Calculation of specific activity when element consists of two isotopes

[mpoesr]

The element rubidium consists of two isotopes; the stable Rb-85 and the radioactive Rb-87 (abundance 27.83%). What is the specific activity of natural rubidium chloride? if you use A(r)=85.47 g.mol-1 for Rb-element, A(r)= 86.91 g.mol-1 for Rb-87, and M(r) = 120.92 g.mol-1 for RbCl.

 

[Astronuc]

Specific activity is simply activity divided by mass, and activity is simply the product of the decay constant and the number of atoms of particular radionuclide present.

So if one is measuring an alloy or compound of radionuclides and stable atoms, then the specific activity would be the ratio of the activity (∑λ_iN_i)/(∑A_jNj), where N is the number of atoms of a given isotope. The numerator includes on the activity of radionuclides, whereas the denominator includes the mass of all nuclides.