$\beta$-delayed fission and $\alpha$ decay of ${}^{196}\mathrm{At}$

A nuclear-decay spectroscopy study of the neutron-deficient isotope ${}^{196}\mathrm{At}$ is reported where an isotopically pure beam was produced using the selective Resonance Ionization Laser Ion Source and On-Line Isotope Mass Separator (CERN). The fine-structure $\alpha$ decay of ${}^{196}\mathrm{At}$ allowed the low-energy excited states in the daughter nucleus ${}^{192}\mathrm{Bi}$ to be investigated. A $\beta$-delayed fission study of ${}^{196}\mathrm{At}$ was also performed. A mixture of symmetric and asymmetric fission-fragment mass distributions of the daughter isotope ${}^{196}\mathrm{Po}$ (populated by $\beta$ decay of ${}^{196}\mathrm{At}$) was deduced based on the measured fission-fragment energies. A $\beta \text{DF}$ probability $P_{\beta \mathrm{DF}}\left({}^{196}\mathrm{At}\right)=9\left(1\right)\times {10}^{-5}$ was determined.

Figure 1

Calculated $Q_{\mathrm{EC}}$(At) (solid circle and square symbols) and $B_f$(Po) (open symbols) values from the FRDM/FRLDM [14, 15] and from the TF model [16]. The $Q_{\mathrm{EC}}$ values from AME2012 [17] are shown by the solid triangles.

Figure 2

(a) Part of the $\alpha$-decay spectrum measured in the GPS experiment and summed from the Si1 and Si2 detectors. The peaks are marked with their energies (in keV) and the isotopes they belong to. (b) The same for the HRS experiment, but only part of the data collected during $\sim 30$ min is shown, for which respective coincidence $\alpha \text{−}\gamma$ data are also available and shown in panel (c); see text. (c) The prompt $\alpha \text{−}\gamma$ coincidence plot for the $\alpha$ decays from (b), measured within the time interval of $\mathrm{\Delta }T(\alpha -\gamma )\le 600\text{ns}$.

Figure 3

Projections on the $E_{\gamma }$ axis of the $\alpha \text{−}\gamma$ matrix, shown in Fig. 2, obtained by gating on (a) the region “G1,” (b) the region “G2,” (c) the region “G3,” and (d) the region “G4.” The $\gamma$-ray peaks are labeled by their energies in keV.

Figure 4

Decay scheme of ${}^{196}\mathrm{At}$ deduced in this work. The yet-unobserved 188(8)-, 209(8)-, and 95-keV $\gamma$ decays are shown by dashed lines. For consistency of the discussion, the decay schemes of two isomeric states in the daughter isotope ${}^{192}\mathrm{Bi}$ are also shown, including the tentative $I^{\pi }$ values and configuration assignments taken from Ref. [36]. Owing to the yet-unknown relative excitation energy of the two $\alpha$-decaying states in ${}^{192}\mathrm{Bi}$, we presently denote them as “m1” and “m2.” The $\alpha$-decay energies of 6060(5) and 6245(5) keV shown for ${}^{192}\mathrm{Bi}^{m1}$ are also taken from Ref. [36] and are slightly different (within the experimental uncertainty) from the values of 6063(5) and 6244(10) keV measured in this work and shown in Fig. 2; see the main text.

Figure 5

The time distribution for the 7053-keV $\alpha$ decay of ${}^{196}\mathrm{At}$, following the implantation during two consecutive proton pulses PP1 and PP2, separated by 1.2 s. The decay part of the time distribution within the time interval of 37–41 s is fitted with an exponential function (shown in red), resulting in a value of $T_{1/2}\left({}^{196}\mathrm{At}\right)=371\left(5\right)$ ms. The fit started 1000 ms after the PP2 proton beam impact, when the so-called ISOLDE “beam gate” was closed; thus, no new activity was deposited anymore on the carbon foil. The data from Si1 and Si2 are used.

Figure 6

Half-life determination of ${}^{196}\mathrm{Po}$. The decay curve of the 6521-keV $\alpha$ decay of ${}^{196}\mathrm{Po}$ in Si3 and Si4 at the decay position of the WM was fitted using an exponential (shown in red), resulting in a value of $T_{1/2}\left({}^{196}\mathrm{Po}\right)=5.75\left(12\right)\text{s}$. Note that the decay measurement stops at $\sim 42$ s, which corresponds to the length of the proton synchrotron booster supercycle in this experiment, at which point the WM moves again by introducing the newly irradiated foil from the implantation position.

Figure 7

Calibrated energy spectrum of single fission events in the $\beta$ DF of ${}^{196}\mathrm{At}$, measured in the detectors Si1 and Si2 during the GPS run.

Figure 8

Comparison of $\beta \mathrm{DF}$ data for ${}^{196}\mathrm{At}$ (left column, present work) and for ${}^{180}\mathrm{Tl}$ (right column, data from Ref. [26]). (Top row) The 2D energy distributions of coincident fission fragments in the two silicon detectors. (Middle row) The total kinetic-energy distributions. The red dashed line in the top two rows corresponds to respective most probable TKE values of ${}^{196}\mathrm{Po}$ and ${}^{180}\mathrm{Hg}$; see text. (Bottom row) The FF mass distributions. The mass distributions shown by the black lines are for all observed events, while those shown by the dot-dashed green line and the dashed blue line correspond to events with the TKE values below and above the most probable TKE values, respectively.