$\alpha$ decay studies of very neutron-deficient francium and radium isotopes

Very neutron-deficient francium and radium isotopes have been produced in fusion evaporation reactions using ${}^{63}\mathrm{Cu}$ and ${}^{65}\mathrm{Cu}$ ions on ${}^{141}\mathrm{Pr}$ targets and ${}^{36}\mathrm{Ar}$ ions on ${}^{170}\mathrm{Yb}$ targets. The gas-filled recoil separator RITU was employed to collect the fusion products and to separate them from the scattered beam. The activities were implanted into a position-sensitive silicon detector after passing through a gas-counter system. The isotopes were identified using spatial and time correlations between the implants and decays. Two new α decaying radium isotopes, ${}^{201}\mathrm{Ra}$ and ${}^{202}\mathrm{Ra}$, were identified. The α decay energy and half-life of ${}^{203}\mathrm{Ra}$ were measured with improved precision. The α decay properties measured for the francium isotopes ${}^{201}\mathrm{Fr},{}^{202}\mathrm{Fr},{}^{203}\mathrm{Fr}$, and ${}^{204}\mathrm{Fr}$ were confirmed, in many cases with improved precision. For the first time, a $(\pi s_{1/2}^{-1})1/2^{+}$ proton intruder state was identified in francium isotopes, namely in ${}^{201}\mathrm{Fr}$ and tentatively in ${}^{203}\mathrm{Fr}$. The measured decay properties for the neutron-deficient odd-mass Fr isotopes suggest an onset of substantial deformation at $N=112$.

Figure 1

Energy spectra of α particles measured in the silicon detector and vetoed with the gas counter using the ${}^{65}\mathrm{Cu}+{}^{141}\mathrm{Pr}$ reaction: (a) all decays and (b) decays following recoil implants within 5 s.

Figure 2

Parent and daughter α-particle energies for all chains of the type $\mathrm{ER}-{\alpha }_m-{\alpha }_d$ observed in the ${}^{65}\mathrm{Cu}+{}^{141}\mathrm{Pr}$ irradiation. Maximum search times were 5 s for the $\mathrm{ER}-{\alpha }_m$ pair and 20 s for the ${\alpha }_m-{\alpha }_d$ pair.

Figure 3

Energy spectra of α particles measured in the silicon detector and vetoed with the gas counter using the ${}^{63}\mathrm{Cu}+{}^{141}\mathrm{Pr}$ reaction: (a) all decays and (b) decays following recoil implants within 5 s.

Figure 4

Parent and daughter α-particle energies for all chains of the type $\mathrm{ER}-{\alpha }_m-{\alpha }_d$ observed in the ${}^{63}\mathrm{Cu}+{}^{141}\mathrm{Pr}$ irradiation. Maximum search times were 5 s for the $\mathrm{ER}-{\alpha }_m$ pair and 20 s for the ${\alpha }_m-{\alpha }_d$ pair.

Figure 5

Energy spectra of α particles measured in the silicon detector and vetoed with the gas counter using the ${}^{36}\mathrm{Ar}+{}^{170}\mathrm{Yb}$ reaction: (a) all decays; (b) decays following recoil implants within 2 s; (c) as (b) but followed by a second decay within 5 s.

Figure 6

Parent and daughter α-particle energies for all chains of the type $\mathrm{ER}-{\alpha }_m-{\alpha }_d$ observed in the ${}^{36}\mathrm{Ar}+{}^{170}\mathrm{Yb}$ irradiation. Maximum search times were 2 s for the $\mathrm{ER}-{\alpha }_m$ pair and 5 s for the ${\alpha }_m-{\alpha }_d$ pair.

Figure 7

(Color online) Experimental excitation energies for the low-lying states in odd-mass bismuth, astatine, and francium isotopes in the region of interest. The $9/2^{-}$ states, in most of the cases the ground states, are not shown.

Figure 8

Suggested α decay schemes belonging to odd-mass francium isotopes are shown.