First Experiment on Fission Transients in Highly Fissile Spherical Nuclei Produced by Fragmentation of Radioactive Beams

We report on a novel experimental approach for studying the dissipative spreading of collective motion in a metastable nuclear system, using, for the first time, highly fissile nuclei with spherical shape. This was achieved by fragmentation of 45 radioactive heavy-ion beams at GSI, Darmstadt. The use of inverse kinematics and a dedicated experimental setup allowed for the identification in atomic number of both fission fragments. From the width of their nuclear-charge distributions, a transient time of $(3.3\pm 0.7)\times {10}^{-21}\mathrm{s}$ is deduced for initially spherical nuclei.

Figure 1

Top: Illustrative drawing of the radioactive-beam production (left) and of the subsequent fragmentation-fission reaction (right). Typical time scales ($t$ and $t$’ refer to the primary and secondary reaction, respectively) are indicated. Bottom: Experimental setup involved along the outlined reaction scenario, consisting of the FRS (left) and the fission setup (right).

Figure 2

Experimental correlation between the nuclear charges ($Z_1$, $Z_2$) of the fission fragments for a ${}^{222}\mathrm{Th}$ beam. The color scale is logarithmic.

Figure 3

Calculated correlation between the initial excitation energy of the system and the $Z_1+Z_2$ sum for the ${}^{215}\mathrm{Ra}$ beam. The color scale is logarithmic.

Figure 4

Width ${\sigma }_Z$ as a function of $Z_1+Z_2$ for a sample of spherical secondary beams as indicated. The data (dots) are compared with calculations using Bohr-Wheeler [22] (dotted lines) and Kramers [23] (dashed lines) fission-decay width as well as the time-dependent approximation (full lines) from 18. In the two latter cases, $\beta$ is set to $4.5\times {10}^{21}{\mathrm{s}}^{-1}$. Staggering in the calculations is due to statistical fluctuations.