Status and perspectives of the Dubna superheavy element factory

In the Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research (FLNR JINR), construction of a new experimental complex is currently in progress (Superheavy Element Factory), aimed at the synthesis of new superheavy nuclides and the detailed study of those already synthesized. The project includes the construction of a new accelerator of stable and long-lived isotopes in the mass range A = 10–100 with an intensity of up to 10 pμA and energy up to 8 MeV/nucleon; construction of a new experimental building and infrastructure for housing the accelerator with five channels for the transportation of beams to a 1200-m2 experimental hall that is equipped with systems of shielding and control for operations with radioactive materials; development of new separators of reaction products; upgrade of the existing separators and development of the new detection modules for the study of nuclear, atomic, and chemical properties of new elements. The first experiments are planned for 2018.


Introduction
The six heaviest chemical elements with atomic numbers 113 to 118 that fill the 7 th row of Mendeleev's Periodic Table were synthesized in reactions of 48 Ca ions with actinide targets in the experimental studies carried out over the recent years.Over 50 new isotopes of elements 104 to 118 with maximum neutron excess were for the first time produced and their decay properties were determined in these investigations [1,2].The new isotopes considerably filled up the Chart of the Nuclides and expanded it up to Z = 118 and N = 177 superheavy elements.Fundamental conclusions of the modern theory concerning the limits of existence of nuclear matter have for the first time received experimental confirmation.
The discovery of the new domain (island) of stability and the very fact of existence of superheavy elements (SHE) have posed a number of new questions associated with fundamental properties of nuclear matter: • Can even heavier nuclei exist?Is the "Island of Stability of SHE" the last one on the Chart of the Nuclides?• Can the superheavy nuclei be formed in the process of nucleosynthesis like those stable and long-lived nuclei in the groups of Pt, Pb, and U-Th found in Nature?• What is the electron structure of a superheavy atom in strong Coulomb field?
• What is the limit of Mendeleev's

Nobel Symposium NS160 -Chemistry and Physics of Heavy and Superheavy Elements
In search for answers to these and many related questions we have to considerably widen our knowledge about the subject of investigation.Obviously, to reach this aim we need to greatly broaden the field of work and involve new facilities.However, judging by first attempts, going beyond the area of already synthesized superheavy elements is quite a difficult task.

SHE factory
Direct synthesis of elements with Z >118 in fusion reactions means using projectiles heavier than Ca, since the capability of high-flux reactors to produce target material is limited to Cf isotopes.A number of attempts undertaken in various laboratories and aimed at the synthesis of elements 119 and 120 did not yield any result.Limits of production cross sections of nuclei with Z = 120 in, for example, the reaction 54 Cr+ 248 Cm [3] and nuclei with Z = 119 via 50 Ti+ 249 Bk [4] were ten to twenty times lower than those of production of isotopes of elements 114 and 115 in experiments with 48 Ca.Advance in the other direction -towards the desired area of more neutron-rich isotopes of the known SHE closer to the predicted shell closure at N = 184 -is possible only with use of radioactive projectiles that are more neutron-rich than 48 Ca.Unfortunately, the intensities of beams of radioactive nuclei at the most advanced accelerators and even those in the design stage are extremely low for performing this kind of experiments.
Moving in the opposite direction -towards neutron deficit in superheavy nuclei -leads to strong decrease of production cross section when going off the shell N = 184.
Under these circumstances, it seems appropriate to investigate in detail the already synthesized nuclei: to determine their masses, forms, and fission barriers, find their excited states, etc.Yet here we need to considerably improve the statistics.Individual events of formation and decay of superheavy nuclides that are registered in present experiments with a rate of one per day to one per month (this is considered a great achievement of the last years) are far from being sufficient for these purposes.Experimental results should be presented not by single events but by hundreds and thousands.Only then one will see rare decay channels of the superheavy nuclei or study in detail nuclear transitions to various states, which means to make complete measurements in spite of extremely low cross sections of producing superheavy elements.
Note that the present situation in some aspects reproduces the beginning of work on synthesis of superheavy elements twenty years ago when a new approach to the synthesis of SHE (reactions 48 Ca+actinides) was found and a significant breakthrough in experimental technique was made.
We came to the conclusion that at present, based on accumulated knowledge in study of

• reactions of synthesis of new elements (fusion reactions with evaporation of neutrons);
• cross sections of production of the nuclei of SHE reaction channels at various beam energies (excitation functions); • competing reaction channels (background conditions); • decay characteristics of the isotopes of SHE (decay modes, energies, decay times, etc.), as well as the achievements of the last twenty years in the fields of • technique of physical experiment; • physics and technology of charged particle accelerators and plasma physics; • new highly sensitive precision detectors for nuclear radiation; • production of new target materials in high-flux nuclear reactors and development of new target technologies, etc.  it is possible to increase the production rate of the known isotopes of SHE by one to two orders of magnitude and carry out experiments aimed at the synthesis of new elements with Z>118 by employing a new experimental complex, the "SHE Factory", and thus make a second breakthrough in the world of the heaviest nuclei and elements.The ideas incorporated in the project of the SHE Factory are associated with developing the experimental basis in several directions.These directions are:

Nobel Symposium NS160 -Chemistry and Physics of Heavy and Superheavy Elements
• construction of new powerful accelerator of stable and long-living isotopes in mass range A = 10-100 with intensity up to 10 p A and energy up to 8 MeV/nucleon (see Fig. 1   All major parts of new cyclotron DC-280 are produced and delivered to Dubna (Fig. 2).
In general, studies of heavy and superheavy nuclei are related to atomic physics, nuclear physics, astrophysics, and radiochemistry.They cover a wide range of phenomena, the study of which is far more than experiments that are to be carried out at the SHE Factory.However, when experiments are performed with extremely heavy nuclei at the limit of stability and thus require extremely high sensitivity, advantages of the SHE Factory become highly important.This is the specificity of the new complex.Its program of experimental research will be directly connected with the achievements of world science and is designed for broad international cooperation.
A long-term program of research of SHE should be developed with regard to the development of the new complex, its phased commissioning, production of the primary beam, and start of the first experiment in 2018.Experimental research at the existing complex ECR-4M+U-400+DGFRS and at the functioning setups will continue at the same pace until the first full-scale experiment at SHE Factory (2017-2018).However, priority is given to the studies that will be continued at the SHE Factory.

Table 1 .
Expected parameters of the DC280 cyclotron.
and Table 1 for more details); • construction of a new experimental building and infrastructure for placing the accelerator with five channels for transportation of beams to a 1200-m 2 experimental Nobel Symposium NS160 -Chemistry and Physics of Heavy and Superheavy Elements