Radioactive decay

Let the number of nuclei (dN) in a radioactive source decaying in a short time dt be dN.
dN is proportional to:-
N - the number of radioactive nuclei present at that moment
dt - the time over which the measurement is made
the element, represented by a constant (l) called the disintegration (or decay) constant

dN = - lNdt space Activity: dN/dt = - lN

The activity is measured in units called Becquerels (Bq) where 1 Bq = 1 disintegration per second.
The disintegration constant or decay constant (l) can be defined as the probability of a nucleon decaying in the next second.

Number of nuclei (N) = Noe-lt space Activity (A) = Aoe-lt space Mass (m) = moe-lt

A useful alternative formula for radioactive decay is: A = A_o/2ⁿ

where n is the number of half lives that have passed and A_o and A are the original and final activities.