Extraction of fission mode properties from fragment mass distributions

Background: Fission modes are typically characterized 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 evaluate an ``effective potential'' from the yields.Results: Several Gaussian functions are usually required to fit yields from nonharmonic potentials associated with a unique theoretical mode. ``Effective fission modes'', defined from the wells of the effective potentials, can be very different to the Gaussian modes. For instance, the S1 and S2 Brosa modes contribute to the same effective potential well at scission.Conclusions: Although Gaussian fits have proven useful to identify the role of shell effects in fission, they are not always associated with fission modes originating from valleys in the potential energy surfaces. Instead, properties of fission modes are better investigated with effective potentials extracted from the yields, preferably with a broad range of excitation energies.