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 (12.6 ± 1.6)% obtained. The cross-section for the cosmic-ray-induced production by fast muons was measured to be (0.64 ± 0.33) 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 (4-5) · 10^{3 63}Ni 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.