A search for production of the superheavy elements with atomic numbers 119 and 120 was performed in the 50Ti+249Bk and 50Ti+249Cf fusion-evaporation reactions, respectively, at the gas-filled recoil separator TASCA at GSI Darmstadt, Germany. Over four months of irradiation, the 249Bk target partially decayed into 249Cf, which allowed for a simultaneous search for both elements. Neither was detected at cross-section sensitivity levels of 65 and 200 fb for the 50Ti+249Bk and 50Ti+249Cf reactions, respectively, at a midtarget beam energy of Elab = 281.5 MeV. The nonobservation of elements 119 and 120 is discussed within the concept of fusion-evaporation reactions including various theoretical predictions on the fission-barrier heights of superheavy nuclei in the region of the island of stability. (Less)

/pdf/search-for-elements-119-and-120-405i1hxz1y.pdf

Search for elements 119 and 120

The electron-capture decay followed by a prompt fission process was searched for in the hitherto unknown most neutron-deficient Md isotope with mass number 244. Alpha decay with $\ensuremath{\alpha}$-particle energies of 8.73--8.86 MeV and with a half-life of ${0.30}_{\ensuremath{-}0.09}^{+0.19}\text{ }\text{ }\mathrm{s}$ was assigned to $^{244}\mathrm{Md}$. No fission event with a similar half-life potentially originating from spontaneous fissioning of the short-lived electron-capture decay daughter $^{244}\mathrm{Fm}$ was observed, which results in an upper limit of 0.14 for the electron-capture branching of $^{244}\mathrm{Md}$. Two groups of fission events with half-lives of ${0.9}_{\ensuremath{-}0.3}^{+0.6}\text{ }\text{ }\mathrm{ms}$ and ${5}_{\ensuremath{-}2}^{+3}\text{ }\text{ }\mathrm{ms}$ were observed. The ${0.9}_{\ensuremath{-}0.3}^{+0.6}\text{ }\text{ }\mathrm{ms}$ activity was assigned to originate from the decay of $^{245}\mathrm{Md}$. The origin of eight fission events resulting in a half-life of ${5}_{\ensuremath{-}2}^{+3}\text{ }\text{ }\mathrm{ms}$ could not be unambiguously identified within the present data while the possible explanation has to invoke previously unseen physics cases.

/pdf/search-for-electron-capture-delayed-fission-in-the-new-1nnsb00uxt.pdf

Search for Electron-Capture Delayed Fission in the New Isotope Md 244

In an experiment performed at Lawrence Berkeley National Laboratory's 88-inch cyclotron, the isotope ^{244}Md was produced in the ^{209}Bi(^{40}Ar,5n) reaction. Decay properties of ^{244}Md were measured at the focal plane of the Berkeley Gas-filled Separator, and the mass number assignment of A=244 was confirmed with the apparatus for the identification of nuclide A. The isotope ^{244}Md is reported to have one, possibly two, α-decaying states with α energies of 8.66(2) and 8.31(2) MeV and half-lives of 0.4_{-0.1}^{+0.4} and ∼6  s, respectively. Additionally, first evidence of the α decay of ^{236}Bk was observed and is reported.

/pdf/identification-of-the-new-isotope-244-md-4g3x43ke0u.pdf

Identification of the New Isotope 244 Md

Spontaneous fission half-lives of the heaviest nuclei: Semi-empirical predictions

In the present work, a $K$ isomeric state in $^{250}\mathrm{No}$, which is more stable against fission than the ground state, was experimentally studied. The aim was to measure the fission branch of this isomeric state. In total, 780 fission events attributed to the decay of $^{250}\mathrm{No}$ were detected. Among them 133 cases were attributed to the ground-state decay with a half-life of 4.0(4) $\ensuremath{\mu}\mathrm{s}$, which was populated by the deexcitation of the isomeric state via electromagnetic transitions with a half-life of 23(4) $\ensuremath{\mu}\mathrm{s}$. In addition, in two more cases, this long-lived isomeric state was populated in the deexcitation of a hitherto unknown, yet higher-lying and short-lived isomeric state with a half-life of $0.{7}_{\ensuremath{-}0.3}^{+1.4}\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{s}$. No direct fission from the long-lived isomeric state, i.e., with a lifetime of longer than 40 $\ensuremath{\mu}\mathrm{s}$, was identified. This results in an upper limit of 0.035 for the branching ratio for fission. This is a significantly more strict limit than the previously known value of 0.5. Nonobservation of fission branching of the long-lived isomer is discussed relative to theoretical predictions and within various semiempirical ways, which resulted in an attribution of a lower limit of ${10}^{4}$ for the fission-hindrance factor, caused by the $K$ quantum number. The presences of multiple high-$K$ isomeric states seemingly is a widespread phenomenon in deformed heavy nuclei. 

Search for fission from a long-lived isomer in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:mi>No</mml:mi><mml:mprescripts /><mml:none /><mml:mn>250</mml:mn></mml:mmultiscripts></mml:math> and evidence of a second isomer

Nuclear fission is one of the radioactive decay modes of heavy nuclei, and is a limiting factor in the existence of the superheavy elements. For decades, both experimental and theoretical works devoted to the study of fission have been performed, and are still ongoing. Among them, the so-called $\beta$ (electron capture)-delayed fission is of special interest. In this work, all known cases of electron-capture delayed fission in heavy nuclei were used to establish a semi-empirical approach that could quantitatively estimate the probability of the process. This framework was constructed based on a qualitative discussion of the fission-barrier shape, in contrast to a parabolic shape. The results show a strong effect of the fission-barrier shape on the delayed fission process. The predictions for yet unmeasured cases of electron-capture delayed fission in nuclei with atomic numbers in the wide range ( $79\leq Z\leq 119$) including the superheavy nuclei are given. It is suggested that $ \beta$-delayed fission manifests itself as one of the main decay modes of the superheavy nuclei, if $ \beta$ decay takes place.

$\beta$β(EC) delayed fission in the heaviest nuclei

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$\beta$β(EC) delayed fission in the heaviest nuclei