Reinvestigation of the decay properties of ${}^{261,262}\mathrm{Bh}$ at the gas-filled recoil separator SHANS2

The isotopes ${}^{261,262}\mathrm{Bh}$ were reinvestigated by using the cold fusion reaction ${}^{54}\mathrm{Cr}+{}^{209}\mathrm{Bi}$. The evaporation residues were separated by the gas-filled recoil separator SHANS2 (Spectrometer for Heavy Atoms and Nuclear Structure-2) and identified by means of the recoil-$\alpha$-correlation method. The $\alpha$-decay properties of ${}^{261,262}\mathrm{Bh}$ were measured and the results are consistent with the previously reported values. Based on the measured half-lives, the existence of two distinct states of ${}^{262}\mathrm{Bh}$ was confirmed, whereas ${}^{261}\mathrm{Bh}$ lacked any isomer with $\alpha$-decay properties. Two spontaneous fission events potentially from ${}^{262}\mathrm{Bh}$ and/or ${}^{261}\mathrm{Bh}$ were detected and the production cross sections at beam energies of 264.1 and 270.4 MeV were obtained.

Figure 1

(a) $\alpha$-decay spectrum of ${}^{262}\mathrm{Bh}$ for full-energy events on the DSSD. (b) Two-dimensional scatter plot of lifetime versus energy of $\alpha$ decays of ${}^{262}\mathrm{Bh}$. Histograms depicting the distribution of lifetimes for ${}^{262}\mathrm{Bh}$(1) and ${}^{262}\mathrm{Bh}$(2) are presented in the inset graph, where the red curves are the fitted decay-time distributions.

Figure 2

(a) Two-dimensional scatter plot of $\alpha \text{−}\gamma$ prompt coincidence from ${}^{262}\mathrm{Bh}$. (b) The $\gamma$ spectrum from the $\alpha \text{−}\gamma$ prompt coincidence events.

Figure 3

Two-dimensional scatter plot of $\alpha$-particle energies from the parent ${}^{262}\mathrm{Bh}$ and the daughter ${}^{258}\mathrm{Db}$ for the ER-$\alpha 1\text{−}\alpha 2$ correlation. The searching time window is 1 s for the ER-$\alpha 1$ pair and 30 s for the $\alpha 1\text{−}\alpha 2$ pair. Only those events detected with full energy by the DSSD at the beam energy of 264.1 MeV are included.

Figure 4

The $\alpha$-decay energy spectra of ${}^{261}\mathrm{Bh}$, ${}^{257}\mathrm{Db}$, and ${}^{253}\mathrm{Lr}$. The lifetime distribution of ${}^{261}\mathrm{Bh}$ is presented in the inset graph; the red curve shows the calculated decay-time distribution with a half-life of 8.9 ms.

Figure 5

The excitation functions of ${}^{262}\mathrm{Bh}$ and ${}^{261}\mathrm{Bh}$. $E_{\cot }$ represents the beam energy of ${}^{54}\mathrm{Cr}$ at the center of target in the laboratory system. The theoretical curves were calculated by using the hivap code and the points are experimental data from GSI [5, 6], LBNL [18], and this work. The masses of ${}^{54}\mathrm{Cr}$, ${}^{209}\mathrm{Bi}$, and the compound nuclei ${}^{263}\mathrm{Bh}$ are taken from AME2020 [30]. The Coulomb barrier $V_{\mathrm{BASS}}$ of the ${}^{54}\mathrm{Cr}+{}^{209}\mathrm{Bi}$ reaction system was calculated by using the BASS nuclear-nuclear interaction potential [31].