[sverige]

N(t)=N_0 e^(-λt)
N(t_(1/2) )=N_0 e^(-λt_(1⁄) )
N_0/2=N_0 e^(-λt_(1⁄) )
〖-ln〗⁡(2)=-λ.t_(1⁄2)
λ=(ln⁡(2))/t_(1⁄2) λ in s^(-1) or in general 〖time〗^(-1)