Where the file nprod.tot, nprod.tot.063, and nprod.tot.065 are the total cross section data for alpha energy from 0.1-8.8 MeV. The unit of the cross section is mb. I didn't use them in the calculation because we are doing the neutron energy spectrum which need differential cross section data.The file name nspec*.*.tot is the differential cross section data for alpha energy *.*MeV with the nature copper target. If there is a suffix of .063 or .065 that means the specific isotope Cu63 or Cu65. The alpha energy is listed from 0.1MeV to 8.8 MeV. If you cannot find those lower energies, that's because they are not be able to produce any neutrons due to the coulomb barrier. The neutron energy(MeV) and the cross section data(mb) are listed in the differential data file.