Decay properties of the neutron-deficient isotope ${}^{242}\mathrm{Es}$

The radioactive decay properties of ${}^{242}\mathrm{Es}$ were studied at the gas-filled recoil separator TASCA. This isotope was produced in the $3n$ evaporation channel of the fusion reaction of ${}^{48}\mathrm{Ca}+{}^{197}\mathrm{Au}$. A half-life of 16.9(8) s was deduced from 662 $\alpha$ decays of ${}^{242}\mathrm{Es}$, resulting in an $\alpha$-decay branching of 41(3)%. Twenty-six fission events with a half-life of $18.2_{-3.0}^{+4.5}\mathrm{s}$ were assigned to originate from the electron-capture delayed fission of ${}^{242}\mathrm{Es}$. The probability for the electron-capture delayed fission was measured to be 0.015(4), which improves and resolves ambiguities in available experimental data. We discuss all known cases for electron-capture delayed fission in Es, Bk, and Am isotopes and compare experimental data with predictions from a recent semiempirical model. A cross section of 27(3) nb was measured for the production of ${}^{242}\mathrm{Es}$.

Figure 1

Energy spectra of the stop detector ($X$ strips) measured during the beam-off periods from the ${}^{48}\mathrm{Ca}+{}^{197}\mathrm{Au}$ reaction in run 1 (a), run 2 (b), and run 3 (c). In the inset of (a), the energy spectrum measured in run 1 is shown in a broader energy range. Isotopic assignments of peaks are marked. See texts for details.

Figure 2

The time distributions of $\alpha$ and fission events correlated with preceding implantation signals. The random correlation (a), $\alpha$ events assigned to the decay of ${}^{242}\mathrm{Es}$ (b), $\alpha$ events assigned to the decay of ${}^{242}\mathrm{Cf}$ (c), and fission events assigned to the ECDF of ${}^{242}\mathrm{Es}$ (d) are shown. Fits of the experimental data by the time-density distribution function according to Ref. [24] are shown. See text for details.

Figure 3

Upper panels: Probabilities of ECDF in Es, Bk, and Am isotopes. Solid and open circles mark the experimental [3, 26] and calculated [4] $P_{\mathrm{ECDF}}$, respectively. A retardation factor $P_{\mathrm{R}}$ used for the description of the delayed fission probabilities according to Ref. [4] and corresponding to calculated values (open circles) is given for each element. The presently measured value for ${}^{242}\mathrm{Es}$ is shown by a solid square. Arrows indicate the present upper limit for ${}^{238}\mathrm{Bk}$ and the recently reported lower limit for ${}^{230}\mathrm{Am}$ [27]. Lower panels: Theoretical $B_{\mathrm{f}}$ and $Q_{\mathrm{EC}}$ values taken from Refs. [28, 29] are shown as a function of the neutron number of the EC-decaying odd-odd nucleus. Dashed vertical lines mark the crossing of the $Q_{\mathrm{EC}}$ and $B_{\mathrm{f}}$ curves. See text for details.