Answer to Question #2540 Submitted to "Ask the Experts"

Q

We have a licensee who disposed of his Pu:Be source from his a neutron howitzer. It was stored in a 5-10 gallon paraffin wax source holder and although the source is gone, he wants to know if some of the wax may have become radioactive with carbon-14 or hydrogen-3. Do you think this is possible? Since they have a scintillation detector, should they treat it as a leak test and scrape some inner wax into a cocktail?

A

The paraffin will absorb neutrons and can become activated. Tritium (hydrogen-3) formation is more likely because it can be formed by neutron capture reactions in hydrogen. Carbon-14 is less likely because it is formed by a neutron capture/proton ejection reaction with nitrogen. According to the Hawley's Condensed Chemical Dictionary, paraffin contains only carbon and hydrogen (that is, no nitrogen), so I would not expect to see any carbon-14 formation at all.

According to the Chart of the Nuclides, hydrogen has a relatively high cross-section for neutron absorption, but deuterium (hydrogen-2) has an absorption cross-section that is very low (in the range of tens of millibarns). This means that, while it is possible to form some tritium, it is unlikely that much will be formed in this shielding.

Although I have not heard of this being a problem, doing a sample certainly won't hurt anything. I'd suggest weighing a gram of paraffin and either powdering it or dissolving it into the scintillation cocktail to sample. This will give you an activity concentration in terms of dpm per gram. Good luck!

Andrew Karam, CHP, PhD