Experimental Evidence for Common Driving Effects in Low-Energy Fission from Sublead to Actinides

Isotopic distributions of fragments from fission of the neutron-deficient ${}^{178}\mathrm{Hg}$ nuclide are reported. This experimental observable is obtained for the first time in the region around lead using an innovative approach based on inverse kinematics and the coincidence between the large acceptance magnetic spectrometer $\mathrm{VAMOS}++$ and a new detection arm close to the target. The average fragment $N/Z$ ratio and prompt neutron $M_n$ multiplicity are derived and compared with current knowledge from actinide fission. A striking consistency emerges, revealing the unexpected dominant role of the proton subsystem with atomic number between the $Z=28$ and 50 magic numbers. The origin of nuclear charge polarization in fission and fragment deformation at scission are discussed.

Figure 1

(a) Schematic of the experimental setup for detecting the coincident fission partners (Ff1 and Ff2). (b) Correlation between the fragment energy loss $\mathrm{\Delta }E$ and total energy $E_{\mathrm{tot}}=\mathrm{\Delta }E+E_{\mathrm{res}}$ detected in $\mathrm{VAMOS}++$. Some $Z$ lines are indicated for reference. (c) Postneutron $A$ distributions for selected $Z$ values. Preneutron (d) $A$ and (e) TKE distributions.

Figure 2

(a) Postneutron $⟨N⟩/Z$ as a function of $Z$. The uncertainty is dominated by statistics, with a remainder due to systematics errors ($Z$ gating and account of acceptance). Shaded yellow bands delimit the regions of the preneutron $⟨N⟩/Z$, with the green thick dashed line corresponding to the macroscopic prediction. (b) $⟨M_n⟩$ multiplicity as a function of postneutron mass. The error bars are mainly contributed by statistics that dominates the determination of $⟨A_{\mathrm{pre}}⟩$. Dashed vertical lines locate symmetry.

Figure 3

(a) Preneutron $⟨N⟩/Z$ for ${}^{178}\mathrm{Hg}$ (black dots) compared with ${}^{239}\mathrm{U}$ [37] (blue open squares) and ${}^{240}\mathrm{Pu}$ [38] (red open triangles). The macroscopic predictions are depicted for each system with a line. (b) $⟨N⟩-⟨N_{\text{macro}}⟩$ difference as a function of $Z$, see text. The downsloping orange line is to guide the eyes.

Figure 4

Neutron $⟨M_n⟩$ multiplicity as a function of $Z$ for ${}^{178}\mathrm{Hg}$ (black dots) compared to various actinides (colored lines) [37, 48, 49]. Error bars are not shown for the latter for legibility. The inset shows $⟨M_n⟩$ as a function of $N$.