<P> Time - variable quantities: </P> <Ul> <Li> Total activity--A, is the number of decays per unit time of a radioactive sample . </Li> <Li> Number of particles--N, is the total number of particles in the sample . </Li> <Li> Specific activity--S, number of decays per unit time per amount of substance of the sample at time set to zero (t = 0). "Amount of substance" can be the mass, volume or moles of the initial sample . </Li> </Ul> <Li> Total activity--A, is the number of decays per unit time of a radioactive sample . </Li> <Li> Number of particles--N, is the total number of particles in the sample . </Li>

Which one of the following is not a mode of radioactive decay