After how many half-lives can short half-life brachytherapy sources be disposed of?

The disposal of short half-life brachytherapy sources is primarily determined by the principle of radioactive decay, which states that after a certain number of half-lives, the quantity of the radioactive material becomes negligible and is considered safe for disposal. Generally, it is accepted that after 10 half-lives, about 99.9% of the radioactive material will have decayed.

However, in medical settings, particularly those dealing with brachytherapy, a more conservative approach tends to be employed for safety margins and regulatory compliance. Disposing of radioactive sources after 20 half-lives ensures that the remaining activity is extremely low, reducing any potential risks to safety and health, including concerns about residual radiation exposure to both waste handlers and the public.

Thus, opting for 20 half-lives provides a significant buffer and aligns with guidelines that aim to ensure public safety and environmental protection. This is why that answer is marked as correct in this context.