That would make sense, since the longest lifetime of an unstable copper isotope is ~13 hours, and so chemical separation plus a week of waiting would give you a low background. By comparison, iron has Fe-60, a naturally occurring gamma-ray emitter with a 2.6M year half-life, and 10 million years is too long to wait. For iron shielding, you'd need isotopic separation to remove the Fe-60, which is wicked expensive.