Neutron Production for Different Radiators

In the following figures the results for the prediction of the neutron yield from Bremsstrahlung-photons are shown. They were obtained using an 20 MeV electron beam impinging onto two different radiators made of Aluminum and Niobium with a thickness of 1% radiation length. The neutron energy is given by: En=Eγ-Sn assuming that the daughter nucleus will be in the ground state after neutron emission.


Photo-neutron production cross section σ(γ,n) for both absorbers and Lorentz fits to these data. The upper curve shows the results for Niobium and the lower curve those for Aluminum.
Bremsstrahlungs-photon flux, and flux multiplied with the (γ,n) cross section for both absorbers. The photon flux from Aluminum equals almost the one from Niobium radiator. Obviously, the threshold behavior of the resonance isn't parameterized properly with a simple Lorentz-fit.
Resulting neutron spectrum for both absorbers. The total neutron yield for the Niobium radiator is higher by a factor of 92. The huge difference between Niobium and Aluminum stems from the much higher Giant Resonance cross section in Nb.
Resulting neutron time distribution at the target location. The time-of-flight for photons between radiator and target is subtracted.