A New Perspective of the Study of Superheavy Nuclei

What is the heaviest element that can, if it is very short lived, exist in nature? This simple, yet profound question has urged us to synthesize unknown superheavy nuclei (SHN) at terrestrial accelerator facilities all over the world. The synthesis of SHN is notoriously difficult due to its really tiny cross sections on the order of picobarn to femtobarn. While the cold fusion reactions, which take advantage of the stabilization effect of doubly-magic ²⁰⁸Pb or its neighbor ²⁰⁹Bi, reducing excitation energy of a compound system, have been successful to synthesize SHN up to the element 113, nihonium, it suffers from an exponential decrease of the cross section with increasing the proton number Z. On the other hand, the hot fusion reactions employ a neutron-rich calcium isotope, ⁴⁸Ca, as a projectile with an actinide target, leading to higher excitation energy as compared to the cold one, while subsequent neutron evaporation successfully cools it down. The latter sustain picobarn-level cross sections even for Z >113 and up to the element 118, oganneson, have been synthesized so far. Although those artificially synthesized SHN are short-lived, their chemical as well as nuclear properties are of great interests, offering a unique opportunity to challenge our theoretical understanding.

If it is really difficult to synthesize SHN experimentally, why don't we look for other possibilities, e.g., naturally existing SHN somewhere in the universe? That is the topic I would like to discuss in this talk. Recently, we have investigated effects of a superstrong magnetic field (as large as 10¹⁸ G) on compositions of the outer crust of neutron stars and found that extremely neutron-rich SHN, including yet unknown elements, emerge as an equilibrium composition of the outer crust of a strongly-magnetized neutron star [1]. The main cause of the emergence of SHN is the Landau-Rabi quantization of electron motion perpendicular to the magnetic field, which enhances the electron (and, thus, proton) fraction, allowing for the outer crust to extend at a higher pressure (density) region. In this talk, I will discuss implications of the finding, building a new bridge between the studies of SHN and neutron stars.