Scientists at JINR created a novel specialised setup for the detailed study of multinucleon transfer reaction products and conducted a first series of experiments. Multinucleon transfer reactions are promising for synthesising and studying the properties of heavy and superheavy nuclei. The experiments will allow for a deeper understanding of the dynamics of such reactions and for their more efficient use in synthesising new nuclei, including those that are inaccessible in other reactions.

Although significant results have been achieved in synthesising new superheavy elements in complete fusion reactions of two heavy nuclei, synthesis of more neutron-rich isotopes is not feasible in complete fusion reactions involving stable beams and actinide targets, which triggers a search for alternative approaches for synthesising these nuclei.

In recent years, it has become evident that multinucleon transfer (MNT) reactions, leading to the formation of two heavy fragments with full momentum transfer, are promising for synthesising heavy and superheavy nuclei and studying their properties. The bulk of fragments — projectile-like fragments (PLFs) and target-like fragments (TLFs) — are centred around the projectile and target nucleus masses, respectively. In MNT reactions, the excitation energy of a heavy fragment can be substantially lower than that in complete fusion, which provides favourable conditions for synthesising heavy isotopes of transuranium elements that arouse specific interest owing to the limitations of their synthesis in fusion reactions. Significantly, heavy isotopes of a number of transuranium elements, ²⁵⁴Cf, ²⁵⁵Es, ²⁵⁶Fm, which cannot be produced in complete fusion reactions, were synthesised in MNT reactions.

Modern theoretical calculations based on the multidimensional Langevin-type dynamical approach have shown that the cross section for the formation of primary transtarget fragments in the reactions of heavy-ion beams and actinides is a few millibarns. The fission barrier of these TLFs is mainly defined by the shell correction (Bf ≈ 5 MeV), and the probability of sequential fission during deexcitation is high, whereas their survival after the evaporation of light particles is several orders of magnitude lower.

²³⁸U ions are apparently the most promising bombarding particles for synthesising superheavy elements in MNT reactions. However, uranium beams can nowadays be produced only in several research centres; thus, thorough studies of such reactions as ¹³⁶Xe+²³⁸U and ²⁰⁹Bi+²³⁸U, wherein uranium is used as a target material, allow for a deeper understanding of MNT mechanisms and help plan future experiments on the synthesis of new heavy isotopes in reactions with ²³⁸U beams.

Because sequential fission of TLFs is more feasible than their survival after deexcitation, two fragments (PLF + TLF) and three fragments (PLF + both fragments of sequential fission of TLF) need to be identified. To study the properties of MNT fragments, such as their production cross sections, excitation energies, and survival probabilities, the primary and secondary mass and energy distributions of PLFs formed in the ¹³⁶Xe,²⁰⁹Bi+²³⁸U reaction at the ¹³⁶Xe and ²⁰⁹Bi beam energies of 1.11 and 1.85 GeV, respectively, were measured in coincidence with either survived TLFs or with both fragments of sequential fission of excited TLFs. The interaction energies for both reactions under investigation correspond to those that are 40–50% higher than the Coulomb barrier. The angles of grazing collisions for ¹³⁶Xe and ²⁰⁹Bi are about 37° and 34°, respectively, in the laboratory frame. The angular momenta of both reactions are roughly the same.

The measurements were carried out at the U400 cyclotron at the Flerov Laboratory of Nuclear Reactions. The 200-µg/cm^{2 238}U target deposited on a 30 µg/cm² carbon backing was irradiated with ¹³⁶Xe and ²⁰⁹Bi ions. The reaction products were measured with the CORSET spectrometer using two experimental techniques: two-arm time-of-flight (ToF-ToF method) measurements for studying two-body coincidences and correlated three-arm time-of-flight and energy measurements (ToF-E method) for studying three-body coincidences. Figure 1 shows a schematic diagram of the setup for measuring MNT fragments formed in the ¹³⁶Xe+²³⁸U reaction.

The theoretical calculations made using the Langevin-type dynamics for nucleus-nucleus collisions agree well with the direct measurements and confirm the reliability of the results and the experimental data analysis, especially in respect of some derivatives.

Fig. 1. Schematic of the experimental setup for the study of two- and three-body kinematics in the ¹³⁶Xe+²³⁸U reaction at E_lab = 1.11 GeV. A start and a stop detectors based on microchannel plates are denoted as St and Sp. SCD stands for semiconductor detectors

Figure 2 shows one of the main results of the investigation: the secondary mass distribution for PLFs in the case of the coincidence detected with both fragments of complementary TLF fission (3-body coincidences) and the PLF mass distribution for the case when a survived heavy fragment is detected (2-body coincidences). It can be seen that for the MNT fragments with masses over 212 u, the formed MNT fragments are deexcited predominantly via sequential fission, thereby leading to the formation of three reaction products. The solid lines are the theoretical calculations of mass distributions. While theory predicts higher cross sections for the survived heavy MNT fragments with masses of 220–250 u, as compared to the experimental values, the predicted mass yields for MNT fragments undergoing fission are in line with the measured spectrum.

Fig. 2. (a) mass distributions of secondary PLFs for two- and three-body coincidences (blue and pink symbols) formed in the ¹³⁶Xe+²³⁸U reaction at E_lab = 1.11 GeV; (b) mass distributions of primary TLFs for two- and three-body coincidences complementary to PLF at the same registration angles. The lines are the theoretical calculations

In this study, the three-body events formed in the MNT reaction were measured for the first time, which allowed us to restore the primary mass distribution of fissioning MNT fragments. The comparison of the primary mass distributions for the survived and fissioning heavy MNT fragments showed that the survival probabilities decrease with increasing masses and amounts to about 3×10^-3 for fragments with mass of 250 u. The heaviest fragments observed in the mass distribution of fissioning MNT fragments have masses around 263 u (Lr isotopes) and cross sections of a few hundred microbarns. Thus, the transfer of about 25 nucleons from the projectile to the target nucleus was found.

The ²⁰⁹Bi+²³⁸U reaction experimental data are processed. The comparative analysis of the experimental data for the ¹³⁶Xe,²⁰⁹Bi+²³⁸U reactions will help clarify the effect of the projectile nucleus on the yield of transuranium fragments in MNT reactions, their excitation energy, and survival probability.

Such experiments aimed at the simultaneous measurement of mass and energy distributions of PLFs in coincidence with either survived TLFs or with both fragments of the sequential TLF fission have never been carried out before. This technique paves the way for a detailed experimental study of the properties of MNT fragments.

The work was supported by the Grant No. 075-10-2020-117 from the Ministry of Science and Higher Education of the Russian Federation.