Folding angle and excitation energy of fragments from ²³⁵U(n_th,f) and ²⁵²Cf(sf) reactions

Coincident fragments from ²³⁵U(n_th,f) and ²⁵²Cf(sf) reactions were investigated with a doublearm fission-fragment spectrometer and PIN-diode arrays. Based on the measurement of kinetic energy, velocity and direction of complementary fragments the total kinetic energy, the total mass, the individual fragment masses, the total number of emitted neutrons as well as the folding angle were deduced event by event. A nearly linear correlation between average folding angle and average total excitation energy of the fragments (and, consequently, the average number of neutrons) was found. Fragment deflection by prompt neutron emission is accordingly described by a complex statistical evaporation model in connection with a semi-empirical calculation of energy partition in nuclear fission. In addition, the folding-angle distribution due to ternary fission is estimated. All experimental fragment distributions and correlations are well repro- duced by the model calculations.