Neutron emission during fission and its impact on fission-fragment mass distribution studied by Langevin model

TL;DR

The paper tackles how fission-fragment mass distributions (FFMDs) in actinides depend on excitation energy and shell effects. It extends Langevin-based fission dynamics by allowing neutron emission to occur at any stage of the fission path, coupling neutron decay widths and shell corrections to shape evolution. The results show that neutron emission during fission can revive shell effects and restore the asymmetric FFMD at high excitation, reproducing experimental trends across multiple isotopes and energies, unlike models that neglect emission. The work provides a unified, computationally efficient framework for multichance fission and yields insights into pre-scission neutron production and fission time scales.

Abstract

Actinide nuclei exhibit mass-asymmetric fission at low energy due to shell structure. The fission-fragment mass distributions produced at high energy tend to have a symmetric shape due to smearing of shell effects. On the other hand, the distribution can be changed by neutron emission occurring before fission, as this decreases the excitation energy of the fissioning nucleus, and thus revives the shell structure. In so called multichance fission, neutron emission is considered prior to fission at the initial nuclear shape, and competition between fission and neutron emission is determined with the framework of the statistical model. In the present work, we describe fission in the Langevin equations, and neutron emission is treated throughout the fission process. The calculation reproduces experimentally observed mass distributions, and for a wide range of initial compound-nucleus excitation energy up to 60 MeV. The results show that, while neutron emission dominates at the ground-state shape, it occurs along the shape evolution path to the scission point.

Figures (5)

• Figure 1: Comparison of the measured FFMDs Hirose2017-nsVermeulen2020-lf (black dots with error bars) for uranium, neptunium, and plutonium isotopes with the Langevin calculation. The red and green solid lines are the results with and without neutron emission during fission, respectively. The figure is arranged so that the isotones of the three compound nuclei species are vertically aligned. Average excitation energies are shown on the right side of the figure.
• Figure 2: The number of neutron emission $\nu_\mathrm{pre}$ of (a) ${}^{234-240}\mathrm{U}$, (b) ${}^{236-242}\mathrm{Np}$, and (c) ${}^{238-244}\mathrm{Pu}$ with the initial excitation energy $E^* = \qtyrange[range-units=single]{15}{55}{\MeV}$ calculated by the present Langevin model (red square). Values from GEF code (black circle) are also shown.
• Figure 3: Sample trajectory of the time evolution of the excitation energy ($E^*$) (black), the intrinsic excitation energy $E_\mathrm{int}$ (blue) and the number of emitted neutrons (red) for $^{238}\mathrm{U}$ at $E^* = 45\MeV$. The black shapes represent the nuclear shape at each point. The arrow indicates the time at which the nucleus passes through the second saddle point.
• Figure 4: Probability of neutron emission plotted on the (a) $z-\delta$ and (b) $z-\alpha$ planes for compound nucleus $^{238}\mathrm{U}$ at $E^* = 45\MeV$. The ground state and second minimum are represented by $\times$ and $+$, respectively. The first and second saddle points are indicated by $\bigcirc$ and $\triangledown$. The scission line is shown with the gray dashed line.
• Figure 5: Potential energy surface on the (a) $z-\delta$ ($\alpha = 0.0$) and (b) $z-\alpha$ ($\delta = 0.2$) planes for ${}^{238}\mathrm{U}$. The ground state and second minimum are represented by $\times$ and $+$, respectively. The first and second saddle points are indicated by $\bigcirc$ and $\triangledown$. The scission line is shown with the gray dashed line.