$\beta$-delayed fission and $\alpha$ decay of ${}^{178}$Tl

A detailed nuclear-decay spectroscopy study of the neutron-deficient isotope ${}^{178}$Tl has been performed using the highly selective Resonance Ionization Laser Ion Source and ISOLDE mass separator (CERN), which allowed a unique isobarically pure beam of ${}^{178}$Tl to be produced. The first identification of the $\beta$-delayed fission of this isotope was made and its probability $P_{\beta \text{DF}}{(}^{178}\text{Tl})=0.15\left(6\right)\%$ was determined. An asymmetric fission fragment mass distribution of the daughter isotope ${}^{178}$Hg (populated by the $\beta$ decay of ${}^{178}$Tl) was deduced based on the measured fission fragment energies. The fine-structure $\alpha$-decay pattern of ${}^{178}$Tl allowed the low-energy states in the daughter nucleus ${}^{174}$Au to be studied.

Figure 1

(a) Part of the $\alpha$-decay spectrum collected in the silicon detectors, from the combined statistics (Run I and Run II). The $\alpha$ peaks are denoted with their energies in keV and the isotope to which they belong. (b) The $\alpha$-$\gamma$ coincidence plot for the $\alpha$ decays from (a), measured within the time interval $\Delta T(\alpha -\gamma )\le 600$ ns. Panels (c) and (d) give the projections on the $E_{\alpha }$ axis from the regions “A” and “B” of the inset to panel (c). The latter shows an expanded part of $\alpha$ decays from panel (b) being in coincidence with the 163- and 173-keV decays.

Figure 2

(a) Projection on the $E_{\gamma }$ axis of the region “G1” of the $\alpha$-$\gamma$ plot shown in Fig. 1b; (b) the same, but for the region “G2.” The sum of the two projections is displayed in (c). The $\gamma$-ray peaks are labeled by their energies in keV. An uncertainty of 0.5 keV is assumed on the $\gamma$-ray energy.

Figure 3

Decay scheme of ${}^{178}$Tl deduced in this work. The intensities for the 6595- and 6693-keV $\alpha$ decays were corrected to account for the effect of $\alpha -e^{-}$ summing in the silicon detectors (see text). The decay schemes of two isomers in the daughter nucleus ${}^{174}$Au, compiled from Refs. [10, 11], are given to aid the discussion, although the decays from the level labeled “m2” were not seen in our work.

Figure 4

The sum time distribution for all $\alpha$ decays of ${}^{178}$Tl, determined using as reference the times of two consecutive proton pulses indicated as first PP and second PP in the figure, respectively. See Ref. [18] for the description of the timing system used in the experiment. The decay part of the time distribution was fitted with an exponential function, resulting in a value of $T_{1/2}{(}^{178}\text{Tl})=252\left(20\right)$ ms.

Figure 5

(a) Energy spectrum for $\beta \text{DF}$ of ${}^{178}$Tl in the fission energy range from the combined statistics (Run I and Run II). (b) Energy spectrum for $\beta \text{DF}$ of ${}^{180}$Tl; data are taken from Refs. [6, 7].

Figure 6

Nilsson orbitals, relevant for the region of ${}^{178}$Tl and ${}^{174}$Au, calculated with a Woods-Saxon potential [34]: (a) for neutrons and (b) for protons.

Figure 7

Systematics of the total kinetic energies as a function of the fissility parameter (Viola systematics) [49]. The Viola fit is shown by the solid line. Owing to their very similar TKE values, ${}^{178,180}$Hg are practically indistinguishable on this plot.