Fusion reaction ${}^{48}\mathrm{Ca}+{}^{249}\mathrm{Bk}$ leading to formation of the element Ts ($Z=117$)

The heaviest currently known nuclei, which have up to 118 protons, have been produced in ${}^{48}\mathrm{Ca}$ induced reactions with actinide targets. Among them, the element tennessine (Ts), which has 117 protons, has been synthesized by fusing ${}^{48}\mathrm{Ca}$ with the radioactive target ${}^{249}\mathrm{Bk}$, which has a half-life of 327 d. The experiment was performed at the gas-filled recoil separator TASCA. Two long and two short $\alpha$ decay chains were observed. The long chains were attributed to the decay of ${}^{294}\mathrm{Ts}$. The possible origin of the short-decay chains is discussed in comparison with the known experimental data. They are found to fit with the decay chain patterns attributed to ${}^{293}\mathrm{Ts}$. The present experimental results confirm the previous findings at the Dubna Gas-Filled Recoil Separator on the decay chains originating from the nuclei assigned to Ts.

Figure 1

(a) Average beam currents for each run (254 MeV, 258 MeV, and 252 MeV). (b) Evolution of isotopic thicknesses of ${}^{249}\mathrm{Bk}$ and ${}^{249}\mathrm{Cf}$ with average total thickness of 0.48(5) mg/${\mathrm{cm}}^2$ during the irradiations by ${}^{48}\mathrm{Ca}$ beam with energies $E_{\mathrm{lab}}=254.0$, 258.0, and 252.1 MeV. Arrows indicate the dates when the long ($\#1$ and $\#2$) and short ($\#3$ and $\#4$) chains were detected.

Figure 2

Schematic illustrations of the focal plane detection setup of TASCA and the Combined ANalog and DIgital (CANDI) data acquisition system coupled to the DSSD stop, box, veto, and MWPC detectors. Data acquisition (DAQ1 and DAQ2) was triggered (Trg) by both $X$ and $Y$ strip signals in ‘OR’ logical mode. For details, see text.

Figure 3

Low (a) and (b) high energies spectra measured during the 270 MeV run. All, anti-MWPC, and beam-off events are shown. In (a) the spectrum of beam-off events detected solely with the stop detector or no coincidence box detector signal within energies of 3–14 MeV is shown. The spectrum of these events with the additional requirement of having only single-signal traces (digital part of CANDI) is also shown. The algorithm for testing a presence of multisignal traces has a time limit down to 0.3 $\mu \mathrm{s}$ in between signals.

Figure 4

Upper panel: Horizontal ($X$ strips) distributions of all, anti-MWPC and beam-off low energy events, and fissions measured during the 254 MeV run are shown. Energy windows for selected type of events are given in brackets. Lower panel: $X-Y$ map of spatial distribution of fission events detected during the entire experiment.

Figure 5

Stop detector energies of anti-MWPC (vertical lines) and beam-off (horizontal lines) events in the pixels $X/Y=103/41$ (a) and 111/19 (b) are shown as function of time, start from the beginning of the experiment. In the left panels, the time scale covering the entire experiment is shown. Shaded regions mark time windows where the chains $\#1$ and $\#2$ and fissions were detected. An expanded view of the time around these shaded regions is shown in the right panels (c,d). Identified beam-off $\alpha$-like events with energies of 7–12 MeV are marked by squares. In the right panels, the $\alpha$ members of cluster-events reconstructed from stop and box detectors are shown by triangles. For details, see text.

Figure 6

Suggested decay sequence of the long $\alpha$-decay chains observed during the 254 MeV-run (chain $\#1$) and 258 MeV-run MeV (chain $\#2$). Experimental energies in MeV and $\mathrm{\Delta }t$ are given for all events. Boxes with black triangles indicate events observed during beam-off periods. The measured fission-fragment energies are from stop+box detectors. Corresponding traces of all members of the chains are shown on the right side of the chains. A summary resulting from $\alpha$-decay chains observed at DGFRS [23] is also shown.

Figure 7

Suggested decay sequence of two short $\alpha$-decay chains observed during the 254 MeV-run (chain $\#3$) and 258 MeV-run MeV (chain $\#4$). Summaries of the 16 short decay chains observed at DGFRS and attributed to the decay of ${}^{293}\mathrm{Ts}$ [20, 21, 22, 23] are also shown for comparative discussion. Energies and correlation times of all individual members are given for two DGFRS chains that end by fission of either ${}^{277}\mathrm{Mt}$ or ${}^{277}\mathrm{Hs}$. For each member of the “14 chains”, an average half-life was calculated from those data, where the correlation time was measured relative to the mother nucleus. Isotopic assignments of chains $\#3$ and $\#4$ are made based on the DGFRS findings. In the case of chain $\#4$ only one of several possible scenarios is given. Boxes with black triangles indicate events observed during beam-off periods. The measured fission fragment energies are from stop+box detectors. The corresponding traces of all members of the chains are shown on the right side of the chains. For more details, see text.

Figure 8

Cross sections for the production of Ts nuclei in the ${}^{48}\mathrm{Ca}+{}^{249}\mathrm{Bk}$ reaction observed in the present work and from DGFRS [20, 23]. (a) $3n$ channel from the fusion reactions where long-decay chains were assigned to ${}^{294}\mathrm{Ts}$. Numbers of observed chains are given. (b) Cross-section values corresponding to short-decay chains assigned to ${}^{293}\mathrm{Ts}$ according to DGFRS are shown together with the sum of the observed number of chains terminating at ${}^{281}\mathrm{Rg}$ and ${}^{277}\mathrm{Mt}/{}^{277}\mathrm{Hs}$. Cross sections of the present chains ending with short-lived fission activity are also given. Solid and dashed lines mark the theoretically predicted cross section values adopted from Refs. [50] and [51], respectively. Excitation energies were calculated using the mass-table data from Refs. [32, 33].

Figure 9

Half-lives of potential candidates for two regions of nuclei with different atomic mass numbers ($200\le A<240$ and $240\le A<260$) produced in the ${}^{48}\mathrm{Ca}+{}^{249}\mathrm{Bk}/{}^{249}\mathrm{Cf}$ reactions are shown by open symbols as function of their $\alpha$-particle energies. Only those $\alpha$-decay branches with absolute intensities and $\alpha$-particle energies exceeding 10% and 7.6 MeV, respectively, are compiled. Isotopes identified in the analysis of the present data are marked by plus signs. A time level of 0.4 s at which most of the data can be separated into two groups is indicated by the dashed line.

Figure 10

Horizontal ($X$-strip) distributions of ER–${\alpha }_1$–${\alpha }_2$ chains containing (a) two beam-on $\alpha$-like, (b) one and (c) two beam-off $\alpha$-like events.

Figure 11

Traces of various signals detected with small energetic signals. The origins of the triggering signals are given together with the energies for the smaller ones. Time differences between two signals are also given. For details, see the text.