I’ve been reading about HL, but I still don’t get why would you use it. So, if half life of coffee is 5h, how that info is relevant when we know that full life is 10 (roughly)? On top of that, how do you get the half-life of a material other than waiting to be completely ‘dead’ and say, ok full life is X, the half life is X/2.
Also, let’s take uranium which in Earth’s crust has a half-life of almost 4.5 billion years.. how did we get this number?
Thank you!
In: Chemistry
Let’s say that in the normal environment, there is one part of isotope A, and one of A’. Animals and plants while alive ingest these isotopes and so have a 1:1 ratio of them in their system.
After they die, they stop ingesting anything at all. The isotopes keep decaying and as A’ decays much faster than A, the ratios change.
In the environment however, the ratio remains 1:1 due to the processes forming the isotopes.
After time we measure the ratios. If the ratio has skewed 2:1 in favour of A, then we know one half-life of A’ has passed. Different ratios indicate different time spans.
Generally speaking when isotopes are used for dating, two or more different isotopes with different half-lives are measured to be as accurate as possible.
Remember this process is exponential, so from 1kg of isotope, a half-life passes and you have 0.5kg left. Another half life and you have 0.25 kg, then 0.125 and so on. You don’t get to zero.
To measure say Uranium you take a known amount of a known isotope, you know the radiation it emits, you measure the radiation and you say ‘in time *x* it emitted *y* radiation from *z* moles of isotope’ and you have everything you need to calculate the half life. Just like you don’t need to weigh an entire mountain if you know its size and density already, a measured sample is enough.
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