The radioactive isotope 229Th is an α emitter with a half-life t½ Th = 7,300 y. Its daughter, 225Ra, is a β emitter with a half-life t½ Ra = 14.8 d, which is much shorter than the parent's.
(a) Show that the number of 229Th nuclei obey the differential equation
dNRa/dt = NTh/τTh - NRa/τRa, where NTh is the number of 229Th nuclei and NRa is the number of 225Ra nuclei, and τTh and τRa are their mean lives, respectively.
(b) Suppose that at time t = 0 there are NTh0 nuclei of the parent isotope 229Th and no nuclei of the daughter isotope 225Ra. Find NRa for t > 0.
(c) Show that after several years, NRa = NTh τRa/τTh.
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