Eleven new heaviest isotopes of elements $Z=105$ to $Z=117$ identified among the products of ${}^{249}\mathrm{Bk}$$+$${}^{48}\mathrm{Ca}$ reactions

The heaviest isotopes of elements $Z=117$ to $Z=105$, ${}^{294}117$, ${}^{293}117$, ${}^{290}115$, ${}^{289}115$, ${}^{286}113$, ${}^{285}113$, ${}^{282}\mathrm{Rg}$, ${}^{281}\mathrm{Rg}$, ${}^{278}\mathrm{Mt}$, ${}^{274}\mathrm{Bh}$, and ${}^{270}\mathrm{Db}$, were identified by means of the Dubna gas-filled recoil separator among the products of the ${}^{249}\mathrm{Bk}$ $+$ ${}^{48}\mathrm{Ca}$ reaction. The details of the observed six decay chains, indicating the production and decay of isotopes ${}^{293}117$ and ${}^{294}117$, are presented and discussed. The decay energies and resulting half-lives of these new nuclei show a strong rise of stability with increasing neutron number, validating the concept of the island of enhanced stability for superheavy nuclei [Oganessian et al., Phys. Rev. Lett. 104, 142502 (2010)].

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Figure 1

Diagram of a recycled Cm target for HFIR irradiation.

Figure 2

Schematic of the Dubna gas-filled recoil separator (dipole magnet $D$ followed by the quadrupole doublet $Q_1$ and $Q_2$). Also shown is a schematic of the detector station at the separator focal plane.

Figure 3

Energy spectra [27] recorded during the 252-MeV ${}^{48}\mathrm{Ca}$ $+$ ${}^{249}\mathrm{Bk}$ run ($E^{*}=39$ MeV). (a) Total energy spectra of beam-on $\alpha$-like signals and beam-off $\alpha$ particles. (b) Total fission-fragment energy spectra, both beam on and beam off. The arrows show the energies of events observed in the correlated decay chains; see Table .

Figure 4

(a) Energy spectrum of all ERs with $Z$ $=$ 112–118 produced in ${}^{48}\mathrm{Ca}$-induced reactions [5, 8, 9, 10, 11, 12, 20, 21, 22]. The energy distribution of ${}^{252}\mathrm{No}$ implants measured in the calibration reaction ${}^{206}\mathrm{Pb}$ $+$ ${}^{48}\mathrm{Ca}$ is shown by the dashed line for comparison. Energies of $Z$ $=$ 117 ERs are shown by the black histogram. (b) Relative position deviations of all events in the decay chains observed in the reaction ${}^{48}\mathrm{Ca}$ $+$ ${}^{249}\mathrm{Bk}$ (grey histogram) with a Gaussian fit (FWHM $~$ 0.54 mm, dashed line). (c) The fission-fragment sum energy spectrum of ${}^{252}\mathrm{No}$. Energies of SF nuclei in the decay chains attributed to ${}^{293,294}117$ are shown by arrows. (d) Time intervals $t$ of all events in the observed decay chains originating from ${}^{293}117$ multiplied by the average decay probability $\overline{\lambda }$ ($\overline{\lambda }=\ln 2/{\overline{T}}_{1/2}$) are compared to the average half-lives of respective nuclei ($t\overline{\lambda }=t\ln 2/{\overline{T}}_{1/2}=t/\overline{t}$). The dashed line is the expected time spectrum for correlated decays in $t\overline{\lambda }$ dimensionless units.

Figure 5

(a) $\alpha$-decay energy and (b) half-lives vs neutron number for the isotopes of odd-$Z$ elements with $Z$ $=$ 107–117 (new results in red and results from ${}^{48}\mathrm{Ca}$-induced reactions [5, 20, 21, 22] in black). All the nuclides with $N>$ 165 have been produced in ${}^{48}\mathrm{Ca}$-induced reactions. $T$${}_{\alpha }$(exp) values are given for nuclei belonging to the ${}^{293}117$ decay chain (five events). The limit for $T$${}_{\alpha }({}^{281}$Rg) was estimated from the measured half-life and number of observed nuclei.

Figure 6

Deviation between experimental and calculated $\alpha$-decay energies for even-$Z$ (top panel) and odd-$Z$ (bottom panel) nuclei (new results in red for nuclei originating from ${}^{293}117$ and ${}^{294}117$; results from ${}^{48}\mathrm{Ca}$-induced reactions [5] in brown). Theoretical data [55, 56] were corrected by a method suggested in [26].

Figure 7

Partial $\beta {}^{+}$/EC-decay half-lives vs decay energy for known odd-odd isotopes of Np-Db (open symbols). The systematics [58] for odd-odd and odd-A isotopes are shown by lines. Experimental half-lives for terminal isotopes for which spontaneous fission was detected in the decay chains produced in ${}^{48}\mathrm{Ca}$-induced reactions with ${}^{237}\mathrm{Np}$, ${}^{243}\mathrm{Am}$, and ${}^{249}\mathrm{Bk}$ targets are given by colored squares. Error bars correspond to $95\%$ C.L. if a single event was detected and $68\%$ C.L. in other cases. $\beta {}^{+}$/EC-decay energies for ${}^{266-268,270}$Db and ${}^{281}\mathrm{Rg}$ were calculated from Refs. [26, 55].