A hospital stores short-lived radiopharmaceutical waste in a shielded vault until the activity falls to the legal limit of 1.0×103Bq. On a particular day the combined activity of waste placed in the vault is 8.0×105Bq. The dominant radioisotope present has a single half-life of 28d.
(i) Determine the decay constant of the isotope.
(ii) For how many whole days must the waste remain stored before the activity first becomes equal to or less than the permissible level?
(iii) How many half-lives will have elapsed in the time calculated in part (ii)?