Cumulative Fission Yield Measurements from 235U(nth,f) with the FIPPS Instrument

Abstract

This work reports high-precision cumulative yield measurements of key isotopes from ²³⁵U(n_th,f) reactions using the FIPPS (FIssion Product Prompt γ-ray Spectrometer) at ILL, representing the first dedicated cumulative fission yield campaign in the facility.

In this work, advanced spectroscopic techniques were employed to reduce nuclear data uncertainties, while evaluating FIPPS capabilities for fission yield measurements.

A pre-irradiated ²³⁵U target was exposed to a high neutron flux, and the resulting γ-rays were recorded using a 16-element HPGe Clover array. A 7-day irradiation at a thermal neutron flux of ~3.3 x 10⁷ n/s/cm² was followed by a 23-day decay period.

The multi-parameter FIPPS setup, providing data from 64 detector channels, enabled detailed reconstruction of the entire fission process from irradiation to decay. Our analysis framework combined machine learning-based spectral analysis with γ-emission simulations, establishing new benchmarks for fission yield measurements. The approach demonstrates the strength of integrating high-precision γ-spectroscopy with advanced computational methods. Cumulative yields for key isotopes were determined with small uncertainties and showed good agreement with evaluated libraries. The developed methodology offers a robust foundation for future fission product studies and nuclear data improvements, applicable to both short- and long-lived isotopes of nuclear relevance.