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Some calculations help

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The question goes like this:


The measured activity of a radionuclide was taken at 3-day intervals. The Following readings are uncorrected for background radiation.


Time(Days) :0 3 6 9 12 15

Count Rate (Per Minute):145 100 71 52 39 30


The count rate was measured again 2 months after the first reading and foungd to be 12 counts per minute.




Well........How do i work it out

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I think the answer is 12.

because of such a rapid decay rate over 15 days, it would be practicaly a Zero count from the original isotope 2 months later.

but I`m no mathematician! :)


I plot, the differences between the numbers.






and thats just over 15 days, so I Guesstimate, in say 60 days, it would be 1 or less.

PLEASE don`t write that as your answer! I only had a go because I thought it was a neat question :)



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Well I wouldn`t take my answer as gospel though :)

there`s a good many better qualified REAL mathematicians on here, IF your in no rush, wait to see what they say 1`st, I`de hate to think I gave you wrong information and have you chasing me all over the internet with a shotgun :)

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12 is right. Look at the data and you can estimate what the half life is, even if you initially assume zero background. This will overestimate the half-life, and you should be able to convince anyone that waiting 60 days is many half-lives. e.g. if you wait 5 half-lives, you are down to about 3% of your original activity. If you wait 10 half-lives, it's about 0.1%. So the 12 cpm at two months is essentially all background.

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