Nuclear orientation in the reaction ${}^{34}\mathrm{S}+{}^{238}\mathrm{U}$ and synthesis of the new isotope ${}^{268}\mathrm{Hs}$

The synthesis of isotopes of the element hassium was studied using the reaction ${}^{34}\mathrm{S}+{}^{238}\mathrm{U}\to {}^{272}\mathrm{Hs}{}^{*}$. At a kinetic energy of $163.0$ MeV in the center-of-mass system we observed one $\alpha$-decay chain starting at the isotope ${}^{267}\mathrm{Hs}$. The cross section was $1.8_{-1.5}^{+4.2}$ pb. At $152.0$ MeV one decay of the new isotope ${}^{268}\mathrm{Hs}$ was observed. It decays with a half-life of $0.{38}_{-0.17}^{+1.8}$ s by $9479\pm 16$ keV $\alpha$-particle emission. Spontaneous fission of the daughter nucleus ${}^{264}\mathrm{Sg}$ was confirmed. The measured cross section was $0.{54}_{-0.45}^{+1.3}$ pb. In-beam measurements of fission-fragment mass distributions were performed to obtain information on the fusion probability at various orientations of the deformed target nucleus. The distributions changed from symmetry to asymmetry when the beam energy was changed from above-barrier to sub-barrier values, indicating orientation effects on fusion and/or quasifission. It was found that the distribution of symmetric mass fragments originates not only from fusion-fission, but has a strong component from quasifission. The result was supported by a calculation based on a dynamical description using the Langevin equation, in which the mass distributions for fusion-fission and quasifission fragments were separately determined.

Figure 1

Decay chains observed in the ${}^{34}\mathrm{S}+{}^{238}\mathrm{U}$ reaction at the center-of-mass energies of 163.0 and 152.0 MeV at the center of the target. The strip numbers of the stop detector are given. Data inside the boxes show the measured implantation energies, lifetimes, and total energies of $\alpha$ decays or sf. In the case of escaped particles, the energy deposition in the stop detector is given in parentheses. Below the boxes are given the vertical positions determined independently from the top and bottom signals of the strip, segment number of the box detector in the case of escaping particles, energies of coincident $\gamma$-ray event in the fourfold clover detector, and status of the beam in the case of radioactive decays.

Figure 2

$\alpha$-decay $Q$ values of even-even nuclei in the region of the single-particle energy gap for neutrons at $N=162$. Open circles connected with lines represent calculated values [31]. Experimental values are marked with open squares. The $Q_{\alpha }$ value of the new isotope ${}^{268}\mathrm{Hs}$ is marked with the solid square. Double circles represent calculated $Q_{\alpha }$ values of nuclei and their daughter products, which could be studied in experiments using the reactions ${}^{34,36}\mathrm{S}+{}^{248}\mathrm{Cm}$,${}^{244}\mathrm{Pu}$, and 4$n$-evaporation channels.

Figure 3

Experimental fission cross sections for FMT events of the reaction ${}^{34}\mathrm{S}+{}^{238}\mathrm{U}$ as a function of the center-of-mass energy $E_{\mathrm{c}.\mathrm{m}.}$ and the excitation energy of the compound nucleus $E^{*}$ (scale at the top). Coulomb barriers for polar and equatorial collisions are indicated by arrows. The solid line represents the capture cross sections ${\sigma }_{\mathrm{cap}}$ calculated by the CC code [34]. The dotted curve represents results of the one-dimensional barrier penetration model. The dash-dotted line is the calculated fusion cross sections ${\sigma }_{\mathrm{fus}}$ based on the Langevin description coupled with the CC model. (Bottom) Experimental cross sections for the production of ${}^{268}\mathrm{Hs}$ (square) and ${}^{267}\mathrm{Hs}$ (circle). Lines represent results of statistical model calculations for 3$n$ to 6$n$ evaporation channels based on different assumptions for fusion (see text for details).

Figure 4

Mass distributions for FMT fission fragments for the reaction ${}^{34}\mathrm{S}+{}^{238}\mathrm{U}$. Energies $E_{\mathrm{c}.\mathrm{m}.}$ are given in each section of the figure. The data are disentangle in two parts, symmetric fission (dashed lines) and asymmetric fission (dotted lines). The sum of both components is given by the solid lines. The histograms show model calculations for FMT fission fragment distributions using the Langevin equation. The calculated fusion-fission parts are shown by the filled areas. The calculations were multiplied by the factor shown in parentheses such that the total cross section agrees with the experimental value to compare the shape of the mass distribution with the experiment.