Catapult neutrons from neck snapping in fission

Abstract: Dynamical fission calculations show that the post-scission configurations resemble two collinear pear-shaped fragments whose juxtaposed surface bulges subside relatively quickly, as the fragments acquire smoother shapes. The associated rapid speed of the healing bulge surface may boost nucleons in the fragment to energies sufficient for emission. The present study explores this mechanism by following the fate of nucleons that are reflected off the inwards moving bulge surface. The simulations suggest that the mechanism may produce high-energy neutrons at the level of a few per cent.

• The paper introduces the catapult mechanism, explaining the high-energy tail of prompt neutrons in fission through rapid surface bulge dynamics.
• It employs a classical trajectory model with Gaussian bulge profiles to quantify neutron yields (2.7–4.8%) and mean energies (7.3–12.4 MeV).
• The findings enhance reactor simulations and nuclear forensics by distinguishing scission neutrons from standard evaporation signatures.

Catapult Neutron Emission Mechanism in Nuclear Fission

Background and Motivation

Prompt neutron emission during nuclear fission is conventionally understood as predominantly originating from statistical evaporation off the highly excited fragments after scission. However, extensive experimental investigations have consistently hinted at the existence of a sub-population of neutrons—termed “scission neutrons”—with energies significantly exceeding those of evaporated neutrons. The generation mechanism for these scission neutrons has remained unresolved, with observed yields typically amounting to a few percent of the total prompt neutron yield. The present paper advances the theoretical understanding of these scission neutrons by focusing on the "catapult" mechanism: the boosting of nucleons via reflection from rapidly inwards-moving surface bulges produced by neck rupture at scission.

Physical Scenario and Mechanistic Model

Upon neck rupture, the fragments are left with pronounced protrusions—pear-shaped bulges—that quickly subside as surface energy is minimized. This morphological relaxation occurs with rapid surface velocities. A nucleon impinging upon such a moving surface may—upon reflection—gain kinetic energy proportional to the surface velocity. Employing a classical trajectory model and parameterizing the bulge shape with Gaussian profiles, the study calculates the probability for a neutron to become unbound and subsequently escape, quantifying yields and spectra as functions of bulge geometry and fragment properties.

This approach complements prior quantum-mechanical Time-Dependent Density Functional Theory (TDDFT) simulations, which are computationally intensive and have revealed two distinct scission neutron classes: early transverse emission from the neck and later emission from fragment ends—consistent with the catapult mechanism (2604.10790).

Numerical Results

Simulations affirm the viability of the catapult effect, yielding the following numerical findings for typical fission fragments:

• Yield: The multiplicity of catapult neutrons is calculated to be in the range of 2.7–4.8% per fragment, strongly dependent on bulge height and width. For an average fission event (e.g., $^{235}$U($n_{th}$,f)), the total catapult neutron fraction is approximately 3.1% of the prompt yield.
• Energy Spectrum: The mean energy of the emitted catapult neutrons lies in the range of 7.3–12.4 MeV, significantly harder than the statistical evaporation spectrum, which is typically dominated below $\approx$9 MeV. The catapult emission dominates the neutron spectrum above this cutoff, as validated against both event generators ({\sc freya}) and recent high-threshold dosimetry measurements (2604.10790).
• Angular Distribution: Boosted neutrons are preferentially emitted away from the fragment axis due to trajectory refraction at the surface, a pattern distinguishing them from evaporation neutrons.

Structural and Parametric Dependence

The catapult yield and spectrum are moderately sensitive to fragment morphology:

• Bulge Geometry: Higher and wider bulges correlate with increased yield and energy. Narrower bulges result in lower yield but harder spectra.
• Fragment Deformation: Greater axis ratios $(c/a)$ of spheroidal shapes marginally enhance both yield and mean energy. Spherical fragments exhibit the lowest yields and energies.
• Temperature Effects: Changing the nuclear temperature at scission (by ±20%) results in only weak variations in catapult yield, underscoring the robustness of the mechanism.

Theoretical and Practical Implications

The catapult mechanism is universal to all fission scenarios—spontaneous or induced—where significant neck-related surface distortions occur. Its signature (a small, highly energetic neutron tail) should facilitate experimental identification, with direct implications for improved modeling of neutron yields and spectra in nuclear data evaluations, reactor simulations, and nuclear forensics. The study provides quantitative guidance for future experimental campaigns, particularly leveraging angular and energy-resolved neutron spectroscopy above standard evaporation energies.

Theoretically, the results emphasize the need for coupled classical-quantum frameworks to capture both collective dynamics and single-nucleon excitation/escape during rapid shape transitions. The moderate sensitivity to geometric parameters suggests that detailed measurements of fragment shapes and associated neutron yields could constrain the dissipative and surface energy aspects of scission dynamics.

Outlook

Moving forward, the integration of TDDFT-based microscopic calculations with tractable classical simulations, such as those presented, will be essential for refinement. Further exploration of fragment-dependent features, escalation in bulge morphologies by manipulating TKE (total kinetic energy) partitioning, and precise measurement of neutron angular correlations and energy spectra at energies beyond 10 MeV are indicated as promising avenues. Given the practical importance in reactor design and nuclear safety, improved quantification of the catapult neutron yield may lead to refined nuclear databases and enhanced predictive fidelity in fission-based applications.

Conclusion

The study rigorously substantiates the catapult neutron emission mechanism as a significant contributor to the high-energy tail of prompt neutron spectra in nuclear fission, with robust numerical estimates for yield and spectral properties. Its universality and energetic distinction from evaporation neutrons provide clear avenues for experimental verification and modeling integration. The findings support the interpretation of recent high-energy neutron measurements and enable more precise characterizations of scission dynamics within the broader framework of nuclear fission theory and applications (2604.10790).