Observation of New Isotopes in the Fragmentation of ${}^{198}\mathrm{Pt}$ at FRIB

Five previously unknown isotopes (${}^{182,183}\mathrm{Tm}$, ${}^{186,187}\mathrm{Yb}$, ${}^{190}\mathrm{Lu}$) were produced, separated, and identified for the first time at the Facility for Rare Isotope Beams (FRIB) using the Advanced Rare Isotope Separator (ARIS). The new isotopes were formed through the interaction of a ${}^{198}\mathrm{Pt}$ beam with a carbon target at an energy of $186\mathrm{MeV}/u$ and with a primary beam power of 1.5 kW. Event-by-event particle identification of $A$, $Z$, and $q$ for the reaction products was performed by combining measurements of the energy loss, time of flight, magnetic rigidity $B\rho$, and total kinetic energy. The ARIS separator has a novel two-stage design with high resolving power to strongly suppress contaminant beams. This successful new isotope search was performed less than one year after FRIB operations began and demonstrates the discovery potential of the facility which will ultimately provide 400 kW of primary beam power.

Figure 1

Region of the nuclear chart, $69\le Z\le 78$ and $112\le N\le 122$, showing the known nuclei and the eight nuclides that were recently discovered at NSCL [11] and FRIB (present work) by the fragmentation of ${}^{198}\mathrm{Pt}$ beams. Previously known isotopes observed in the present work whose formation involved one- and two-neutron pickup relative to the projectile are marked.

Figure 2

Schematic view of the experimental setup used in the current work. A ${}^{198}\mathrm{Pt}$ beam was accelerated by the FRIB linear accelerator and fragmented in a carbon target at the start of the ARIS fragment separator. Fragments were transported to the last diagnostic block (DB5) of the separator and stopped in a silicon detector telescope. The positions of various diagnostic devices are shown.

Figure 3

(a) The separation of charge states is demonstrated in the $q$ vs $Z\text{−}q$ spectrum of a setting optimized to observe ${{}_{70}{}^{186}\mathrm{Yb}}^{68+}$ ions, in which the heliumlike charge states were most prominent. A charge-state selection gate [1.7 to 2.3] on the heliumlike ions was applied to obtain the spectrum in (b), with the $Z$ distribution of the heliumlike ions as a function of $(A/q-3)\times q$, where the mass-to-charge ratio, $A/q$, was constructed from TOF and $B\rho$ measurements. This PID plot, labeled as $Z$ vs $A-3q$, demonstrates the quality of the mass, charge, and elemental separation. The limit of previously observed nuclei is indicated by the solid red line. A $\gamma$-ray spectrum observed in coincidence with the labeled ${{}^{188}\mathrm{Ta}}^{71+}$ ions was used to confirm its identification (see text).

Figure 4

The ionic, $q$ (a), and elemental, $Z$ (b), spectra obtained for fragments that stopped in the fourth detector of the Si telescope during the data taking for the new-isotope search. The spectra were gated to select the heliumlike ions as described in the caption of Fig. 3.

Figure 5

Mass spectra of elements with atomic numbers ranging from 69 to 71 obtained with the new-isotope-search settings of the separator. A gate condition on $Z$ with a width [$Z-0.3$, $Z+0.3$] was applied to generate these spectra in addition to the selection of the heliumlike charge state (see Fig. 3). The standard deviation obtained for Gaussian functions having a constant width (dashed red lines) is indicated for each element. The dashed blue line marks the limit of known isotopes for each element.