; it's a "stoichiometric relationship".

I agree that only the relative amounts are needed for dating purposes. Not disputing that at all.

The paragraph you quoted above was from my addressing a completely different question posed by faith24, namely, whether we could compensate for the situation where lead other than decay products was present simply by measuring the remaining amount of uranium present. My answer is that you cannot compensate in that fashion.

*Edited by NoNukes, : Add a little detail*