Level structure of217Rn and221Ra investigated in the alpha-decay225Th →221Ra →217Rn

The nuclei²²¹Ra and²¹⁷Rn have been investigated in the α-decay chain²²⁵Th→²²¹Ra →²¹⁷Rn through γ-ray and conversion-electron studies. The short-lived²²⁵Th nuclei (T _1/2=8min) were produced in the²²⁶Ra(α, 5n) reaction, and γ-rays and conversion electrons were measured — between the irradiation periods — in coincidence with αparticles. In²²¹Ra the five lowest levels are interpreted as members of aK=5/2 paritydoublet with ΔE_π=104 keV. These levels, as well as a higher-lying K^π=3/2⁺ band, are consistent with an octupole deformation of²²¹Ra, as expected from theoretical considerations. In²¹⁷Rn only three excited levels are observed, with a favoured α-decay to a 5/2⁺ excited level thus establishing positive parity for the ground state of²²¹Ra.