Spectroscopy along Flerovium Decay Chains: Discovery of ${}^{280}\mathrm{Ds}$ and an Excited State in ${}^{282}\mathrm{Cn}$

A nuclear spectroscopy experiment was conducted to study $\alpha$-decay chains stemming from isotopes of flerovium (element $Z=114$). An upgraded TASISpec decay station was placed behind the gas-filled separator TASCA at the GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt, Germany. The fusion-evaporation reactions ${}^{48}\mathrm{Ca}+{}^{242}\mathrm{Pu}$ and ${}^{48}\mathrm{Ca}+{}^{244}\mathrm{Pu}$ provided a total of 32 flerovium-candidate decay chains, of which two and eleven were firmly assigned to ${}^{286}\mathrm{Fl}$ and ${}^{288}\mathrm{Fl}$, respectively. A prompt coincidence between a 9.60(1)-MeV $\alpha$ particle event and a 0.36(1)-MeV conversion electron marked the first observation of an excited state in an even-even isotope of the heaviest man-made elements, namely ${}^{282}\mathrm{Cn}$. Spectroscopy of ${}^{288}\mathrm{Fl}$ decay chains fixed $Q_{\alpha }=10.06(1)\mathrm{MeV}$. In one case, a $Q_{\alpha }=9.46(1)$-MeV decay from ${}^{284}\mathrm{Cn}$ into ${}^{280}\mathrm{Ds}$ was observed, with ${}^{280}\mathrm{Ds}$ fissioning after only $518\mu \mathrm{s}$. The impact of these findings, aggregated with existing data on decay chains of ${}^{286,288}\mathrm{Fl}$, on the size of an anticipated shell gap at proton number $Z=114$ is discussed in light of predictions from two beyond-mean-field calculations, which take into account triaxial deformation.

Figure 1

(a) Summary of the decay chains of ${}^{286,288}\mathrm{Fl}$ (cf. Supplemental Material [21] and Table 2). (b),(c) Digitized preamplifier pulses of events associated with decay chains 08 and 02. Numbers in the panels are calibrated energies in mega-electron volts. Correlation times are given between recoil implantation (orange squares), $\alpha$ decays (yellow squares), and fission (green squares). The 0.36(1)-MeV event in an upstream DSSD (blue) is in prompt coincidence with the 9.60(1)-MeV event in the implantation DSSD. Black triangles in the lower right corner of a square indicate detection during beam-off periods. (d) Suggested decay sequence of chain 02 through excited states in ${}^{282}\mathrm{Cn}$. Tentative levels and transitions are dashed.

Figure 2

(a) Comparison of observed and predicted $Q_{\alpha }$ values of the $\alpha$-decay sequence ${}^{292}\mathrm{Lv}$, ${}^{288}\mathrm{Fl}$, and ${}^{284}\mathrm{Cn}$. The theoretical numbers stem from macroscopic-microscopic (MM) models [48, 49], axially symmetric (SLY4 and UNEDF1 [50], Gogny [51]) and triaxial (RMF [52]) density functional theory (DFT), and a triaxial beyond-mean-field (TBMF) description [54]. (b) Proposed excited states in ${}^{282}\mathrm{Cn}$ compared with predictions of two TBMF calculations [54, 55] for ${2_1}^{+}$ (diamonds) and ${0_2}^{+}$ states (circles) in $N=170–176$, ${}^{282-288}\mathrm{Cn}$. The predicted $E(2^{+})$ values for ${}^{282,284}\mathrm{Cn}$ are nearly identical for both descriptions and were therefore placed side by side.