[填空题]
$_{55}^{124}\text{Cs}$ has a half-life of 30.8 s.
(a) If we have $8.8\ \mu\text{g}$ initially, how many Cs nuclei are present? $N_0$ = $ \times 10^{10}\ \text{nuclei}$
(b) How many are present 2.0 min later? N = $ \times 10^{15}\ \text{nuclei}$
(c) What is the activity at this time? $\lambda N$ = $ \times 10^{13}\ \text{decays/s}$
(d) After how much time will the activity drop to less than about 1 per second? t = $min$
