Quantifying radionuclide signatures from a γ–γ coincidence system

• Monte-Carlo based software developed to easily create/update a coincidence signal library for environmental radionuclides.
• Coincidence library utilised to accurately quantify gamma coincidence signatures.
• All coincidence signature probabilities are corrected for cascade summing, conversion electron emission and pair production.
• Key CTBTO relevant radionuclides have been tested to verify the calculated correction factors.
• Accurately quantifying coincidence signals during routine analysis will allow dramatically improved detection limits.

A method for quantifying gamma coincidence signatures has been developed, and tested in conjunction with a high-efficiency multi-detector system to quickly identify trace amounts of radioactive material. The γ–γ system utilises fully digital electronics and list-mode acquisition to time–stamp each event, allowing coincidence matrices to be easily produced alongside typical ‘singles’ spectra. To quantify the coincidence signatures a software package has been developed to calculate efficiency and cascade summing corrected branching ratios. This utilises ENSDF records as an input, and can be fully automated, allowing the user to quickly and easily create/update a coincidence library that contains all possible γ and conversion electron cascades, associated cascade emission probabilities, and true-coincidence summing corrected γ cascade detection probabilities. It is also fully searchable by energy, nuclide, coincidence pair, γ multiplicity, cascade probability and half-life of the cascade. The probabilities calculated were tested using measurements performed on the γ–γ system, and found to provide accurate results for the nuclides investigated. Given the flexibility of the method, (it only relies on evaluated nuclear data, and accurate efficiency characterisations), the software can now be utilised for a variety of systems, quickly and easily calculating coincidence signature probabilities.