First online multireflection time-of-flight mass measurements of isobar chains produced by fusion-evaporation reactions: Toward identification of superheavy elements via mass spectroscopy

Using a multireflection time-of-flight mass spectrograph located after a gas cell coupled with the gas-filled recoil ion separator GARIS-II, the masses of several $\alpha$-decaying heavy nuclei were directly and precisely measured. The nuclei were produced via fusion-evaporation reactions and separated from projectilelike and targetlike particles using GARIS-II before being stopped in a helium-filled gas cell. Time-of-flight spectra for three isobar chains, ${}^{204}\mathrm{Fr}-{}^{204}\mathrm{Rn}-{}^{204}\mathrm{At}-{}^{204}\mathrm{Po}, {}^{205}\mathrm{Fr}-{}^{205}\mathrm{Rn}-{}^{205}\mathrm{At}-{}^{205}\mathrm{Po}-{}^{205}\mathrm{Bi}$, and ${}^{206}\mathrm{Fr}-{}^{206}\mathrm{Rn}-{}^{206}\mathrm{At}$, were observed. Precision atomic mass values were determined for ${}^{204–206}\mathrm{Fr}, {}^{204,205}\mathrm{Rn}$, and ${}^{204,205}\mathrm{At}$. Identifications of ${}^{205}\mathrm{Bi}, {}^{204,205}\mathrm{Po}, {}^{206}\mathrm{Rn}$, and ${}^{206}\mathrm{At}$ were made with $N\lesssim 10$ detected ions, representing the next step toward use of mass spectrometry to identify exceedingly low-yield species such as superheavy element ions.

Figure 1

Schematic overview of the experimental setup. Red arrows show the path of ions through the system. Red lettering gives time scale.

Figure 2

Time-of-flight spectra observed using ${}^{169}\mathrm{Tm}$(${}^{40}\mathrm{Ar}, X$) reaction at (a) 4.825 MeV/nucleon and (b, c) 5.16 MeV/nucleon. Ions made (a) $n=148$ laps, (b) $n=223$ laps, and (c) $n=224$ laps in the MRTOF-MS. The (a) $A/q=205$ and $A/q=206$ and the (b, c) $A/q=204$ and $A/q=205$ spectral peaks were observed simultaneously in single spectra. See text for details.

Figure 3

Deviation of each isotope's measured mass from literature values. Error bars for this work have been scaled by the Birge ratio, when available, and include 75 keV/$c^2$ systematic uncertainty.