Thick-target Yield of $^{65}$Cu($α,n)^{68}$Ga Near Threshold and Implications for Nuclear Medicine, Deep Underground Detector Backgrounds, and Nucleosynthesis

Abstract

We report thick target yields of $^{65}{\rm Cu}(α,n)^{68}{\rm Ga}$ from $α$ energies $E_α$$=$6.5--8~MeV measured via neutron-counting at the Edwards Accelerator Laboratory at Ohio University. Our results agree with prior results based on single-foil irradiations, while disagreeing with thick-target yields inferred from stacked-foil irradiations by up to an order of magnitude. Based on comparisons of our and prior measurements to Hauser-Feshbach calculations, we estimate the thick-target yield for $^{68}{\rm Ga}$ from $E_α$=14~MeV impinging on 100\%-enriched copper ($^{65}{\rm Cu}$) is 161$^{+5}_{-24}$~MBq/$μ$Ah (for He$^{++}$), which is in agreement with the current IAEA estimate of 156~MBq/$μ$Ah. At $E_α$$=$8~MeV, where radioimpurity-free production of $^{68}{\rm Ga}$ could be realized with a natural copper target, our yield estimate is nearly 4$\times$ lower than the IAEA estimate. We find that the $^{65}{\rm Cu}(α,n)$ cross section estimates presently employed for neutrino and dark matter detector backgrounds and the $^{65}{\rm Cu}(α,n)$ astrophysical reaction rate recently recommended for nucleosynthesis calculations are in agreement with our measurement results.