Subtraction Method for an Effective Quasi-monoenergetic Neutron Beam by Using Continuous Energy Spectra

The cross sections of 89 Y( n , 3 n ) 87 m ,87 g Y and 209 Bi( n , 4 n ) 206 Bi reactions at a neutron energy of 30 MeV are measured by making use of neutron beams of continuous energy spectra and a subtraction method. By impinging proton beams of 30 and 35 MeV to a thick beryllium target, neutron beams of continuous and broad energy spectra are produced and are guided to Y and Bi sample targets. The difference between the two neutron spectra generated by two neighboring proton energies is found to be peaked in a narrow energy range and thus can be regarded as quasi-monoenergetic, which can be used to extract ( n, xn ) cross sections. The uncertainty in the neutron fluence is reduced by analyzing the activities of aluminum and niobium reference samples placed on top of the Y and Bi samples. The use of a subtraction method by employing neutron beams of continuous energy spectra gives us the 89 Y( n , 3 n ) 87 m ,87 g Y and 209 Bi( n , 4 n ) 206 Bi cross sections in fair agreement with the existing experimental data and nuclear data libraries.