The first experimental evidence for the (M1 + E2) mixed character of the 9.2 keV transition in 227Th

The 9.2 keV nuclear transition in ²²⁷Th was studied in the ${\beta }^{-}$-decay of ²²⁷Ac by means of the internal conversion electron spectroscopy to clarify the spin-parity assignment of the ground state and the two lowest excited states of ²²⁷Th. The transition multipolarity was proved to be of mixed character M1 + E2 and the spectroscopic admixture parameter ${\delta }^2(E2/M1)=0.695\pm 0.248$ ($\left|\delta \right(E2/M1\left)\right|=0.834\pm 0.149$) was determined. Nonzero value of $\delta (E2/M1)$ questioned the present theoretical interpretation of low-lying levels of ²²⁷Th. Calculations performed prefer the $1/2^{+}$, $3/2^{+}$, and $3/2^{+}$ sequence instead of the adopted $1/2^{+}$, $5/2^{+}$ and $3/2^{+}$ one for the 0.0, 9.2, and 24.3 keV levels, respectively.