Angular Momentum Removal by Neutron and γ-Ray Emissions during Fission Fragment Decays

Area of Science:

• Nuclear Physics
• Nuclear Reactions
• Nuclear Structure

Background:

• Understanding angular momentum in nuclear fission is crucial for nuclear structure studies.
• Previous assumptions suggested limited angular momentum removal during neutron and gamma-ray emission.
• Experimental measurements of fission fragment spin have shown complex behaviors, including sawtooth patterns.

Purpose of the Study:

• To investigate the angular momentum removal mechanisms from fission fragments (FFs) via neutron and gamma-ray emission.
• To evaluate the impact of angular momentum removal on FF spin distributions and their implications for observed correlations.
• To compare model predictions with experimental data on FF spin measurements.

Main Methods:

• Simulations were performed to model neutron and statistical gamma-ray emission.
• Angular momentum changes per fission event were calculated.
• Model predictions were compared with experimental data, including discrete transitions and isomeric states.

Main Results:

• Approximately half of emitted neutrons carry angular momenta greater than or equal to 1.5ℏ.
• Neutron and statistical gamma-ray emissions significantly alter FF spin by 3.5-5ℏ on average, with large standard deviations.
• The model reproduces experimental spin measurements, particularly for light FFs, and improves for heavy FFs when excluding isomeric states.

Conclusions:

• Angular momentum removal during neutron and gamma-ray emission is substantial and has wide distributions.
• These wide distributions can obscure underlying correlations in initial FF spins.
• The observed sawtooth behavior in average FF spin after statistical emissions may not directly reflect the scission mechanism.