You guessed it, the notebooks of Marie Curie.
Via Open Culture, here's a report on the papers and other belongings of the discoverer of polonium and radium, Marie Curie who worked in her future husband Pierre's lab. (I love that movie).
Her notebooks, her clothing, her furniture, pretty much everything surviving from her Parisian suburban house, is radioactive, and will be for 1,500 years or more.
If you want to look at her manuscripts, you have to sign a liability waiver at France’s Bibliotheque Nationale, and then you can access the notes that are sealed in a lead-lined box.