(e,e'f) Coincidence experiments on 235U and 238U

Abstract

Coincidence experiments for (e, e'f) on the actinide nuclei 235U and 238U have been performed at the Mainz Microtron (MAMI A) concentrating on three subjects: multipole strength distributions and form factors for the lowest multipolarities, the mass split in the fission decay of various giant multipole resonances, and the separation of near barrier fission channels. Data were taken at four values of momentum transfer ( q eff ≈ 0.20, 0.28, 0.53, and 0.71 fm −1 for 238U, q eff ≈ 0.20, 0.44, 0.57, and 0.71 fm −1 for 235U) for excitation energies ω = 4–22 MeV. The fission fragments have been detected using the Giessen PPAC-Ball. A model-independent multipole analysis yields both form factors and strength distributions for E1, E2/E0, and E3 excitations. The extracted E1 strength distributions in the fission channel are in good agreement with recent photofission data. The extracted E2/E0 strength distributions in the fission channel show two distinct resonant structures centered at ω ≈= 10 and 14 MeV, attributed to concentrated collective E2 and E0 strength, respectively. Up to 12 MeV for 238U (19±2)% and for 235U (39±2)% of the isoscalar E2 energy-weighted sum rule is exhausted. The extracted form factors can be described within a hydrodynamical model by use of parameters c tr c 0 = 1.2 and 1.0 for E1 and E2, respectively. The mass split in the fission decay of giant multipole resonances has been studied via the measurement of time-of-flight difference spectra of both fission fragments. A model-independent multipole decomposition yields the fission width ratios Γ S/Γ A as functions of excitation energy for separated multipoles (E1, E2/E0, and E3). An increased symmetric contribution is evident for the isoscalar giant quadrupole resonance as compared to giant E1 and E3 resonances. Complete in- and out-of-plane fission fragment angular correlations for 238U have been measured for the two lowest values of momentum transfer. A combined analysis with the extracted E1 and E2/E0 multipole strength distributions yields strength distributions for 6 near-barrier fission channels ( J π , K)=(0 +,0), (1 −,0), (1 −, 1), (2 +,0), (2 +, 1), and (2 +, 2) up to 8 MeV. An estimation of the respective fission barriers deduced from the increase of the fission cross sections is given.