Absolute Age: Radiometric Dating

[kingwinner]

1) "A half-life is the time it trakes for half the mass of a given amount of a radioactive element to decay into its daughter elements"

 

Is it true that a sample decays "bit by bit"? Why only part of it decays (to daughter elements) and some other part is kept complete untouched (still being the parent isotope)? Won't all parts decay at the same time??

 

 

2) "If you were to begin with 10 g of U-238, after 9 billion years, or 2 half-lives, one fourth, or 2.5 g, of the original U-238 would remain. Three fourths would now be the daughter element Pb-206."

 

The decaying process of U-238 has many intermediate products (like Th-234, Pa-234,etc) before becoming Pb-206. Then is the bolded part true? (25% U-238, 75% Pb-206) How about the intermediate products, would they exist significantly, or would they just exist in an insignificant amount that it can be ignored?

[kingwinner]

Can anyone please explain?:cup: :cup:

[kingwinner]

I am OK and understand Question 1 now!

[kingwinner]

Can someone please explan Question 2 for me? I don't get it!:hihi: Would U-238 change into Pb-206 (with many intermediate products in between) in just a flash of time that none of the intermediate products exist?

[MortenS]

Yes, the amounts of the decay products are neglibible with respect to the ratio between Pb-206 and U-238. Just look up the decay rates for all the intermediaries and you will quickly discover that there will be close to nothing of the nuclides with half-lives less tahn a year, because they will decay faster than they are formed. There will be U-234, Th-230 and Ra-226 (and to a lesser extent Pb-210) in the sample, but the amounts are so small in relation to the amount of Pb-206 and U-238 that their collective amount will be less than 2 parts per hundred thousand of the original amount after two half-lifes.

 

To get a feeling of the decay process, take a look at the following java applet:

 

http://www.eserc.stonybrook.edu/ProjectJava/Radiation/