Mass distribution and mass-resolved angular distribution in the ${}^{16}\mathrm{O}+{}^{238}\mathrm{U}$ reaction at sub-barrier energy

Background: The angular distribution of fission fragments or products is a sensitive probe to investigate non-compound-nucleus (NCN) fission. However, the gross angular distribution cannot distinguish between different NCN fission processes. Both preequilibrium fission and quasifission contribute to NCN fission and would both result in deviations from the statistical saddle point model (SSPM). Mass-resolved angular distributions of the fission product offers the possibility to distinguish between the two mechanisms [Sodaye et al., Phys. Rev. C 95, 014612 (2017); Tripathi et al., Phys. Rev. C 88, 024603 (2013); Vorkapic and Ivanisevic, Phys. Rev. C 55, 2711 (1997)].

Purpose: Mass and mass-resolved angular distributions of fission products have been measured in the ${}^{16}\mathrm{O}+{}^{238}\mathrm{U}$ reaction at $E_{\mathrm{lab}}=85.3\mathrm{MeV}$ to investigate the dominant NCN fission mechanism at sub-barrier energy.

Method: Mass and mass-resolved angular distributions were measured by recoil catcher technique followed by offline γ-ray spectrometry.

Results: The mass distribution was observed to be predominantly asymmetric. The angular anisotropy of the fission products was observed to be independent of their mass.

Conclusions: The mass independence of the angular anisotropy suggested quasifission to be the dominant NCN fission process. This was further confirmed from the underestimation of angular anisotropy by theoretical calculations even after including the contribution from preequilibrium fission.

Figure 1

Mass distribution in the ${}^{16}\mathrm{O}+{}^{238}\mathrm{U}$ reaction at $E_{\mathrm{lab}}=85.3\mathrm{MeV}$ ($E_{\mathrm{c}.\mathrm{m}.}/V_{\mathrm{c}}=0.95$).

Figure 2

Plot of angular distributions [$W\left({\theta }_{\mathrm{c}.\mathrm{m}.}\right)$ vs $\mathrm{co}{\mathrm{s}}^2\left({\theta }_{\mathrm{c}.\mathrm{m}.}\right)$] for different fission products in the ${}^{16}\mathrm{O}+{}^{238}\mathrm{U}$ reaction at $E_{\mathrm{lab}}=85.3\mathrm{MeV}$ ($E_{\mathrm{c}.\mathrm{m}.}/V_{\mathrm{c}}=0.95$). Data for fission products with $A/Z\ge 2.491$ are shown by open circles.

Figure 3

Plot of angular anisotropy values as a function of fission product mass number. Circles are experimental points. Data for fission products with $A/Z\ge 2.491$ are shown by open circles. Open stars and triangles are SSPM [33, 34] calculations for compound-nucleus fission. Filled stars and inverted triangles are the anisotropy values calculated after including the contribution from preequilibrium fission (see text for details).

Figure 4

(a) Moment of inertia of the fissioning nucleus in the vicinity of the liquid drop model saddle point (filled circles) and in the vicinity of the second saddle of the shell corrected potential energy surface (open circles). (b) Deformation (potential) energy of the fissioning nucleus as a function of fission product mass calculated using the liquid drop model (filled circles) and the liquid drop model with shell correction (open circles).