Binary fragmentations of excited nuclear systems in the 372 MeV ${}^{56}\mathrm{Fe}+{}^{232}\mathrm{Th}$ reaction

The binary fragmentation of the excited superheavy system of $Z=116$ formed in the reaction 372 MeV ${}^{56}\mathrm{Fe}+{}^{232}\mathrm{Th}$ has been investigated. The fragment masses and kinetic energies were determined through measurement of the fragment velocities by the time-of-flight method. Neutron spectra were measured in coincidence with the fragments at several angles with respect to the fragment direction, which were analyzed to deduce the total as well as the pre-scission neutron multiplicities. We analyzed the correlations between fragment mass and kinetic energy, gated and nongated by the coincidence with neutrons, to learn about the dynamics of the reaction with respect to the two-body exit channels. The events in the near-mass-symmetric valley region appear to receive a significant contribution from the asymmetric mode of fission of the superheavy compound nucleus $Z=116$, which can be due to the influence of the closed proton and neutron shells of $Z=50$ and $N=82$ in the light fragments. From the observed number of pre-scission neutrons, it is inferred that the time scales of the fissionlike reactions leading to near-mass-symmetric splits are rather large, of the order of several times ${10}^{-20}s$. The average number of prompt neutrons emitted in the spontaneous fission of such a superheavy nucleus is $\nu =(12\pm 1)$, as deduced from the neutron measurements.

Figure 1

(a)Fragment-mass and (b) TKE distributions for the 372 MeV ${}^{56}\mathrm{Fe}+{}^{232}\mathrm{Th}$ reaction. Dots mark the binary-fragment events; open circles are used for fragment-neutron coincidences. The fragment masses corresponding to the closed proton $Z=50$ shell, closed neutron $N=82$ shell, and symmetric split are marked by arrows in Fig. 1a.

Figure 2

Fragment-mass distributions for different TKE bins for the reaction 372 MeV ${}^{56}\mathrm{Fe}+{}^{232}\mathrm{Th}$.

Figure 3

Fragment TKE distributions for selected fragment-mass windows; the arrows mark the calculated $E_{c.m.}+Q_{\mathrm{gg}}$ values for each mass window.

Figure 4

Variation of fragment average total kinetic energy $\langle \text{TKE}\rangle$ with the mass of the detected fragment. The dashed curve corresponds to Viola's systematics [21]. The solid curve indicates predictions of the HICOL dynamical model [22].

Figure 5

Observed neutron spectra at different angles with respect to the beam direction for the fragment-mass window $A_{\mathrm{FF}1}=130–144$ amu. Multisource fits to spectra are also shown: Solid curves correspond to the total neutron emission, and dotted curves correspond to the pre-scission neutron emission.

Figure 6

Total and pre-scission neutron multiplicity as a function of light fragment mass $A_{\mathrm{FF}1}$ deduced from the fits to the measured fragment-neutron correlations.