Assessing improvements in sensitivity and measurement timescales using gamma-gamma coincidence spectrometry for assay measurements

Abstract A gamma-gamma ( $${{\upgamma }}-{{\upgamma }}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>γ</mml:mi> <mml:mo>-</mml:mo> <mml:mi>γ</mml:mi> </mml:mrow> </mml:math> ) coincidence spectrometer was modelled using the Geant4 toolkit and validated against experimental measurements. Incorporation of a decay-time estimation method enabled the generation of time-stamped list mode (T-List) data, facilitating a quantitative assessment of the performance advantages of high-resolution $${{\upgamma }}-{{\upgamma }}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>γ</mml:mi> <mml:mo>-</mml:mo> <mml:mi>γ</mml:mi> </mml:mrow> </mml:math> coincidence spectrometry relative to conventional high-resolution $${{\upgamma }}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>γ</mml:mi> </mml:math> -spectrometry. The detection limit sensitivity for specific radionuclides of interest in the presence of a specified contaminant were investigated and the relative benefits of coincidence analysis discussed. The detection limit for $$^{\text {134}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mtext>134</mml:mtext> </mml:mmultiscripts> </mml:math> Cs, using the 605-795 keV signature, is shown to be independent of $$^{\text {137}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mtext>137</mml:mtext> </mml:mmultiscripts> </mml:math> Cs activity but sensitive to $$^{\text {132}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mtext>132</mml:mtext> </mml:mmultiscripts> </mml:math> I due to the presence of abundant high-energy coincident photons. Similarly, the potential acceleration of analytical timelines through the combination of partial radiochemical separation and $${{\upgamma }}-{{\upgamma }}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mi>γ</mml:mi> <mml:mo>-</mml:mo> <mml:mi>γ</mml:mi> </mml:mrow> </mml:math> coincidence spectrometry is explored. For example, $$^{\text {156}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mtext>156</mml:mtext> </mml:mmultiscripts> </mml:math> Eu is predicted to be detectable in fission product matrices containing other lanthanide radionuclides using $${{\upgamma }}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>γ</mml:mi> </mml:math> – $${{\upgamma }}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>γ</mml:mi> </mml:math> coincidence spectrometry, despite remaining below conventional $${{\upgamma }}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>γ</mml:mi> </mml:math> -spectrometric detection limits, with quantifiable results obtainable several hours earlier than via full radiochemical separation followed by standard $${{\upgamma }}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>γ</mml:mi> </mml:math> -spectrometry. In the analysis of compositionally complex samples, this approach also enables the identification of radionuclides that adversely affect detection limits, thereby highlighting candidates for removal by radiochemical separation.