Factual error: Iodine-131 does not decay to normal Iodine. It decays to a stable isotope of the noble gas Xenon.
Factual error: I-131 (or any radioactive material) "at the end of its half life" does not instantly become non-radioactive, as was depicted in this episode. Radioactive decay is a continuous process; the half-life has no "beginning", but tells you how much time after "now" for however much is there at any time "now" to be reduced to one half that amount. Wait enough half-lives and the amount will fall to an inconsequential amount, but one is never enough.
Factual error: Firstly, I-131 has an eight day half life. It is thus impossible that she is poisoned on day 1 (a dose enough to kill her), and then the following day there is no detectable rad. reading at the source. The eight day half-life also comes into play with the hand that becomes "neutralized" to radioactivity. Secondly, the discussion re: alpha, beta, and gamma radiation is true, however if breathed/ingested, I'd sure rather it be a gamma emitter than an alpha emitter, as alphas will deposit more energy (this is contrary to what was stated).
Answer: Yes, sizes of jails differ.