The Ni-63 produced by the Cu-63(n, p)Ni-63 reaction provides a unique measu
re to estimate the fast-neutron fluence of the Hiroshima/Nagasaki atomic bo
mb. In the similarity of the fission neutron spectrum of Cf-252 to that of
U-235, We have measured activation cross sections of the Cu-63(n, p)Ni-63 r
eaction averaged for the Cf-252 fission spectrum.