New Insights into the ${}^{243}\mathrm{Am}+{}^{48}\mathrm{Ca}$ Reaction Products Previously Observed in the Experiments on Elements 113, 115, and 117

Results of a new series of experiments on the study of production cross sections and decay properties of the isotopes of element 115 in the reaction ${}^{243}\mathrm{Am}+{}^{48}\mathrm{Ca}$ are presented. Twenty-one new decay chains originating from ${}^{288}115$ were established as the product of the $3n$-evaporation channel by measuring the excitation function at three excitation energies of the compound nucleus ${}^{291}115$. The decay properties of all newly observed nuclei are in full agreement with those we measured in 2003. At the lowest excitation energy $E^{*}=33\mathrm{MeV}$, for the first time we registered the product of the $2n$-evaporation channel, ${}^{289}115$, which was also observed previously in the reaction ${}^{249}\mathrm{Bk}+{}^{48}\mathrm{Ca}$ as the daughter nucleus of the decay of ${}^{293}117$. The maximum cross section for the production of ${}^{288}115$ is found to be 8.5 pb at $E^{*}\approx 36\mathrm{MeV}$.

Figure 1

Cross sections for the $2n$, $3n$, and $4n$ evaporation channels of the reaction ${}^{243}\mathrm{Am}+{}^{48}\mathrm{Ca}$. Vertical error bars correspond to statistical uncertainties [11]. Horizontal error bars represent the range of excitation energies populated for one beam energy. For reference purposes we show the energy at the Bass barrier [12] (black arrow). Open arrows represent excitation energies corresponding to the maxima of the $3n$-evaporation channel measured in the reactions ${}^{242}\mathrm{Pu}+{}^{48}\mathrm{Ca}$ [9] (left) and ${}^{245}\mathrm{Cm}+{}^{48}\mathrm{Ca}$ [10] (right). The dashed line through the $3n$ data is drawn to guide the eye.

Figure 2

Observed decay chains interpreted as originating from the isotopes ${}^{293}117$ (average of five events) [4, 5], ${}^{289}115$ (single event), and ${}^{288}115$ (average of 24 events—from 21 new and 3 prior decay chains). The measured $\alpha$-particle energies (average values) and lifetimes are shown. For the ${}^{289}115$ decay chain positions of detected events are given in blue.

Figure 3

$\alpha$-particle and fission-fragment energy spectra registered by the focal-plane detector (left-hand panel) and decay-time distributions on a logarithmic scale (right-hand panel) for isotopes originating from ${}^{288}115$. The arrows show the energies of the three events obtained in the first experiment in 2003 [1, 2]. The fission-fragment energies were not corrected for the pulse height defect of the detectors. SF events detected by only the focal-plane detector are shown by the open histogram; those detected by both focal-plane and side detectors, the shaded histogram. The smooth curves are the time spectra for exponential decays associated with the half-lives $T_{1/2}$ shown in the figures.