Mark - A nerdly question:)

Submitted by Anonymous on Sat, 2011-06-18 10:41

Can you point me in a direction for converting "activity" to "quantity"?

For example, if we know that a given sample of soil had an activity of 2bq/kg of Cs-137, how can I derive the mass of Cs-137? Is there a table or converter for different isotopes?

Thanks!

BC

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Look up the specific

Submitted by Anonymous (not verified) on Sat, 2011-06-18 11:09.

Look up the specific activity of Cs137 on wolfram alpha. That gives the activity per gram of Cs137. That should do it.

Exactly. Searching for an

Submitted by bandstra on Sun, 2011-06-19 00:00.

Exactly. Searching for an isotope in Wolfram Alpha will yield its specific activity. For Cs-137, you will find 3.214E12 Becquerels/gram.

To get the fraction of Cs-137 in a sample, divide the measurement by the specific activity:
    (2 Bq/kg)/(3.212e12 Bq/g)
    Answer: 6.227E-16

To get the mass of Cs-137 in a sample of a given size (say, 1 kg), multiply the above by the mass:
    (2 Bq/kg)/(3.212e12 Bq/g)*(1 kg)
    Answer: 6.227E-16 kg

By the way, the specific activity of an isotope is not a fundamental property; it can be derived from the isotope's half-life and molar mass (i.e., 137 for Cs-137):
    1/(137 g/mol)/(Avogadro constant)*ln(2)/(30.08 yr) in Bq/g
    Answer: 3.212E12 Bq/g

Mark [BRAWM Team Member]