Scission configuration of ${}^{239}\mathrm{U}$ from yields and kinetic information of fission fragments

The simultaneous measurement of the isotopic fission-fragment yields and fission-fragment velocities of ${}^{239}\mathrm{U}$ has been performed for the first time. The ${}^{239}\mathrm{U}$ fissioning system was produced in one-neutron transfer reactions between a ${}^{238}\mathrm{U}$ beam at 5.88 MeV/nucleon and a ${}^9\mathrm{Be}$ target. The combination of inverse kinematics at low energy and the use of the $\mathrm{VAMOS}++$ spectrometer at the GANIL facility allows the isotopic identification of the full fission-fragment distribution and their velocity in the reference frame of the fissioning system. The proton and neutron content of the fragments at scission, their total kinetic and total excitation energy, as well as the neutron multiplicity were determined. Information from the scission point configuration is obtained from these observables and the correlation between them. The role of the octupole-deformed proton and neutron shells in the fission-fragment production is discussed.

Figure 1

Fission-fragment mass and atomic number distributions measured in the $\mathrm{VAMOS}++$ focal-plane detection-setup. Lower panels show the resolution of the measurements in relative full width at half maximum.

Figure 2

Fission fragment velocity of ${}^{239}\mathrm{U}$: (a) Velocity of fission fragments in the laboratory reference frame. The horizontal/vertical axis corresponds to the projection of the velocity parallel/perpendicular to the forward trajectory. The dashed lines indicate the mean value of the heavy and light fragments. (b) Distribution of the fission-fragment velocity reconstructed in the reference frame of the fissioning system.

Figure 3

Yields and kinetic energies of fission fragments of ${}^{239}\mathrm{U}$ as a function of post-neutron-evaporation mass: (a) Post-neutron-evaporation mass yields. Present data (circles) are compared with evaluated data from ENDF/B-VIII.0 [36] (red lines) and with the gef (v. 2019/1.1) [37] (blue line). (b) Post-neutron-evaporation kinetic energy of fragments as a function of post-neutron-evaporation mass. Present data (circles) are compared with the gef calculation (blue lines).

Figure 4

Neutron excess and neutron multiplicity of fission fragments of ${}^{239}\mathrm{U}$: (a) Average neutron excess of fission fragments pre- and postneutron evaporation as a function of the atomic number. (b) Average neutron multiplicity as a function of the atomic number. Present data (symbols) are compared with the gef (v. 2019/1.1) calculation [37] (lines).

Figure 5

Average total kinetic energy of fission fragments of ${}^{239}\mathrm{U}$ as a function of the fission-fragment atomic number. Present data (circles) are compared with previous measurements from neutron-induced fission [42] (dotted green line) and with gef (v. 2019/1.1) [37] (solid blue line). The average $Q$ value of the reaction as a function of the atomic number is also presented (squares).

Figure 6

Elemental fission yields and neutron excess of ${}^{239}\mathrm{U}$ and ${}^{240}\mathrm{Pu}$: (a) Normalized yield as a function of the atomic number. Present data (black circles) are compared with a previous measurement of ${}^{240}\mathrm{Pu}$ [43] (red squares). (b) Pre-neutron-evaporation neutron excess of fission fragments as a function of the atomic number. Present data (black circles) are compared with a previous measurement of ${}^{240}\mathrm{Pu}$ [38] (red squares) and with a scission-point model based on the liquid-drop behavior (dashed lines, colors indicate the systems). Octupole-deformed shells $Z=52,56$ and $N=56,84,88$ are indicated by solid black lines (see text for details).

Figure 7

Average total excitation energy and elongation at scission: (a) Total excitation energy of the fragments at scission with respect to the ground state of the fissioning system. Present data (black circles) are compared with a previous measurement of ${}^{240}\mathrm{Pu}$ [38] (red squares) and with gef calculations (v. 2019/1.1) [37] (solid and dotted blue lines). (b) Elongation of the fissioning system at scission as a function of the atomic number.