WWR-M reactor neutron spectrum study at 4-18 MeV by the differential recoil proton method

To clarify a question of neutron excess in the high-energy part of U-235 prompt fission neutron spectrum, the reactor neutron spectrum measurements in the wide energy range by differential recoil proton method have been carried out at angles of scattering 30 degrees, 35 degrees, 40 degrees, 45 degrees, and 50 degrees. The neutron spectrum was approximated by Maxwell distribution E similar to E-1/2 exp (-alpha E) from which the "statistical" error of the parameter a was determined. The measured spectrum is in good agreement with the 235U fission spectrum in the energy interval 6-12 MeV and it has the following parameters: alpha = (0.776 +/- 0.015) MeV-1, temperature T = (1.289 +/- 0.026) MeV, and average energy (E) over bar = (1.934 +/- 0.034) MeV. The analysis of the high-energy range (above 12 MeV) of the measured spectrum indicates its harder character (the average energy (E) over bar is within the limits of 2.72-3.01 MeV). The observed effect of neutron excess in this part of the spectrum is in favour of the hypotheses on high-energy scission neutron emission in nuclear fission.