Why Are Madame Curie's Notebooks Still Radioactive?

[vee-bee]

How are Madame Curie's notebooks still radioactively dangerous when Hiroshima is not?

 

[Astronuc]

How are Madame Curie's notebooks still radioactively dangerous when Hiroshima is not?

The explosion at Hiroshima was an airburst, some much of the U-235 and fission products were vaporized in the atmosphere, and they would have drifted away to the ocean. Much of the contaminated areas were exposed to rain, so the rain would wash away the debris. Otherwise the debris would have been buried or moved to some other area. Likely most radionuclides formed from neutron absorption would have been short-lived and would have decayed by now.

The notebook probably was contiminated with dust or solution. And she was working with radium and polonium, and possibly actinides, which would have long half-lives.

How does one know that Madame Curie's notebooks are still radioactively dangerous? What is the activity from them?