Formation rate, daughter activity

Case II. Secular Equffibrium. In a situation where the parent has a half-hfe that is much greater than that of the daughter, the activity from the decay of the parent does not decrease to any great extent on a timescale that is several half-frves of the daughter. Therefore, A2 >>Ai. Because the formation of the daughter occurs hy the decay of the parent, the rate of formation of the daughter can he expressed as... 

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