Barium 140 decays to 140La with a half-life of 12.8 days, and the 140La decays to stable 140Ce with a half-life of 40.5 h. A radiochemist, after precipitating 140Ba, wishes to wait until he has a maximum amount of 140La before separating the 140La from the 140Ba. 

a) How long must he wait? 

b) If he started with 1000 Mbq (27 mCi) of 140Ba, how many micrograms of 140La will he collect?

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