Whoever put those notes in a lead box and is responsible for granting access knows exactly how much radiation is coming off of them, and how its amount and composition change every year.
Once you have that data, it shouldn't be too difficult for someone with relevant expertise to figure out roughly how much of each radioactive substance is in there and how they will decay over time.
Once you have that data, it shouldn't be too difficult for someone with relevant expertise to figure out roughly how much of each radioactive substance is in there and how they will decay over time.