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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:
dt
t dN
tA
)(
)(
−=
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