Yu.N. Kopatch

G.S. Ahmedov, D. Berikov, S.B. Borzakov, I.I. Chuprakov, G.V. Daniljan, S. Enkhbold, Fan Lyong Tuan, N.A. Fedorov,
Yu.M. Gledenov, D.N. Grozdanov, N.A. Gundorin, A.P. Kobzev, M. Kulik, V.L. Kuznetsov, E.V.Kuznetsova, Zh.V. Mezentseva, S.V. Mironov, V.V. Novitsky, I.A. Oprea, K.D. Oprea, Yu.N. Pokotilovskij, A.B. Popov, P.V. Sedyshev, M.V. Sedysheva, O.V. Sidorova, N.V. Simbirtseva, V.R. Skoj, A.M. Suhovoj, S.A.Telezhnikov, T.Yu. Tretyakova, Vu Dyk Kong, Sh.S. Zeynalov, 24 engineers, 4 workers

A group of participants near the experimental facility with the POLI diffractometer at the FRM II reactor. From left to right: Earl Babcock, Zahir Salhi, Daniyar Berikov, Aleksey Gagarsky, Vadim Novitsky, Yuri Kopatch.

• Measurement of TRI and ROT effects for gamma-rays and neutrons in the fission of uranium by polarized neutrons;
• Measurement of yields and angular correlations of light charged particles in ternary and quaternary fission of ²⁵²Cf using Timepix detectors;
• Determination of characteristics of excitation levels of nuclei in the (n, 2n) and (n, n'γ) reactions with neutrons with an energy of 14 MeV;
• Measurements of angular and energy distributions of prompt fission neutrons (PFN) in ²³⁵U(n,f) and ²³⁹Pu(n,f) reactions in the resonance region using a position-sensitive twin ionization chamber and 32 scintillation counters;
• Determination of model concepts of modern values of the level density and radiation widths of nuclei of various shapes and types in the capture of slow neutrons;
• Carrying out an experiment to search for a singlet deuteron;
• Measurement of fast neutron cross sections for ⁶Li(n,α) ³H and ⁹¹Zr(n,α) ⁸⁸Sr reactions.

A series of works on measuring the rotation effect (ROT-effect) of a fissile nucleus in the angular distributions of prompt γ-quanta has been carried out. This effect is expressed as a shift in the anisotropic angular distribution of γ-quanta (Figure 1) emitted using excited fission fragments with some small angle δθ towards the fission axis in the case of reverse direction of neutron beam polarization.

Figure 1. Experimentally measured angular distribution of prompt γ-quanta relative to the fission axis: the curve on the chart is the result of the approximation of this distribution by the function N(θ) = N(90o)·(1 + A·cos2θ). The red and green lines are measurements using neutrons with energies of 60 and 270 meV, respectively

In fact, it is not the angular distribution of γ-quanta that shifts using the angle δθ, but the fission axis shifts relative to the initial direction of the deformation axis of the fissile nucleus, i.e. the coordinate axis shifts and the angular distribution of γ-quanta is registered relative to it (Figure 2).

Figure 2. A visual diagram of generating a shift in the angular distribution of fission gamma-quanta

Since the fissile nucleus rotates at the moment of rupture, the fragment trajectories, being rectilinear without nucleus rotation, become hyperbolic that results in a deviation of the fission axis from the initial direction of the fissile nucleus deformation axis. In the experiment, it occurs as an asymmetry of counting of signal coincidences from gamma-quantum detectors and detectors of fragments with respect to the direction of the neutron beam polarization (Figure 3). The latter also determines the direction of the fissile nucleus polarization and consequently, the direction of its rotation.

Figure 3. Basic experimental results. Asymmetries D(θ) as a function of the angle for prompt fission γ-quanta. The solid line shows the result of approximation of the obtained angular dependence D(θ) by the function F = Rγsin(2θ)

The asymmetry of coincidence counting is determined by the ratio:

    D(θ) = [N⁺(θ) - N⁻(θ)]/ [N⁺(θ) + N⁻(θ)].                                           (1)

Here, N⁺(θ) and N⁻(θ) are the number of coincidences of the signals from gamma-quantum detectors and detectors of fragments (Figure 3), measured at two opposite directions of neutron beam polarization. Gamma-quantum detectors and detectors of fragments are positioned at an angle θ to each other in the plane orthogonal to the axis of the longitudinally polarized neutron beam.

Schematic diagram of the detector part of the experimental facility. 1—fission chamber, 2—inlet thin-walled aluminum window of the chamber, 3, 4— detectors of fission fragments based on position-sensitive low-pressure multiwire proportional counters (start and stop detectors), 5—holder, 6—plastic detectors.

It can be shown that

  D(θ) = δθ ∙ F'(θ)/F(θ),                                                                         (2)

F(θ) is a function describing the angular distribution of γ-quanta and F'(θ) is its derivative in the average position of the γ-quantum detector positioned at an angle θ.

Thus, by measuring the asymmetry and determining the angular distribution function of γ-quanta, one can determine the angle to which the fragment trajectory is shifted and consequently, the angular velocity and rotation direction of the fissile nucleus (Table 1).

Table 1. Basic results of ROT asymmetry for prompt gamma quanta of 235U fission using polarized neutrons

	0.3 eV	0.06 eV
А	0.163 ± 0.013	0.1570 ± 0.0053
Rγ	-5.4 ± 2.5	-17.3 ± 2.8
δ	0.021 ± 0.009	0.069 ± 0.008

In total, 3 series of experiments have been carried out, differing from each other in the energy of polarized neutrons, the neutron polarization method, detectors of fission fragments and their positioning, as well as the data acquisition and processing system. All measurements were carried out at the research neutron source Heinz Mayer-Leibniz (the FRM II reactor) at the Technical University of Munich in Garching (Germany).