Scientific work “High-resolution magnetic analyzer MAVR for studying characteristics of nuclear reactions” by Yu. E. Penionzhkevich, V. A. Maslov, S. M. Lukyanov, N. K. Skobelev, I. V. Kolesov, G. G. Gulbekyan, et al. has been awarded the JINR Encouraging Prize for 2020. The authors tell more about the work.

One of the scientific activities of the Laboratory of Nuclear Reactions JINR is studying the mechanisms of nuclear reactions with stable and radioactive beams of accelerated nuclei. Research is also underway on the properties of nuclei located at the boundaries of nucleon stability. The experiments are carried out on the FLNR cyclotrons – U-400, U-400M, as well as in cooperation with other research centres in GANIL (France), RIKEN (Japan), and the Cyclotron Laboratory (Jyväskylä, Finland). In all the experiments, magnetic spectrometers and separators are used which make it possible to obtain a high degree of purification of reaction products from a primary beam, a high momentum resolution, and thus, high energy and mass resolutions of these reaction products. Taking into account the large momentum of the reaction products formed in reactions with heavy ions, it is necessary to register them at forward angles. Therefore, the main methodological problem in this case is the separation of the reaction products from the nuclei of the primary bombarding beam, the intensity of which is several orders of magnitude higher. Thus, the purification factor of the used magnetic analyzers should be no worse than 10^-3. This condition is met by the high-resolution magnetic analyzer (MAVR) created at FLNR, JINR. The MAVR analyzer has a relatively large solid angle (15 msr), high momentum resolution (10^-4), and a high dispersion along the focal plane (1.9 m). With this analyzer, it is possible to register the products of nuclear reactions with energies up to 30 MeV/nucleon and a high resolution in mass and charge, which is especially important for separation and identification of heavy products of nuclear reactions. Registration of the products of nuclear reactions is carried out using a multi-detector MULTI spectrometer installed in the focal plane of the analyzer, which allows registration and identification of the products of nuclear reactions by charge Q, atomic number Z, and mass A with an accuracy of one in terms of mass and charge. Nuclear reaction products are identified by A, Z, and Q by measuring the magnetic rigidity of the analyzer, energy loss in the focal detector (ΔE), time of flight (T), and total particle energy (E).

The MAVR analyzer can operate both on primary beams of heavy ions and on beams of radioactive nuclei of the accelerating complex of the U-400 and U-400R cyclotrons. It can be and is used directly both as an analyzer for measuring the energy spectra of the products of nuclear reactions and as a beam energy monochromator.

To increase the efficiency of registration of low-energy products of nuclear reactions due to their narrower ionic charge distribution in the gas, the analyzer is supposed to be used in a gas-filled version. The main purposes of using the analyzer are the following:

• obtaining data on the mass, charge, and energy distributions of the products of nuclear reactions with stable heavy ions (for example, ⁴⁸Ca, ⁵⁶Fe, ⁶⁴Ni) and with energies of 10≤E≤20 MeV/nucleon formed in the reactions of multi-nucleon and deep inelastic transfer at forward angles;
• measurement of energy spectra of high-energy light charged particles up to the kinematic limit of the two-body reaction channel;
• study of the products of nuclear reactions with high energy resolution on beams of exotic nuclei of light and medium masses produced at the DRIBs accelerator complex in the near-barrier energy region.

The first experiments carried out on beams of heavy ions on U-400 using the MAVR magnetic analyzer showed the following:

1. the parameters declared during its construction were achieved (large solid angle, high momentum resolution);
2. the possibilities of registration and identification of light reaction products in the range of nuclear masses A = 2-60 and registration of heavy transuranium products of fusion reactions and fission fragments were realized;
3. a high factor of purification of reaction products from the primary beam was achieved;
4. it became possible to carry out correlation measurements at forward angles at a relatively high intensity of the primary beam.

Part of the research is aimed at studying the characteristics of multi-nucleon and deep inelastic transfer reactions and reactions with emission of fast charged particles at forward angles in order to find the optimal reactions with the maximum cross sections, and thus providing the maximum yields of nuclei located at the boundaries of nucleon stability. No less important is the question of studying the deep inelastic transfer reactions with ions of mass A > 100. In such reactions, neutron-rich nuclei near the neutron line of stability may be formed with a relatively high probability. It is not excluded that these reactions may be an effective method for the synthesis of heavy elements. In the focal plane, in this case, it is supposed to measure the correlation of high-energy alpha particles with energies ≥80 MeV in coincidence with the cold recoil nucleus. The use of monochromatic heavy ion beams after the reconstruction of the U-400 cyclotron into the U-400R and the magnetic spectrometer MAVR-2 can be an effective method for carrying out further studies in this direction at FLNR.

Photo of the high-resolution magnetic analyzer MAVR from the focal plane.