Measurements of cross sections and decay properties of the isotopes of elements 112, 114, and 116 produced in the fusion reactions<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mmultiscripts><mml:mi mathvariant="normal">U</mml:mi><mml:mprescripts/><mml:none/><mml:mrow><mml:mn>233</mml:mn><mml:mo>,</mml:mo><mml:mn>238</mml:mn></mml:mrow></mml:mmultiscripts></mml:math>,<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mmultiscripts><mml:mi mathvariant="normal">Pu</mml:mi><mml:mprescripts/><mml:none/><mml:mn>242</mml:mn></mml:mmultiscripts></mml:math>, and<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mmultiscripts><mml:mi mathvariant="normal">Cm</mml:mi><mml:mprescripts/><mml:none/><mml:mn>248</mml:mn></mml:mmultiscripts><mml:mo>+</mml:mo><mml:mmultiscripts><mml:mi mathvariant="normal">Ca</mml:mi><mml:mprescripts/><mml:none/><mml:mn>48</mml:mn></mml:mmultiscripts></mml:mrow></mml:math>
Abstract
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 $^{238}\mathrm{U}(^{48}\mathrm{Ca},xn)^{286\ensuremath{-}x}112$ were measured to be ${\ensuremath{\sigma}}_{3n}={2.5}_{\ensuremath{-}1.1}^{+1.8}\phantom{\rule{0.3em}{0ex}}\mathrm{pb}$ and ${\ensuremath{\sigma}}_{4n}={0.6}_{\ensuremath{-}0.5}^{+1.6}\phantom{\rule{0.3em}{0ex}}\mathrm{pb}$; for the reaction $^{242}\mathrm{Pu}(^{48}\mathrm{Ca},xn)^{290\ensuremath{-}x}114$: ${\ensuremath{\sigma}}_{2n}\ensuremath{\sim}0.5\phantom{\rule{0.3em}{0ex}}\mathrm{pb}$, ${\ensuremath{\sigma}}_{3n}={3.6}_{\ensuremath{-}1.7}^{+3.4}\phantom{\rule{0.3em}{0ex}}\mathrm{pb}$, and ${\ensuremath{\sigma}}_{4n}={4.5}_{\ensuremath{-}1.9}^{+3.6}\phantom{\rule{0.3em}{0ex}}\mathrm{pb}$. In the reaction $^{233}\mathrm{U}(^{48}\mathrm{Ca},2--4n)^{277--279}112$ at $E*=34.9\ifmmode\pm\else\textpm\fi{}2.2\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ we measured an upper cross section limit of ${\ensuremath{\sigma}}_{xn}\ensuremath{\leqslant}0.6\phantom{\rule{0.3em}{0ex}}\mathrm{pb}$. The observed shift of the excitation energy associated with the maximum sum evaporation residue cross section ${\ensuremath{\sigma}}_{\mathrm{ER}}(E*)$ 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 ${\ensuremath{\sigma}}_{\mathrm{ER}}$ 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 $^{287}114(\ensuremath{\alpha})\ensuremath{\rightarrow}^{283}112(\ensuremath{\alpha})\ensuremath{\rightarrow}^{279}110(\mathrm{SF})$, in two cases out of 22, we observed decay chains of four and five sequential $\ensuremath{\alpha}$ transitions that end in spontaneous fission of $^{271}\mathrm{Sg}\phantom{\rule{0.3em}{0ex}}({T}_{\ensuremath{\alpha}∕\mathrm{SF}}={2.4}_{\ensuremath{-}1.0}^{+4.3}\phantom{\rule{0.3em}{0ex}}\mathrm{min})$ and $^{267}\mathrm{Rf}\phantom{\rule{0.3em}{0ex}}({T}_{\mathrm{SF}}\ensuremath{\sim}2.3\phantom{\rule{0.3em}{0ex}}\mathrm{h})$, longer decay chains than reported previously. We observed the new nuclide $^{292}116\phantom{\rule{0.3em}{0ex}}({T}_{\ensuremath{\alpha}}={18}_{\ensuremath{-}6}^{+16}\phantom{\rule{0.3em}{0ex}}\mathrm{ms},{E}_{\ensuremath{\alpha}}=10.66\ifmmode\pm\else\textpm\fi{}0.07\phantom{\rule{0.3em}{0ex}}\mathrm{MeV})$ 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.