Question

here is the question:

Answer 1

but how?

Answer 2

@Vincent-Lyon.Fr

Answer 3

@Akhilender What is the significance of 20.4 eV is there a threshold for absorption at this energy? If the energy of the neutron is less than a value such that if absorbed it did not produced an excited state in deuterium the absorption would not occur and the neutron wound just elastically scatter.