On the extraction of fission mode properties from fragment mass distributions

TL;DR

This paper investigates how Gaussian fits of fission fragment mass distributions relate to underlying theoretical modes, highlighting the limitations and proposing methods to better compare experimental data with potential energy surface models.

Contribution

Introduces a simple approach inspired by the scission point model to compare Gaussian fission modes with theoretical modes from potential energy surfaces, considering anharmonic effects.

Findings

Multiple Gaussian functions are needed for non-harmonic potentials.

Effective fission modes can differ significantly from Gaussian modes.

Gaussian fits help identify shell effects in fission.

Abstract

Background: Fission modes are typically characterised by fragment mass and total kinetic energy centroids, around which a distribution of these variables is observed. These distributions are usually fitted with Gaussian functions. Purpose: To investigate how the properties of these ``Gaussian fission modes'' compare with underlying ``theoretical fission modes'' defined from the potential energy surface of the fissioning nuclei. Methods: A simple approach, inspired by the scission point model, is introduced to investigate the impact of anharmonicity of the potential along the scission line on Gaussian mode properties. This approach is also used to evaluated an ``effective potential'' from the yields. Results: Several Gaussian functions are usually required to fit yields from non-harmonic potentials associated with a unique theoretical mode. Similarly, ``effective fission modes'', defined from the wells of the effective potentials, are sometime very different to the Gaussian modes. For instance, the S1 and S2 Brosa modes contribute to the same effective potential well at scission. Conclusions: Gaussian fits are useful to identify the role of shell effects in fission. However, comparisons with theoretical potential energy surfaces are better carried with effective potentials extracted from the yields, assuming that a broad range of excitation energies is available.