Answer to Question #2596 Submitted to "Ask the Experts"

Q

A few weeks ago while cleaning up a couple of shelves in our chemistry stockroom, I noticed a bottle of thorium nitrate. It looks to be about 50 g of the solid. It was stored on the oxidizer shelf, which is correct. It is approximately 5-6 ft from the walkway. I decided to double contain it and I placed the small bottle in a larger amber bottle. It has not been opened or used in about 14 years. How should this solid be stored? Will plastic contain the radation? Will a lead storage box work? I told my chairperson and he was hesitant to store it in lead, stating it may produce x rays. Is this true? And lastly I placed a Geiger counter up to it and noticed it was registering radiation. When I pulled it away a few inches it was the same as background. Why is this? This was just a nonspecific counter. How dangerous is this compound if it remains out of the way and sealed?

A

Store it as you would any potentially hazardous chemical. The plastic it is stored in will stop the alpha particles and the beta particles. It will not stop the gamma rays. Storing it in lead will reduce the gamma ray emissions to one extent or another, although this is probably not necessary. It is true as your chairperson says that x rays can be produced but that is not a concern in your situation. In general, the idea is to avoid shielding beta particles, especially high-energy betas, with a high atomic number material such as lead because of the potential for bremsstrahlung (x ray) production. If I understand your question correctly, the betas would first be stopped by the glass/plastic walls of the container before they could ever reach the lead outer shielding. With regard to your Geiger counter measurements, the intensity of any type of radiation decreases with distance. Assuming that the detector is operating correctly, what this tells you is that the radiation levels are not a concern.

It is not hazardous from a radiological point of view to keep the compound out of the way and sealed. I am unwilling to address any potential chemical hazard.

Paul Frame, CHP, PhD
Oak Ridge Associated Universities.