Transition from asymmetric to symmetric fission in 45-69 MeV proton induced fission of 238U

Thin targets of 238U were bombarded with 45, 55, 65 and 69 MeV protons at the Cyclotron and Radioisotope Center (CYRIC) of Tohoku University in Japan. Gamma rays emitted from the fission products were recorded and analyzed by using gamma spectroscopy. Then, the cross sections for the formation of fission products, fragment mass distributions, and total fission cross sections were measured. Special care was taken to make this measurement precisely. The experimental results show that for 238U(p, f) reaction at Ep > 65 MeV, the asymmetry component in the mass distribution is eliminated, and the symmetric fission remains the dominant fission mode.