Proton-induced fission of ${}^{181}$Ta at high excitation energies

Total fission cross sections of ${}^{181}$Ta induced by protons at different relativistic energies have been measured at GSI Darmstadt using the inverse kinematic technique. These data contribute to solve inconsistencies in previously reported measurements, but also help to benchmark state-of-the-art reaction codes. The energy range covered with these measurements allowed us to investigate the onset and temperature dependence of dissipative and transient effects at small deformation.

Figure 1

Schematic representation of the experimental setup used in the present experiment.

Figure 2

Scatter plot of the amplitudes of the signals registered with the two MUSIC detectors. Nuclei lighter than ${}^{181}$Ta produced in reactions before MUSIC 1 appear in the diagonal region. In the vertical line the ${}^{181}$Ta spot corresponding to noninteracting beam particles is represented, and below are the events corresponding to reactions in the target. The box encloses the fission region and the color code represents counts on a logarithmic scale.

Figure 3

Scatter plot of the energy-loss signals provided by the two paddles of the double plastic scintillator placed downstream from the target with the reaction trigger (top panel, 1 $A$ GeV, full target; bottom panel, 300 $A$ MeV, full target). Both plots are normalized to the same number of counts to highlight the influence of different reaction channels. The color code represents counts on a logarithmic scale.

Figure 4

Same as the top panel in Fig. 3 but conditioned by the fission selection from Fig. 2. The different contours and insets illustrate the background suppression method used to identify fission events as explained in the text.

Figure 5

Projection of the fission region defined in Fig. 4 on the $\Delta E_1+\Delta E_2$ axis represented in the same figure. The contribution of evaporation residues was evaluated by means of a Gaussian fit (dotted line) and subtracted from the total contribution (solid line).

Figure 6

Energy-loss signals provided by MUSIC 1. The region between the dotted red lines corresponds to ${}^{181}$Ta.

Figure 7

Fission cross sections measured in this work (solid circles) in comparison to previously measured data and estimates obtained from the systematics established by Prokofiev (dashed line).

Figure 8

Fission cross sections measured in this work (solid circles) in comparison to model calculations using incl+abla, isabel+abla, and incl+gemini++.

Figure 9

Mass, charge, and excitation energy distributions of the prefragments leading to fission produced at the end of the intranuclear cascade (thick line) and the residual nuclei reconstructed from both fission fragments (thin line) for the reaction ${}^{181}\text{Ta}+p$ at 1 $A$ GeV, predicted by incl+gemini++, and isabel+abla, and incl+ablaconsidering $\beta =4.5\times {10}^{21}{\text{s}}^{-1}$. For the excitation energy distributions, the thin line refers to the system at the saddle point.

Figure 10

Mean value of the mass, charge, and excitation energy of the prefragments leading to fission (thick lines) and the residual nuclei reconstructed using the mass and atomic number of both fission fragments, $A_{\text{res}}$ and $Z_{\text{res}}$ (thin lines) for the reaction ${}^{181}\text{Ta}+p$ as a function of the beam energy calculated with incl+abla (dotted lines) and isabel+abla (solid lines) considering $\beta =4.5\times {10}^{21}{\text{s}}^{-1}$ and incl+gemini++ (dashed lines). For the excitation energy curves, thin lines refer to the saddle point.

Figure 11

Same as Fig. 10 but for the RMS of the distributions.

Figure 12

Fission cross sections measured in this work (solid circles) in comparison to calculations using incl+ablafor different values of the reduced dissipation coefficient and considering the Kramers picture of fission. The cross sections predicted by the systematics of Prokofiev [42] are indicated by crosses.

Figure 13

Average fission lifetime (solid line) compared to the corresponding average transient time (dashed line) as a function of the proton energy for the reaction ${}^{181}\text{Ta}+p$. The shaded area represents the time dispersion induced in both cases by the different temperatures and fission barriers of the fissioning nuclei.