CPB Ljubljana 2018 course book

Radioactive Decay: Activity A

The probability for a radioactive decay per unit time for a specific nuclide is constant and called the decay constant, λ .

Since radioactive decay is a stochastic, spontaneous process it is not possible to identify which particular atoms out of an amount of a specific radionuclide will undergo such decay at a specific time. It is only possible to predict the mean number of disintegrated nuclei at a specific time, i.e. the activity A(t) defined as:

dN

t

(

)

tA )

(



dt

where dN(t) is the number of decays observed during the time interval dt: the minus sign is included since dN(t)/dt is negative due to the decrease of N(t) with time while activity, A(t), is a positive number. The SI unit of activity is the becquerel (Bq, named after the discoverer of radioactivity): 1 Bq=1 disintegration per second =1s -1 . Activity was traditionally measured in units of curies (Ci): 1 Ci = 3.7 10 10 Bq. Definition of 1 Ci: Activity contained in 1g 226 Ra