First Direct Measurement of Isotopic Fission-Fragment Yields of ^{239}U

Area of Science:

• Nuclear Physics
• Nuclear Reactions
• Nuclear Energy

Background:

• Accurate measurements of isotopic fission-fragment yields are crucial for understanding nuclear processes.
• Previous studies on uranium-239 (²³⁹U) fission-fragment yields have reported anomalies requiring further investigation.
• Fast neutron-induced fission is particularly relevant for advanced nuclear reactor designs.

Purpose of the Study:

• To perform the first direct and complete measurement of isotopic fission-fragment yields for ²³⁹U.
• To evaluate nuclear fission models at excitation energies relevant to fast neutrons.
• To investigate the persistence of nuclear shell effects in the tin (Sn) region during fission.

Main Methods:

• ²³⁹U fissioning systems were produced via one-neutron transfer reactions between a ²³⁸U beam and a ⁹Be target.
• Fission fragments were detected and isotopically identified using the VAMOS++ spectrometer at the Grand Accelerateur National d'Ions Lourds (GANIL) facility.
• Measurements were conducted at Coulomb barrier energies with an average excitation energy of 8.3 MeV.

Main Results:

• Direct and complete isotopic fission-fragment yields for ²³⁹U were obtained for the first time.
• The experimental data align with current nuclear model calculations.
• The study does not support the previously reported anomaly in ²³⁹U fission-fragment yields.

Conclusions:

• The findings provide essential data for validating and improving nuclear fission models.
• The results confirm the continued influence of spherical shell effects in the Sn region, even above the fission barrier.
• This research contributes to the understanding of nuclear fission relevant to next-generation nuclear reactors.