21-29 July 2022
US/Pacific timezone

Nuclear Data Gone Awry: Error in the IAEA Beam Monitor Reference Cross Section, $^{\rm nat}$Ni(n,x)$^{61}$Cu, Discovered while Measuring a Discrepant $^{40}$Ar(n,p)$^{41}$Ar Reaction.

27 Jul 2022, 08:09
Folsom ()



Darren Bleuel


To investigate the use of argon as a deuteron beam stopping volume, the $^{40}$Ar(d,p)$^{41}$Ar cross section was measured at average deuteron energies of 3.6 MeV, 5.5 MeV, and 7.0 MeV using an activation method. Natural nickel foils were used as both beam degraders and monitors, as the $^{nat}$Ni(d,x)$^{61}$Cu reaction has historically been one of only a few IAEA-recommended beam monitor reactions to measure deuteron fluence. A discrepancy, well outside reported uncertainties, was observed between the accepted and measured values of the intensity ratio of the two strongest gamma rays following $^{61}$Cu $\beta$ decay. This discrepancy has significant impact since a considerable number of published cross sections measured in ratio to that beam monitor cross section may depend on the choice of either the first or second strongest gamma ray in those calculations. To determine the magnitude of this error, over a hundred separate measurements of the 283 keV to 656 keV gamma-ray emission ratio were collected from seven experiments and a variety of detectors and detection geometries. A weighted average of all these ratios indicates an error in the value listed in the Nuclear Data Sheets of 11%, most probably in the second-highest intensity gamma ray, 656 keV. This has potentially introduced an 11% error in $^{61}$Cu production cross section measurements, cross sections using nickel activation as a deuteron beam current monitor, or in dose rates when $^{61}$Cu is used in nuclear medicine. Adjustment of the $^{nat}$Ni(d,x)$^{61}$Cu cross section, which was primarily based on the erroneous gamma-ray intensity, then agreed closely with our direct charge collection. Following this correction, the $^{40}$Ar(d,p)$^{41}$Ar cross section was determined from activation and found to be ~40% higher than a previous measurement and an order of magnitude higher than TENDL, further demonstrating the need for multiple, independent measurements of often-accepted nuclear data.

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