Re: [Phys-l] thorium-228

[ludwik kowalski <kowalskil@mail.montclair.edu>]

On Dec 27, 2009, at 2:29 PM, Brian Whatcott wrote:

> 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?

Yes, activity (in Ci) is not the same as specific activity in Ci/(unit  
of quantity, such as volume or mass).

Ludwik

 Ludwik's new book (AUTOBIOGRAPHY) see:

> http://pages.csam.montclair.edu/~kowalski/mybook2.html


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