Characterization of the scission point from fission-fragment velocities

The isotopic yield distributions and kinematic properties of fragments produced in the transfer-induced fission of ${}^{240}\mathrm{Pu}$ and fusion-induced fission of ${}^{250}\mathrm{Cf}$, with 9 MeV and 45 MeV excitation energy, respectively, were measured in inverse kinematics with the spectrometer VAMOS. The kinematics of identified fission fragments allow to derive properties of the scission configuration such as the distance between fragments, the total kinetic energy, the neutron multiplicity, the total excitation energy, and, for the first time, the proton- and neutron-number sharing during the emergence of the fragments. These properties of the scission point are studied as functions of the fragment atomic number. The correlation between these observables, gathered in one single experiment and for two different fissioning systems at different excitation energies, give valuable information for the understanding and modeling of the fission process.

Figure 1

Fission fragment velocity as a function of the neutron number of the elements produced in the fission of ${}^{240}\mathrm{Pu}$ (left panels) and ${}^{250}\mathrm{Cf}$ (right panels). The error bars show the second momentum of the velocity.

Figure 2

Average fragment velocity $\langle V\rangle$ as a function of the fragment atomic number $Z$ produced in the fission of ${}^{240}\mathrm{Pu}$ (blue dots) and ${}^{250}\mathrm{Cf}$ (red squares).

Figure 3

The average neutron excess $\langle N\rangle /Z$ of fragments produced in the fission of ${}^{240}\mathrm{Pu}$ (a) and ${}^{250}\mathrm{Cf}$ (b) as a function of their atomic number $Z$ is displayed before (black markers) and after (blue open markers) post-scission neutron evaporation. The pre-evaporation $\langle N\rangle /Z$ it is compared to the estimation of a LD-SP model (red-dashed line). Specific configurations $Z=50,N=82$ and $Z=54,N=88$ are marked with green crosses.

Figure 4

Average neutron multiplicity $\langle \nu \rangle \left(Z\right)$ (a) and total neutron evaporation multiplicity $\langle {\nu }_{1+2}\rangle \left(Z\right)$ (b) for ${}^{240}\mathrm{Pu}$ (blue dots) and ${}^{250}\mathrm{Cf}$ (red squares). Data from fission of ${}^{238}\mathrm{Np}$ with $E^{*}\sim 11$ MeV (blue dashed line) [24] and ${}^{243}\mathrm{Am}$ with $E^{*}\sim 6$ MeV (red dotted line) [26] are shown for reference.

Figure 5

(a) Total kinetic energy of fission fragments at scission of ${}^{240}\mathrm{Pu}$ (blue dots) and ${}^{250}\mathrm{Cf}$ (red squares). The blue dashed line show data from thermal-neutron induced fission of ${}^{239}\mathrm{Pu}$ [27]. (b) Ratio between the distance between fragments at scission and that as spherical touching nuclei for fission of ${}^{240}\mathrm{Pu}$ (blue dots) and ${}^{250}\mathrm{Cf}$ (red squares).

Figure 6

(a), (b) Average difference between the ground states of the fissioning system and the fragments $\langle Q_{\mathrm{g}.\mathrm{s}.}\rangle$ at scission (full symbols) and the $\langle TK{E}^{*}\rangle$ (open symbols). (c), (d) Total excitation energy at scission $\langle TX{E}_{\mathrm{g}.\mathrm{s}.}^{*}\rangle$. (e), (f) Post-neutron evaporation normalized yields for reference. (a), (c), and (e) correspond to ${}^{240}\mathrm{Pu}$ while (b), (d), and (f) correspond to ${}^{250}\mathrm{Cf}$.

Figure 7

Minimum energy released in neutron evaporation and excitation energy shared according with the mass ratio of the fragments for ${}^{240}\mathrm{Pu}$ (blue dots and dashed line) and ${}^{250}\mathrm{Cf}$ (red squares and long-dashed line).