Multinucleon transfer reactions: return to the origins

News, 24 April 2020

JINR Prize 2019 series of publications

Approaching to some kind of a frontier (time, spatial or ideological) people tend to sum up the results and make plans for the future. The peak of such rethinking of everything happening now, including in science, usually takes place at the end of yet another century. The end of the 20th century was not an exception: unresolved problems in physics were listed, and we should work on them in the 21st century. Most of these lists include the problem that can be formulated in the following way: “How did elements heavier than iron emerge in the Universe?”

Today, 20 years after the beginning of the century, astrophysicists understand at large how these elements have emerged but it is not yet clear where. Great hopes are pinned to the discovery of gravitational waves and the subsequent rapid development of a so-called multi-messenger astronomy, i.e. the simultaneous observation of a particular astronomic event using all the tools available to researchers. Registration of the gravitational or neutrino signal is supposed to become a trigger to the beginning of such an observation.

But the question how heavy chemical elements are synthesized has been inherited then by nuclear physicists. It is believed that the heaviest elements (thorium, uranium and heavier ones) are produced as a result of the fast capture of a large number of neutrons by the progenitor nucleus. Such a scenario is called an r-process (rapid process). Neutrons should fall into the core as often as to ensure that this core overloaded with neutrons, and therefore radioactive, continues to capture them without decaying. This will allow producing not only uranium – the heaviest element on Earth – but also the heaviest elements of the island of stability in the artificial synthesis of which Dubna has been so successful.

To make sure that our ideas of the natural nucleosynthesis are right, it is crucially important not only to find out the conditions at which the successful r-process is possible (the search for such events in the Universe is the task of astrophysicists). It is also necessary to know the properties (masses, lifetimes, peculiarities of radioactive decay, etc.) of those neutron-excess nuclei involved in natural nucleosynthesis.

In order to study something, it is necessary to get something at first, and in the quantity sufficient for the study. There are three main methods of artificial synthesis of new nuclei known in nuclear physics: fusion, fission and fragmentation. As a result of the fusion of stable nuclei, neutron-deficient nuclei are synthesized. That is why neutron-excessive nuclei can be produced only in the fission processes of heavier nuclei or in their fragmentation. There are several large, extremely expensive accelerating complexes operating in the world. And new ones are being created to achieve access to nuclei far from being stable. The emphasis nowadays is made on the reactions of fission and fragmentation.

However, it turned out that both these methods do not work when it comes to the production of neutron-rich nuclei with a magical number of neutrons: 126. It is the field extremely important for the understanding of astrophysical nucleosynthesis of heavy elements. They just do not work for nuclei heavier than uranium.

A map of known nuclei with a schematic drawing of the major production methods

The search for a new method of synthesis of nuclei interesting to researchers has once again confirmed the validity of the catchphrase that all new is well overlooked old. More than 50 years ago, V. V. Volkov and his colleagues discovered a new type of nuclear reactions in FLNR and called them the reactions of deep inelastic scattering or the multi-nucleon transfer reactions. The latter name is explained by the fact that interacting nuclei in such reactions can exchange a large number of nucleons thus producing new, yet not known nuclei. This peculiarity allowed the group of V. V. Volkov to discover about 30 new light nuclei in the deep inelastic scattering reactions.

Nevertheless, these new reactions turned out to be extremely inconvenient for experimental physicists. Nuclei leave the target with a very wide range of angles, energies and charge states, and this significantly complicates the task of their collection, separation and identification. The understanding of the mechanism of these reactions at the theoretical level was also insufficient. As a result, the multi-nucleon transfer reactions were almost forgotten for several decades and new (including neutron-excessive) nuclei were produced by Herculean efforts mentioned above.

Scientists returned to the use of multi-nucleon transfer reactions to produce new heavy and superheavy neutron-excessive nuclei about 20 years ago. It was made mostly on the initiative of V. I. Zagrebaev (FLNR Deputy Director until 2015) and W. Greiner (Frankfurt University, Germany). They demonstrated that these reactions could be efficient and competitive for the production of neutron-excessive heavy nuclei.

Nowadays, the multi-nucleon transfer reaction is one of the most popular themes studied in world-leading nuclear centres. New facilities able to work with such “inconvenient” reactions are being designed and constructed. Such facilities aim to apply multi-nucleon transfer reactions to produce and study new nuclei. It is obvious that their effectiveness will much depend on our understanding of how these reactions proceed and what are the kinematic characteristics of the products of such reactions. That is why research (both theoretical and experimental), which is aimed to study the reactions themselves, peculiarities of their procedure, to find out various patterns, gains more importance now.

We (the author of this article and FLNR junior researcher V. V. Saiko) wrote a series of scientific papers in which we managed to create a dynamic model of nucleus-nucleus collisions allowing us to accurately describe multi-nucleon transfer reactions of interacting heavy ions. The series won the second prize of JINR for 2019. Performed research confirmed the effectiveness of multi-nucleon transfer reactions as a method of neutron-excessive nuclei synthesis. Recommendations were made considering the choice of particular combinations of nuclei as well as the performance of appropriate experiments. We expect research to be continued and hope that it will finally help decode one of the mysteries of modern physics about the origin of chemical elements in the Universe.

Alexander Karpov,
FLNR Scientific Secretary