Nuclear isomers

This special issue explores the intricate realm of nuclear isomers 1,2, rare long-lived excited states of nuclei, some of which can store over 10 MeV of energy in a single nucleus.Their distinctive characteristics and intriguing lifetimes spanning from nanoseconds to millions of years, beckon us to uncover their underlying enigmas.Isomers hold brimming potential for basic research in nuclear physics, atomic physics, astrophysics, mathematical physics, and industrial applications such as energy storage, timekeeping, and medical imaging.The genesis of this field can be traced back to a remark by Soddy in 1917 3, "We can have isotopes with identity of atomic weight, as well as of chemical character, which are different in their stability and mode of breaking up."This statement is often recognized as the earliest indication of isomeric existence.Otto Hahn is generally credited with the first experimental observation of an isomer in 234 Pa while working on uranium salts 4.However, prior to Hahn's work, Fajans and Gohring 5 had identified a 1.1 min activity in the UX 2 isotope in a new chemical element "brevium", as they named it, which was to be identified later as an isomeric state of 234 Pa 6.Hahn in his 1921 paper 4 observed for the first time the ground state activity of 6.7 h in 234 Pa, as well as the short-lived activity resulting from the beta decay of 234 Th, whose half-life was unspecified 7.With the appearance of two states belonging to the same nuclide, Hahn recognised that "ein soleher Fall ist bis jetzt bei den radioaktiven Umwandlungen noch nicht beobachtet worden" such a case has not been observed before in radioactive transformations 7.The short-lived activity was the 1.1 min isomer, whose connection to the ground state remained elusive.Consequently, the levels lying above the 1.1 min isomer did not have known energy values.Only recently, Korsakov et al. 8 confirmed the 1.1 min isomeric activity with a tentative spin of (0 -) that lies only 2.6 keV above the 73.9 keV, (3 + ) state causing its decay to be almost impossible.Indeed, it was some years before this "first" case of nuclear isomerism was accepted as being correct.The word "isomer" itself seems to have been used in the literature, for the first time in the nuclear context, by Gamow in 1934 9, after the discovery of neutrons by Chadwick in 1932.Then, in 1935, Kurtchatov et al. 10 and, separately, Szilard and Chalmers 11 discovered isomers in bromine and indium isotopes, respectively.These new isomers, focused on by Bethe in his 1937 review 12, were the first of a wave of isomer discoveries made possible by exploiting Chadwick's neutrons and other experimental advances.With the global interest in isomer physics still growing today, along with new and expanding access to unexplored regions of the nuclear landscape, and with new ideas for applications beyond the domain of nuclear structure physics, this special issue presents a series of articles dedicated to the advances and insights gained in this field in recent years.Nuclear isomers undeniably stand at the forefront of global nuclear science pursuits, with an abundance of isomeric data at our disposal.In the early times of developing state-of-the-art nuclear models, isomers revealed and quantified the shell gaps, basic building blocks of nuclear structure.Based on significant changes in the angular momentum of γ-decaying isomers, von Weizsäcker 13 gave the first explanation for the isomerism phenomenon now referred to as spin isomers.In this issue, Brown 14 provides a theoretical review for the origins of such γ-decaying isomers starting within a spherical shell model basis and adeptly explains many two-particle isomers