Fission Fragment Measurements in Heavy Actinides Using Inverse Kinematics and Surrogate
Techniques

by Ioan Paul Parlea (1^st year PhD student)

In direct kinematics fission, the kinetic energies of the fission products are insufficient to obtain a clear identification of the fission products in A and Z in kinematical measurements, which implies that complete fission product distributions cannot be provided [1]. Additionally, due to radiochemical limitations, heavy targets can only be produced in small quantities and also witho possible contamination. As a result, the overall quality of an experiment is difficult to be ensured.

This prompted the creation of surrogate techniques. They allow researchers to study he same compound nuclei that would be formed in neutron-induced reactions [2]. Modern fission studies often use Multi-Nucleon Transfer reactions as a surrogate method because they can build systematics of fission probabilities across isotopic chains of heavy actinides and allow to precisely reconstruct the excitation energy of the compound nucleus [3].

The experiment under study was carried out at GANIL in 2024 and is based on the inverse-kinematics reaction between a 232Th beam and a 12C target of 100 μg/cm2 thickness. Fission fragments were identified event-by-event in both mass and atomic number using the VAMOS (VAriable MOde Spectrometer), providing full isotopic resolution.

The Second Arm, positioned at ~40° relative to VAMOS, extends the experimental capabilities by measuring the second fission fragment and enabling measurements and coincidence detection, contributing to a more complete kinematic reconstruction of the fission events. In addition, we
measure the Time-of-Flight of the evaporated neutrons.

This work presents calibrated Time-of-Flight measurements, as well as the energies and velocities of prompt neutrons emitted in coincidence with the fission fragments in the laboratory frame. While these emissions originate from both the compound nucleus and the fission fragments, their respective contributions are investigated through energy spectra and multiplicity analyses. These quantities are influenced by Multi-Chance Fission, which leads to a distribution of fissioning systems with decreasing excitation energy [4].

References:
[1] – Andreyev AN, Nishio K, Schmidt KH. Nuclear fission: a review of experimental advances and phenomenology. Rep Prog Phys. 2018 Jan; 81(1):016301. doi: 10.1088/1361-6633/aa82eb. PMID: 28753131.
[2] – Katsuhisa Nishio, 2020 Journal of Physics: Conference Series 1643 012151
[3] – Okumura, S., & Odsuren, M. (2018). Compilation of Experimental Nuclear Reaction Data. https://doi.org/10.61092/iaea.zcyf-xyxc
[4] – K-H Schmidt, B. Jurado. Review on the progress in nuclear fission-experimental methods and theoretical descriptions. Rep. Prog. Phys. 81, 106301 (2018)