Decay spectroscopy of heavy and superheavy nuclei

TL;DR

This review consolidates experimental progress in decay spectroscopy after separation (DSAS) for the heaviest nuclei, spanning $Z=99$ to $Z=118$, with emphasis on how alpha, beta, gamma, conversion-electron, and spontaneous-fission decays reveal single-particle structure, deformation, and isomerism near the island of stability. It details the DSAS methodology, including recoil implantation, positional/time correlations, and advanced detectors (silicon, germanium, CE tagging) that enable rare-event identification and spectroscopic insight. The article synthesizes observations across 204 known isotopes, highlighting systematic trends in shell gaps (notably $Z\approx100$, $N\approx152$), the role of high-$K$ isomers, and the competition among decay modes that constrains spin-parity assignments and fission barriers. It also surveys theoretical advances, such as complete shell-model descriptions and weak-decay calculations, that contextualize measurements and guide future experiments. Finally, it outlines upcoming facilities and technologies poised to extend DSAS capabilities, enabling more comprehensive spectroscopic studies of SHN and tighter tests of nuclear-structure theories under extreme proton vs neutron numbers.

Abstract

After more than half a century since the first predictions of the so-called "island of stability of superheavy nuclei", exploring the limits of nuclear stability at highest atomic numbers is still one of the most prominent challenges in low-energy nuclear physics. These exotic nuclear species reveal their character and details of some of their properties through their induced or spontaneous disintegration. The achievements in the field of superheavy nuclei (SHN) research, which involves studying the production and decay of the heaviest nuclear species, have been reported in a number of review papers. In the introduction of this paper, references are provided to review papers, summarizing the many aspects of SHN research in other disciplines, like chemistry, atomic physics, and earlier work on nuclear structure, including in-beam spectroscopy, and superheavy element (SHE) synthesis. This review is an attempt to summarize the experimental progress that has been made in recent years by employing the versatile tool park of Decay Spectroscopy After Separation (DSAS) for the heaviest isotopes from Z=99 (einsteinium) to Z=118 (oganesson). DSAS, with its major instrumentation components heavy-ion accelerator, separator and decay detection, is the only way to access the heaviest nuclei up to oganesson. While in-beam γ-spectroscopy has reached 256Rf in terms of the highest atomic number Z and mass number A, SHE chemistry succeeded to sort flerovium (Z = 114) as the heaviest element into the periodic table. Laser spectroscopy and precise mass measurements are limited basically to the nobelium/fermium region, with high-precision Penning-trap mass-measurements being performed for 256Lr and 257Rf, and with the 257Db mass obtained, using a multi-reflection time-of-flight mass spectrometer (MRToF MS).

Figures (38)

• Figure 1.1: Christophe Theisen (Photo with permit from Véronique Theisen)
• Figure 2.1: Excerpt of the chart of nuclides showing the heaviest nuclei observed ($Z$=96-118 and $N$=137-177). The colors indicate the decay mode and the subdivision in areas of different sizes the relative decay probabilities. Yellow denotes $\alpha$-decay, green spontaneous fission (SF), red $\beta^+$ or electron capture ($EC$) and blue $\beta^-$ decay. The scheme is adopted from Soti2024. For $^{267}$Ds a faint yellow has been chosen as the observation of its $\alpha$ decay is uncertain (see Ref.s Ghiorso1995Karol2001). For the $\beta$ (EC) decay of $^{266,267,268}$Db a faint red has been chosen as the competition between $SF$ and $EC$ is being debated (see subsection \ref{['Db']}).
• Figure 3.1: Decay spectroscopy after separation and genetic correlations: the recoiling nucleus $_Z^AX$ is implanted in a position-sensitive detector at position ($X_r$, $Y_r$). It subsequently decays via $\alpha$ emission in its neighborhood at position ($X\alpha1$, $Y\alpha1$) to the daughter nucleus ($_{Z-2}^{A-4}Y$) which itself decays to the granddaughter ($_{Z-4}^{A-8}Y$) at position ($X\alpha2$, $Y\alpha2$). In addition to the recoils and $\alpha$ particles $\gamma$-, $x$-rays and $CE$s are detected in coincidence. The technique allows correlations in position and/or time of the recoil implantation and its subsequent decay due to the inclusive detection of the particles and photons involved. (Figure and caption are taken from ref. Ackermann2024)
• Figure 4.1: Nuclear chart and its limits of stability deduced from the liquid-drop model. The green lines correspond to the neutron $S_n=0$ and proton $S_p=0$ drip lines, with $S_n$ and $S_p$ denoting the neutron and proton separation energies, respectively. The blue lines $B_f = 0$ and $t_{1/2} = 10^{-14}$ s correspond to a vanishing fission barrier and a spontaneous fission half-life of $10^{-14}$ s, respectively. The red line corresponds to beta-stable nuclei. Besides magic numbers shown with lines, the horizontal dotted line corresponds to $Z=104$. (Figure and caption are taken from Ref. Ackermann2017)
• Figure 4.2: Definition of the $K$ quantum number as the total projection of the sum $j_i$ of the spin of the nucleon $S_i$ and the orbital angular momentum $l_i$ of all excited 2-quasiparticle states onto the symmetry axis of the nucleus. (Figure and caption are taken from Ref. Ackermann2017)