A group of scientists of the Laboratory of Neutron Physics JINR has carried out a more detailed analysis of the data obtained during experiments in 2016-2018 on measuring the rotation effect of the fission axis, the so-called ROT effect, in the binary fission of ²³⁵U induced by monochromatic polarised neutrons. A new approach to the data allowed scientists to find the coefficient of angular anisotropy of emission of gamma-rays relative to the fission axis in 2020. The Romanian Reports in Physics journal published the scientific article with the obtained results in 2023.

“Initially, the experiment aimed at observing only the ROT effect. However, when we came back to the topic, it was necessary to measure its absolute value,” Daniyar Berikov, a researcher of the FLNP JINR Sector of Investigations of Neutron-Nuclear Interactions says. “This effect is expressed in the shift of the anisotropic angular distribution of γ-rays emitted by excited fission fragments at some small angle δθ relative to the fission axis when the neutron beam polarization direction is reversed. The coefficient of angular anisotropy of emission of γ-rays relative to the fission axis allows scientists to measure an angle at which the trajectory of the fragment is shifted, and, consequently, the angular velocity and direction of rotation of the fissioning nucleus. During the last analysis, we just obtained the results on the angular distribution of gamma rays relative to the fission axis, which are also of scientific value.”

The simplest integral parameter characterizing the anisotropy of the emission of γ-rays is the anisotropy coefficient A, defined as A = W(0)/W(90) — 1. The current paper aimed at finding the value of anisotropy A using the same geometric configuration of detectors and under the same experimental conditions that scientists used to study the ROT effect for prompt γ-rays in the binary fission, as all possible effects connected with the geometry of the experimental facility should be the same in both situations.

Researchers carried out experimental measurements at the Heinz Mayer-Leibniz research neutron source (FRM II reactor) at the Technical University of Munich in Garching (Germany). In the experiment, they used a beam of monochromatic neutrons with an energy of 60 meV provided by the POLI Diffractometer. Scientists used as a fission source the ²³⁵U target with a thickness of 1 mg/cm2 applied to a 30-µm thick aluminium foil. They installed it between a pair of stop detectors (fission fragment detectors), which are 10-section low pressure position sensitive multiwire proportional counters (LPMWPC) placed in a sealed fission chamber made of stainless steel. Specialists measured γ-rays in coincidence with fragments using plastic scintillators placed outside the fission chamber, about 30 cm from the centre of the target, forming 16 different combinations of angles relative to the sections of the fission fragment detector.

They found the coefficient of angular anisotropy using the approximation of the measured angular distribution of prompt γ-rays by the function N(θ) ~ 1+ A·cos2θ.

The angular distribution of prompt γ-rays in binary fission of ²³⁵U

As a result, the coefficient of angular anisotropy of emission of gamma rays relative to the fission axis was A= 0.1570 ± 0.0053, which is in good agreement with the results obtained by other authors for thermal neutrons.