Search for β-delayed fission of ${}^{228}\mathrm{Ac}$

Radium was radiochemically separated from natural thorium. Thin ${}^{228}\mathrm{Ra}$$\stackrel{{\beta }^{-}}{\to }$${}^{228}\mathrm{Ac}$ sources were prepared and exposed to mica fission track detectors, and measured by an HPGe $\gamma$-ray detector. The $\beta$-delayed fission events of ${}^{228}\mathrm{Ac}$ were observed and its β-delayed fission probability was found to be ($5\pm 2)\times {10}^{-12}$.

Figure 1

(a) A sample of fission fragment tracks observed in mica foils exposed by ${}^{228}\mathrm{Ra}$$\stackrel{{\beta }^{-}}{\to }$${}^{228}\mathrm{Ac}$ sources. (b) The observed fission fragment track in the blank experiments. (c) A sample of natural background fission fragment tracks.

Figure 2

The measured $\gamma$-ray spectrum.