Chemical elements are the building blocks of our world. So far, 118 elements have been discovered. According to the liquid drop model of a nucleus, the elements with a proton number of Z=104 and higher could not exist and should split instantaneously . Due to the influence of stabilizing nuclear shell effects, these so-called “superheavy elements” could be formed and studied in the laboratory. Production rates for these elements are low and they follow the general trend that their half-lives decrease with an increasing proton number.
For decades, scientist have been searching for the so-called Island of Stability. The island is formed by nuclear shell closures. It is expected that nuclides around the island are inherently stable against nuclear fission and have relatively long half-lives. Depending on the theoretical models, the island has been predicted around the neutron number N=182 and the proton numbers Z=114, Z=120 or Z=126. The NEXT project will extend the chart of known-nuclides to the neutron-rich side in the heavy element region.
The heaviest element which occurs naturally in macro amounts on our earth is uranium with a proton number Z=92. The formation of elements heavier than bismuth with a proton number Z=83 can be explained by the rapid neutron-capture (r-) process. The r-process requires an environment with a high-density neutron flux. In 2017, the “smoking gun” of the r-process has for the first time been observed by multi-messenger astronomy in a neutron star merger. The exact path of the r-process along the nuclear landscape is still not known, and depends on the nuclear properties of neutron-rich isotopes involved such as half-lives, decay modes, neutron capture cross sections and for heavier elements also on the fission barrier. Nuclear physics studies, theoretical simulations and astronomical observations are the key to explain the formation of elements in our universe and answer the questions if elements heavier than uranium have been formed.
In the NEXT project, we want to address two major questions in nuclear and astrophysics:
A. How can we access neutron-rich heavy nuclei?
B. Is it possible that superheavy elements have been formed in our universe?
To address these questions a setup dedicated to discover and study Neutron-rich, EXotic, heavy nuclei produced in multi-nucleon Transfer reactions (NEXT) will be built at the AGOR cyclotron facility in Groningen. These exotic nuclei are the key to answer the question of the origin of the chemical elements heavier than iron in our universe. Isotopes relevant for r-process will be discovered and their mass and decay properties will be determined. Mass measurements of these exotic nuclei are important benchmark data for nuclear- and nuclear astrophysics theory. Decay modes such as fission have significant influence on the formation of heavy elements in our universe. The proposed mass and decay studies of neutron-rich, exotic nuclei will therefore contribute to our knowledge of nuclear structure and of the element synthesis in astrophysical processes.
In the NEXT project, the world-wide first separator dedicated to multi-nucleon transfer reactions with heavy (actinide) targets will be constructed. The AGOR cyclotron is capable of delivering the required beam with significant intensities and thus, NEXT can provide unique access to exotic nuclei and that is complementary to existing rare-isotope beam facilities.
