Selective excitations in actinide nuclei via alpha pickup

The $\alpha$-cluster pickup reaction ($d,{}_{}{}^6{}_{}{}^{}\mathrm{Li}$) has been studied at $E_d=55\mathrm{}$ MeV on targets of ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$ and ${}_{}{}^{238}{}_{}{}^{}\mathrm{U}$. Members of the ground-state rotational bands in ${}_{}{}^{228}{}_{}{}^{}\mathrm{Ra}$ and ${}_{}{}^{234}{}_{}{}^{}\mathrm{Th}$ are excited and absolute reduced $\alpha$ widths obtained from finite-range distorted-wave analysis are in good agreement with values deduced from $\alpha$ decay. In addition three excited groups of states are very strongly populated in both nuclei with spectroscopic strength per group comparable with those of the respective ground state bands. These groups are apparently excited rotational bands with band heads at $E_x=700\mathrm{}\pm 40, 1070\pm 60, \mathrm{and} 1390\pm 60$ keV in ${}_{}{}^{228}{}_{}{}^{}\mathrm{Ra}$ and $E_x=810\mathrm{}\pm 30, 1150\pm 40, \mathrm{and} 1470\pm 40$ keV in ${}_{}{}^{234}{}_{}{}^{}\mathrm{Th}$. The selective and strong excitation in this particular multi-nucleon transfer reaction of several excited bands is not predicted by existing theoretical models. An attempt has been made to describe the systematics of excited $0^{+}$ states in the actinide region with the interacting boson model. Excitation energies are reasonably well described but intruder states are present and transfer strengths are not reproduced properly. The observation of strong $\alpha$-cluster pickup to excited rotational bands suggests coherent contributions from both neutron and proton pair excitations which can lead to strong four-body correlations and/or to new types of collective excitations which favor quartet structure. It is found that about 25% of the nuclear charge (matter) at $r\simeq 10.6$ fm must be associated with $\alpha$ particles. This high $\alpha$-clustering probability indicates $\alpha$-particle condensation in low-density nuclear matter.