Fast Decision in Favor of the Slow Fission Process

The fission probability $P_{\mathrm{f}}$ of highly excited targetlike nuclei produced in reactions of 2.5 GeV protons on Au, Bi, and U was studied as a function of excitation energy $E^{*}$ whereby $E^{*}$ is deduced eventwise from the multiplicity of evaporated light particles. At the highest $E^{*}$ of 1000 MeV $P_{\mathrm{f}}$ amounts to $\approx 30\%$ with all 3 target nuclei irrespective of the initial fissility. Statistical-model calculations satisfactorily reproduce the observed evolution of $P_{\mathrm{f}}$ with $E^{*}$—provided that no extra transient delay is introduced. Fission thus is decided upon very fast and early in the long deexcitation chain towards scission which comprises as much as $\approx 80\%$ of all evaporated alpha particles.

Figure 1

Measured inelastic or inclusive (○) and fission (▲) differential cross sections $d\sigma /d{E}^{*}$ as a function of $E^{*}$ for $p(2.5\mathrm{GeV})+\mathrm{U}$, Bi, and Au. The histograms show the result of INCL2.0 [10] calculations of the inclusive cross sections.

Figure 2

Logarithmic contour plots used for the selection of fission fragments from the reaction $p+\mathrm{Bi}$. (a) Correlation between the two heaviest fragments $A_1$ and $A_2$. (b) Total kinetic energy (TKE) of the two FFs as a function of their sum mass $A_1+A_2$. (c) Distributions of the folding angle ${\theta }_{\mathrm{fold}}$ between the FFs (in arbitrary $\mathrm{\text{units}}/\mathrm{rad}$) as a function of $E^{*}$.

Figure 3

Comparison of experimental (symbols) and simulated (histograms) fission probability as a function of $E^{*}$. The association with the three target nuclei is given in the figure. The simulation for $p+\mathrm{Au}$ with $a_{\mathrm{f}}/a_n=1.05$ and ${\tau }_{\mathrm{d}}=2\times {10}^{-21}\mathrm{s}$ according to Ref. 5 is shown by the dot-dashed histogram.

Figure 4

Energy spectra of $\alpha$ particles that are emitted into 3 angular domains relative to the motion of the light fission fragment: ${\Theta }_{\alpha \mathrm{\text{−}}\mathrm{LF}}={10}^{°}$ to 40°, 70° to 100°, and 140° to 170°. Three calculated components are fitted to the experimental distribution (circles): the contribution from the compound nucleus prior to scission (thick continuous lines) and from the light (dashed lines) and the heavy (dotted lines) fission fragment. At 70° to 100° a further component for the neck emission is added (thin line). The total calculated spectrum is shown by the histogram. Example from 2.5 GeV $p+\mathrm{Au}$ at $E^{*}=600–900\mathrm{MeV}$.