Measurements of cross sections and decay properties of the isotopes of elements 112, 114, and 116 produced in the fusion reactions ${}^{233,238}\mathrm{U}$, ${}^{242}\mathrm{Pu}$, and ${}^{248}\mathrm{Cm}+{}^{48}\mathrm{Ca}$

We have studied the dependence of the production cross sections of the isotopes ${}^{282,283}112$ and ${}^{286,287}114$ on the excitation energy of the compound nuclei ${}^{286}112$ and ${}^{290}114$. The maximum cross section values of the $xn$-evaporation channels for the reaction ${}^{238}\mathrm{U}({}^{48}\mathrm{Ca},xn){}^{286-x}112$ were measured to be ${\sigma }_{3n}={2.5}_{-1.1}^{+1.8}\mathrm{pb}$ and ${\sigma }_{4n}={0.6}_{-0.5}^{+1.6}\mathrm{pb}$; for the reaction ${}^{242}\mathrm{Pu}({}^{48}\mathrm{Ca},xn){}^{290-x}114$: ${\sigma }_{2n}\sim 0.5\mathrm{pb}$, ${\sigma }_{3n}={3.6}_{-1.7}^{+3.4}\mathrm{pb}$, and ${\sigma }_{4n}={4.5}_{-1.9}^{+3.6}\mathrm{pb}$. In the reaction ${}^{233}\mathrm{U}({}^{48}\mathrm{Ca},2–4n){}^{277–279}112$ at $E*=34.9\pm 2.2\mathrm{MeV}$ we measured an upper cross section limit of ${\sigma }_{xn}⩽0.6\mathrm{pb}$. The observed shift of the excitation energy associated with the maximum sum evaporation residue cross section ${\sigma }_{\mathrm{ER}}(E*)$ to values significantly higher than that associated with the calculated Coulomb barrier can be caused by the orientation of the deformed target nucleus in the entrance channel of the reaction. An increase of ${\sigma }_{\mathrm{ER}}$ in the reactions of actinide targets with ${}^{48}\mathrm{Ca}$ is consistent with the expected increase of the survivability of the excited compound nucleus upon closer approach to the closed neutron shell $N=184$. In the present work we detected 33 decay chains arising in the decay of the known nuclei ${}^{282}112$, ${}^{283}112$, ${}^{286}114$, ${}^{287}114$, and ${}^{288}114$. In the decay of ${}^{287}114\left(\alpha \right)\to {}^{283}112\left(\alpha \right)\to {}^{279}110\left(\mathrm{SF}\right)$, in two cases out of 22, we observed decay chains of four and five sequential $\alpha$ transitions that end in spontaneous fission of ${}^{271}\mathrm{Sg}(T_{\alpha ∕\mathrm{SF}}={2.4}_{-1.0}^{+4.3}\min )$ and ${}^{267}\mathrm{Rf}(T_{\mathrm{SF}}\sim 2.3\mathrm{h})$, longer decay chains than reported previously. We observed the new nuclide ${}^{292}116(T_{\alpha }={18}_{-6}^{+16}\mathrm{ms},E_{\alpha }=10.66\pm 0.07\mathrm{MeV})$ in the irradiation of the ${}^{248}\mathrm{Cm}$ target at a higher energy than in previous experiments. The observed nuclear decay properties of the nuclides with $Z=104–118$ are compared with theoretical nuclear mass calculations and the systematic trends of spontaneous fission properties. As a whole, they give a consistent pattern of decay of the 18 even-$Z$ neutron-rich nuclides with $Z=104–118$ and $N=163–177$. The experiments were performed with the heavy-ion beam delivered by the U400 cyclotron of the FLNR (JINR, Dubna) employing the Dubna gas-filled recoil separator.

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Figure 1

(a) Total beam-on and beam-off $\alpha$-particle energy spectra of events registered by the focal-plane detector and by both the focal-plane and side detectors in the reaction ${}^{242}\mathrm{Pu}+{}^{48}\mathrm{Ca}$. The solid histogram shows the energies of events observed during beam-off periods in the correlated decay chains (see Fig. 2 and Table ). (b) Total fission-fragment energy spectrum; the solid histogram shows the energies of events observed in the correlated decay chains.

