Toward fullerene-based radiopharmaceuticals: high-yield neutron activation of endohedral 165Ho metallofullerenes

C60 and endohedral 165Ho metallofullerenes were irradiated under various neutron flux conditions to maximize the activity of surviving 166Ho metallofullerenes. C60 survived irradiation to a fairly high extent (>80%) at fluences of <1018 n cm−2 (75.8% thermal), but fast-neutron damage (or equivalent oxidative decomposition) progressed rapidly at higher fluences. Under identical conditions the metallofullerenes were degraded more quickly and extensively than was C60, and decomposition proceeded primarily by fast-neutron damage instead of anticipated (n, γ)-metal recoil. Irradiation under more thermal neutron flux conditions (96.2% and 99.8%) significantly improved metallofullerene survival (20–30%), and degradation was demonstrated to proceed through the metal-recoil pathway.