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Specialty Chemistry Forums => Nuclear Chemistry and Radiochemistry Forum => Topic started by: xavier on October 14, 2006, 10:10:50 AM

Title: decay time for technetium and indium
Post by: xavier on October 14, 2006, 10:10:50 AM
I have a problem I can't answer and I need help with it:

How much time is required before 10mCi of Technetium 99m and 25 mCi of Indium 113m decay, assuming equal activities?

I would appreciate anyone who can solve it and send me the answer. :)

thanks
Title: Re: decay time for technetium and indium
Post by: Mitch on October 14, 2006, 02:02:09 PM
You do your HW not us. You can look up the half-lives here http://www.isotope-table.com/
Title: Re: decay time for technetium and indium
Post by: Grejak on October 14, 2006, 02:38:54 PM
Like Mitch, I am opposed to doing someone else’s homework and then sending it off to them.  Especially because you chose to take that course, not me.  However, I am willing to give hints.

Quote from: xavier on October 14, 2006, 10:10:50 AM
I have a problem I can't answer and I need help with it:

How much time is required before 10mCi of Technetium 99m and 25 mCi of Indium 113m decay, assuming equal activities?

I think that is because the problem is very poorly worded.  mCi is a unit of activity, 3.7 x 1010 decays per second, while the question implies that it is a mass or moles etc.  It also reads as though you can assume an arbitrary initial activity and then calculate different times for both 99mTc and 113mIn.  Tell your professor that I said it was not a valid question, I am sure that he/she/it will listen to me…

Just in case that does not work though, I suspect that the question is actually supposed to read something like:

Assuming that you begin with equal activities of 99mTc and 113mIn, how long would it take for the activity of 99mTc to decrease by 10 mCi.  During this time, the activity of 113mIn must also decrease by 25 mCi.

You can use the website provided by Mitch to look up the decay information for 99mTc and 113mIn.  Then you can use the formula A = A0e(-lambda*t) for each isotope.  Since you know the decrease in activity during the time interval, you should be able to give a value of A in terms of A0.

Then you have two variables and two equations, so simple algebra will give you an answer.

Quote
I would appreciate anyone who can solve it and send me the answer. :)

Yeah, I dont quite define "help" as "do it for me".