Question

Given a radioactive nucleus with decay constant lambda and total cross section Ot, determine the condition for which the nucleus will most likely decay before it has a collision with a neutron, if it is irradiated by a neutron beam with intensity Io [neutrons/cm^2-s].

Answer 1

Before the irradiance of neutron beam the radioactibve nucleus will decay by either alpha- or beta- or gamma - decay or the spontaneous fission which is not seen because such nucleus do not exist, condition for it is mass number should be above to 300. After irradiance of neutron beam the nuclear reactions like for fast neutrons (n,p), (n,2n) or (n,alpha) are possible, and for slow neutrons or thermal neutrons the neutron capture is main reaction in the form of (n,gamma). Neutrons with initial energy in 1MeV range would undergo many scattering processes until their energy were reduced to the eV range, where they would have a high probability of resonant or non resonant absorption. Now for radioactive nucleus with absorption of low energy neutrons will give induced fission. If the intermediate state is below the barrie, fission is inhibited and other decay modes, including re-emission of absorbed particle, may dominated. The ability of a nucleus to undergo induced fission will depend critically on the energy of the intermediate system; for some nuclei, absorption of thermal neutron is sufficient to push them over the barrier, while for others, fast neutrons may be required.