Nuclei of lithium-7 can absorb high-energy protons and emit neutrons, but they do not undergo fission when they absorb neutrons.
In contrast, uranium-235 nuclei undergo fission when they absorb neutrons, emitting several more neutrons in the process.
Give a reason why a self-sustaining chain reaction can occur in a sample of uranium-235 but not in a sample of lithium-7.