The 55th meeting of the Programme Advisory Committee for Nuclear Physics took place on 27 January online chaired by Professor M. Lewitowicz. The PAC Chairman presented information on the implementation of the recommendations of the previous PAC meeting. JINR Vice-Director S. N. Dmitriev informed participants of the event about the Resolution of the 130th session of the JINR Scientific Council (September 2021) and on the decisions of the JINR Committee of Plenipotentiaries (November 2021).

Deputy Director of the Laboratory of Neutron Physics E. V. Lychagin made a report on the theme “Investigations of neutron nuclear interactions and properties of the neutron” and the proposal for the extension of scientific and methodical activities within this theme until 2023. It is noteworthy that the following projects are being implemented in the frames of the theme: TANGRA (from 2017), ENGRIN (from 2022), EG-5 modernisation (from 2022).

The PAC members listened to the report with great interest and highly appreciated scientific results achieved by the Laboratory and the fulfilment of numerous interesting ideas for the last two years. For example, in the study of nuclear reactions caused by neutrons, an in-depth analysis was carried out of the results of P-even and T-odd correlation measurements in the ²³⁶U compound nucleus fission at energies of neutrons of 0,06 and 0,27 eV. It allowed comparing rotation angles of axis fission at various energies of neutrons for the first time.

Within the TANGRA project using Romashka detecting systems of 18 BGO detectors and an HPGe detector, specialists measured angle distributions and gamma quanta outputs in the reaction (n,n’γ) for neutrons with an energy of 14 MeV for C, O, Mg, Al, Si, Cr, and Fe nuclei.

Jointly with physicists from the Czech Technical University, the Laboratory measured rare modes of spontaneous ²⁵²Cf fission with a highly active sample (about 400 kBq). To register light particles, the Timepix detectors were used in order to detect the ²⁵²Cf quadruple fission. The statistics collection during two months of measurements made it possible to confidently distinguish in ternary fission p, d, t, α, Li, Be, B, and C as the lightest particle (nucleus) and spectra of registered particles (p, d, t, α). The cluster resolution of the Timepix detector makes it easy to separate the (α,α) and (α,t) pairs from quadruple fission.

The PAC noted that FLNP had reached considerable progress in the development of the first efficient reflectors of slow neutrons based on nanodiamond powders. The Committee considered the key areas of experimental research planned until the end of 2023:

• measurement of gamma-quanta spectra in s- and p-resonances aimed to search for P-even and T-odd effects in the reactions with slow polarised neutrons;
• measurement of mass-energetic and angle distributions of fragments, neutrons, and gamma-quanta of fission;
• search for rare fission modes, etc.

In addition, FLNP will continue methodological works in the following areas:

• modernisation of the EG-5 electrostatic generator, extension of the tool base of the accelerator complex;
• creation and development of neutron and gamma detectors for space crafts.

Extensive work was conducted with the use of various nuclear and physical methods for solving the tasks of ecology, material sciences, archaeology, art history, medicine, etc. These studies were carried out with a large number of scientists from research centres of the JINR Member States. It is planned to continue these activities.

The PAC recommended prolonging the project until the end of 2023 with the first priority.

N. D. Kovrizhnykh reported on new results achieved at the SHE Factory of FLNR. The speaker noted that in 2021, three series of experiments at the new DGFRS-2 gas-filled separator of the SHE Factory were carried out. To determine parameters of the new DGFRS-2 separator, scientists used the fusion reactions of ²⁴³Am, ²⁴²Pu, ²³⁸U with ⁴⁸Ca accelerated ions at the DC-280 cyclotron in which Мс, Fl, Cn isotopes and their daughter nuclei were produced.

In the fusion reaction ²⁴³Am+⁴⁸Ca at five ⁴⁸Ca energies with an intensity of up to 1.3 pµA, 6 new chains of ²⁸⁹Mc (2n-), 58 chains of ²⁸⁸Mc (3n-), two chains of ²⁸⁷Mc (4n channel) were synthesised and a new isotope ²⁸⁶Mc (5n channel) was produced. The ²⁸⁷Mc isotope has been observed in previous experiments at the DGFRS-1 only in three chains, and the ²⁸⁶Mc daughter nucleus was obtained only in two chains.

The ²⁶⁸Db α-decay was first detected. Its branch and half-life were measured, and the new ²⁶⁴Lr isotope was produced. The ²⁷⁹Rg spontaneous fission was registered for the first time. It was shown that the transmission of DGFRS-2 is two times higher than that at DGFRS-1.

In the experiment with ²⁴²Pu, the intensity of ⁴⁸Ca reached 3 pµA; 25 decay chains of ²⁸⁶Fl and 69 of ²⁸⁷Fl were synthesized at two ⁴⁸Ca energies. Indications have been obtained for the existence of two decay branches of ²⁸⁷Fl, ²⁸³Cn, and ²⁷⁹Ds, differing in the ratio of partial probabilities of ²⁷⁹Ds α-decay or spontaneous fission.

In irradiation of ²³⁸U, the intensity of ⁴⁸Ca reached a record value of 6.5 pµA. During September-October 2021, 16 decay chains of ²⁸³Cn were obtained in the experiment.

In the entire series of experiments, 177 decay chains of Mc, Fl, and Cn were registered, the decay properties of about 30 isotopes of elements from Rf to Mc were measured with higher precision. The reaction cross sections were measured at different ⁴⁸Ca energies. It was shown in the experiments that the new DGFRS-2 gas-filled separator reached design parameters, which makes it possible to conduct new experiments on the study of superheavy nuclei at the higher sensitivity level. The PAC congratulated the FLNR team on outstanding impressive results at the SHE Factory in the synthesis of superheavy nuclei and the study of decay properties. The Committee asked the FLNR Directorate to publish the first results of the experiments as soon as possible.

The PAC listened with interest to the scientific report “The double-γ decay width of the nuclear 2⁺₁ state” by A. P. Severyukhin.

Eight young researchers of the Laboratory of Nuclear Reactions made short reports on new results in the fields of nuclear physics at the PAC meeting. The PAC members highlighted the best three of them: “Detailed study of radioactive properties of No isotope decays using α, β, γ spectroscopy” presented by M. Tezekbaeva, “Creation of the facility and development of a method to study the symmetric combinations in manynucleon transfer reactions (²³⁸U+²³⁸U)” by K. N. Novikov, and “Production of radioactive targets for the first experiments at SHE Factory” made by A. Yu. Bodrov. The report by M. Tezekbaeva will be presented at the 131^st session of the Scientific Council of JINR. The PAC members met with the JINR Directorate to discuss a number of issues raised at the meeting.

The next meeting of the PAC for Nuclear Physics is scheduled for 30 June – 1 July 2022.