# Model for series radioactive decay

1. Dec 29, 2008

### KFC

If nucleus A decays to nucleus B in rate a, and B decays to C in rate b, and C is decaying at the rate c. To setup a model for that process, we start from A

$$\frac{dA}{dt} = - a A(t)$$

and for B, part for B is dying out from it's own decaying process but some amount of A will decay into B, so

$$\frac{dB}{dt} = a A(t) - bB(t)$$

But for C, it is quite confusing, the total number of C is proportional to the number of B directly but also to that of A indirectly, of course it will dying out due to it's own decaying process. My question is should I include a term for A(t) in the different equation for C? That is, should I write

$$\frac{dC}{dt} = a A(t) + bB(t) - cC(t)$$

or

$$\frac{dC}{dt} = bB(t) - cC(t)$$

2. Dec 29, 2008

Staff Emeritus
The reason you are having trouble is because you are trying to do too many steps at once. Write each variable's differential equation first, e.g.

$$\frac{dB}{dt} = - \frac{dA}{dt} - bB(t)$$

i.e. the amount added minus the amount lost. Then and only then start plugging in known quantities.

I know it's tempting to take short cuts, but you are really much better off being methodical.

3. Dec 29, 2008

### KFC

Got it. Thanks