Low-lying isovector $2^{+}$ valence-shell excitations of ${}^{212}\mathrm{Po}$

We present the results from an experiment dedicated to search for quadrupole-collective isovector valence-shell excitations, states with so-called mixed proton-neutron symmetry (MSS), of ${}^{212}\mathrm{Po}$. This nucleus was studied in an $\alpha \text{-transfer}$ reaction. The lifetimes of two short-lived excited states, candidates for the one-phonon MSS, were determined by utilizing the Doppler shift attenuation method. The experimental results are in qualitative agreement with a simple $\text{single-}j$ shell model calculation, which, together with the observed lack of quadrupole collectivity, indicates that the isovector nature of low-lying states is a property of the leading single-particle valence shell configuration.

Figure 1

(a) The projection of the $\text{particle-}\gamma \left(\text{at}{142}^{\circ }\right)$ matrix. The vertical dashed lines represent parts of the particle spectrum found to be in coincidence with the $\gamma$ rays from the indicated nuclei. (b) The $\gamma \text{-ray}$ spectrum in coincidence with the group of particles indicated as ${}^{212}\mathrm{Po}$ in panel (a). The transitions connecting the $2_2^{+}$ and the $2_3^{+}$ states to the $2_1^{+}$ state are given in (blue) bold.

Figure 2

An example of line-shape fits of the 785.3-keV $(2_2^{+}\to 2_1^{+})$ (a) and the 952.1-keV $(2_3^{+}\to 2_1^{+})$ (b) transitions obtained with the program apcad. The dashed (blue) lines show the background and the fit regions. The solid (red) line represents the total fit. The 785-keV line is fitted simultaneously with the 780.4-keV line which originates from the decay of the $7^{(+)}$ state of ${}^{212}\mathrm{Po}$ at the excitation energy of 3155 keV $(\tau =0.12(6\left)\mathrm{ps}\right)$ [18]. The dotted lines (green and grass green) represent the individual contributions of 785- and 780-keV lines to the total fit. The vertical dash-dotted lines show the position of the unshifted peaks.

Figure 3

Comparison of experimental low-lying excited states of ${}^{212}\mathrm{Po}$ (a) with the calculated ones (b) (for details see the text). The energies of the levels are given in keV.