Figure 2

Total $\alpha$-particle-like energy spectra of events that stopped the beam during the ${}^{242}\mathrm{Pu}+{}^{48}\mathrm{Ca}$ experiment. The solid histogram shows the energies of events that switched off the beam that were followed by a beam-off SF event or beam-off $\alpha$ particles with $E_{\alpha }=8.9–10\mathrm{MeV}$ and a beam-on SF event, with a total elapsed time interval of up to $14\min$.

Figure 3

(a) Relative position deviations of all events in the observed decay chains. (b) Relative time intervals of all events in the observed decay chains $(\lambda =\ln 2∕T_{1∕2})$ compared with the average half-lives assigned to the appropriate nuclides. Dashed lines are fits to the data.

Figure 4

Time sequences in the average decay chain of ${}^{287}114$ (left) and in selected decay chains observed in the ${}^{242}\mathrm{Pu}+{}^{48}\mathrm{Ca}$ (middle) and ${}^{238}\mathrm{U}+{}^{48}\mathrm{Ca}$ (right) reactions. Measured energies, time intervals, and positions of the observed decay events are shown. Energy uncertainties are shown in parentheses. (1) The energy of this event was detected by side detectors only. (2) The energies of events detected by both the focal-plane and side detectors, respectively, are shown in brackets.

Figure 5

Excitation functions for the $2n$ (▵), $3n$ (∎), $4n$ (엯), and $5n$ (▾) evaporation channels from the complete-fusion reactions ${}^{233,238}\mathrm{U},{}^{242,244}\mathrm{Pu},{}^{248}\mathrm{Cm}+{}^{48}\mathrm{Ca}$. The Bass barrier [17] is shown by an open arrow in each panel; in the topmost panel it is labeled with $B_{\text{Bass}}$. Lines show the results of calculations [18]. Error bars correspond to statistical uncertainties.

Figure 6

(a) $\alpha$-decay energy vs neutron number for isotopes of even-$Z$ elements with $Z⩾100$ (solid circles, even-even isotopes; open circles, even-odd isotopes) [8, 9, 10, 11, 21]. Data at $N⩾163$ that are connected by dashed lines are from [1, 2, 4, 5] and the present work. Solid lines show the theoretical $Q_{\alpha }$ values [24, 25] for even-$Z=100–118$ elements. (b) Experimental values of TKE vs $Z^2∕A^{1∕3}$ (previously measured data from [26] and references therein), open squares; experimental data from the present work, solid squares. The solid line is the linear fit to the data, excluding the mass-symmetric fissioners [27].

Figure 7

(a) Comparison of the cross section as a function of excitation energy $E*$ for quasifission $\left({\sigma }_{\mathrm{QF}}\right)$, fission $\left({\sigma }_{\mathrm{F}}\right)$, and ER $\left({\sigma }_{\mathrm{ER}}\right)$ for the ${}^{238}\mathrm{U}+{}^{48}\mathrm{Ca}$ reaction. Experimental data are from [3] and present work, and the lines show calculations [18]. Note the location of the “polar” and “equatorial” touching configurations with respect to the Bass barrier, and the discontinuous cross section scale. (b) Comparison of hot fusion cross sections for the production of $Z⩾102$ nuclides using a variety of heavy-ion beams (top panel). Note the relatively constant cross sections for $Z⩾112$ nuclides produced with ${}^{48}\mathrm{Ca}$ projectiles. We show the fission barrier heights as a function of neutron number on the bottom panel [28, 36] (lower panel). Solid symbols correspond to the number of neutrons in compound nuclei formed in different reactions. Note the influence of the closed shells—especially the spherical shell closure at $N=184$. There is a correspondence between the increased barrier height in the lower panel and the enhanced cross sections of ER production in ${}^{48}\mathrm{Ca}$-induced reactions in the top panel at the same neutron numbers.