Cu*

A discrepancy, well outside reported uncertainties, has been observed between the accepted and measured intensity ratios of the two strongest gamma rays following ⁶¹Cu β decay.  This discrepancy has significant impact since the ^natNi(d,x)⁶¹Cu reaction has historically been one of the few IAEA recommendations for use as a deuteron flux monitor.  A considerable number of published cross sections measured in ratio to that beam monitor cross section may depend on the choice of using either the first or second strongest gamma ray in those calculations.  To determine the magnitude of this error most precisely, 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 these ratios indicates an 11% error in the value listed in the Nuclear Data Sheets, most probably in the second-highest intensity gamma ray, 656 keV.  This has potentially introduced an 11% error in ⁶¹Cu production cross section measurements, cross sections using nickel activation as a deuteron beam current monitor, and/or in dose rates when ⁶¹Cu is used in nuclear medicine.