Cosmic-ray-induced 63Ni --A potential confounder of fast-neutron-induced 63Ni in copper samples from Hiroshima

Recently, the determination of 63Ni in copper samples has been suggested as a means to assess fast-neutron fluences in Hiroshima and Nagasaki. In those samples, 63Ni (half-life: 100.07 years) was produced by MeV neutrons from the A-bomb explosions via the reaction 63Cu(n,p)63Ni. For large distances from the hypocenters, cosmic-ray-induced production of 63Ni might also be important and, therefore, it is calculated here. The effective probability f* which is required to quantify the cosmic-ray-induced production by stopped muons, was measured, and a value of (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$12.6 \pm 1.6$\end{document})% obtained. The cross-section for the cosmic-ray-induced production by fast muons was measured to be (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$0.64 \pm 0.33$\end{document}) mb, at a muon energy of 100 GeV. To validate the proposed method, cosmic-ray-induced production of 32P in sulfur and of 39Ar in granite was also calculated, and reasonable agreement with literature values was found. Our estimates indicate that as many as \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$(4 -5)\cdot 10^{3}$\end{document}63Ni nuclei per gram copper were produced in a sample that was exposed to cosmic radiation in Hiroshima for about 80 years. A similar concentration due to A-bomb neutrons would be expected in Hiroshima at a distance from the hypocenter of about 1900 m.