A radiopharmacy elutes a Mo-99/Tc-99m generator every morning; the Mo-99 parent has a 66-hour half-life, the Tc-99m daughter a 6-hour half-life, and the column holds the parent for the generator's life. Which statement best explains why useful Tc-99m activity is available again the next morning?
Select an answer to reveal the explanation.
Short Explanation
Mo-99 stays trapped in the column and keeps decaying, so fresh Tc-99m builds up between elutions. Because the parent’s half-life is much longer than the daughter’s, it’s like a slow tap refilling your pitcher overnight.
Full Explanation
The correct statement identifies the source of renewed activity: Mo-99 remains on the column and continues decaying, producing Tc-99m after the morning elution has removed most of the daughter. Because Mo-99’s half-life is much longer than Tc-99m’s, the parent changes relatively little overnight while newly formed Tc-99m accumulates. Activity left on the column after an elution is not a reliable explanation for useful next-day activity: with a six-hour half-life, residual Tc-99m falls to about one-sixteenth after 24 hours, so the stated daughter half-life rules out persistence as the main source. Mo-99 breakthrough into the collected eluate could produce Tc-99m there, but that mechanism requires parent to leave the column, contrary to the scenario’s explicit statement that the column retains it. Transient equilibrium is a useful generator concept, but it does not restore daughter activity to the full parent activity by the next morning; after roughly 24 hours, ingrowth is substantial but remains below the contemporaneous parent activity. Exam caveat: “overnight” is approximate, so the exact daughter activity depends on elapsed time since elution. Practical check: verify the elapsed time and calculate expected Tc-99m ingrowth from the generator’s Mo-99 activity and the two decay constants.