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Re: [Phys-l] thorium-228



ludwik kowalski wrote:

1 mole of water (18 grams) contains 6.02*10^23 molecules. This amounts to 3.34*10^22 molecules per gram of water. How many atoms of Th is there in 1 gram of water, if its concentration is 0.03 ppb?

N'=(3.34*10^22)*(0.03*10^-9)=0.10*10^13=1.0*10^12 (atoms of thorium per gram of water)

For one liter, N=1000*N'=1.0*10^15 atoms

Thus the decay rate is R=k*N=(1.15*10^-8) * (1.0*10^15) = 1.15*10^7 decays per second.

The activity in Curies is:

A= R/(3.7*10^10) = (1.15*10^7) / (3.7*10^10 ) = 3.11*10^-4 Ci
Thank you for working the problem.
I see that, following your earlier definition
of concentration, you use, not conc. by mass, not conc. by volume,
but concentration by number of atoms per number of molecules.
How strange! I expect this represents molar concentration.
or concentration by "least units of interest"....
And I see that you reduced the computation to a standard volume of one liter.
Is it true that Activity (as defined) increases with quantity of isotope?
Or is Activity always defined as per liter?

Brian W