Systematics of spontaneous emission of intermediate mass fragments from heavy nuclei

We have used polycarbonate track-recording films to confirm the rare decay mode of ${}_{}{}^{226}{}_{}{}^{}\mathrm{Ra}$ by ${}_{}{}^{14}{}_{}{}^{}\mathrm{C}$ emission and to set stringent upper limits on ${}_{}{}^{14}{}_{}{}^{}\mathrm{emission}$ rates of ${}_{}{}^{221}{}_{}{}^{}\mathrm{Fr}$, ${}_{}{}^{221}{}_{}{}^{}\mathrm{Ra}$, and ${}_{}{}^{225}{}_{}{}^{}\mathrm{Ac}$. The ${}_{}{}^{14}{}_{}{}^{}\mathrm{emission}$ rate exhibits a pronounced odd-even effect. For Ra isotopes the hindrance factor for odd-even parents relative to even-even parents is at least 10 times higher for ${}_{}{}^{14}{}_{}{}^{}\mathrm{C}$ emission than for α emission.