FE Physics
 Formulas & Explanation
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The activity is the number of disintegrations per second.

A (t)= - ΔN(t)/Δt    or A(t) = | ΔN(t)/Δt |  



A                activity in Bq or s-1

ΔN(t)        number of disintegrated nulei 

Δt              time in s


Remark:    Δt  << T1/2                

T1/2                   half- life                     



Activity is proportional to the number of radioactive nuclei.

 A(t) = λ  N(t)

λ                 decay constant  λ = ln 2/T1/2



The number of radioactive nuclei at time t is :

N(t) = N(0)  (1/2)t / T1/2

N(t)           number radioactive nuclei after time t

N(0)          number radioactive nuclei at t=0 s


Because the activity is proportional to the number of radioactive nuclei we can write :


A(t) = A(0) (1/2)t/T1/2


A(t)           activity after time t

A(o)          activity when t = 0 s