(4) Aktiivsus (A) on mingis kindlas energiaseisundis oleva radionukliidi koguse aktiivsus etteantud ajal: A = dN /dt, kus dN on sellest energiaseisundist toimuvate spontaansete tuumasiirete arvu ooteväärtus ajavahemiku dt jooksul., (4) Activity (A) is the activity per quantity of a radionuclide in a specific energy state at a given time: A = dN / dt, where dN is the expectation value of the number of spontaneous nuclear transformations from that energy state in the time interval dt., defineeritakse selle seaduse ulatuses, defined within the scope of this Act