Correlation between energies and angles of α-particles emitted in thermal-neutron fission of U

The energy distribution and yields of the α-particles emitted in the thermal-neutron fission of ²³⁵U were measured with the same detector system for the cases when the average angles between fragments and α-particles were 90°, 46°, 27° and 11°. The data were analysed by the Monte Carlo method to take into account the effects of the finite size of the source and the various detectors, and the following results were obtained: (i) Even at small angles with respect to the fission axis the yield of the α-particles is about 2–3 % of the yield at 90°, and does not go to zero as would be expected for a true Gaussian angular distribution of a variance which fits the data around 90°. (ii) At these angles where most of the α-particle yield belongs to the non-Gaussian component, the α-particle energy distribution has a significantly higher most probable energy but the FWHM of the energy distribution is not significantly different, (iii) The rms width σ_θ of the Gaussian angular distribution is found to increase for very high (E_α > 20 MeV) α-particle energies and also to a lesser extent for very low (E_α < 15 MeV) energies. The origin of the yield of the α-particles at small angles, and the dependence of the rms width σ_θ on the energy are discussed.