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

Abstract The 9.2 keV nuclear transition in 227Th was studied in the β − -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 δ 2 ( E 2 / M 1 ) = 0.695 ± 0.248 ( | δ ( E 2 / M 1 ) | = 0.834 ± 0.149 ) was determined. Nonzero value of δ ( E 2 / M 1 ) 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.