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The nuclear structure of 225Th has been studied using the reaction 226Ra(α, 5n) 225Th at a beam energy of 50 MeV. In-beam γγ, e−e−, e−n and e−γ coincidences have been measured, along with γ-ray and electron intensities, and these have been used to establish a decay scheme up to spin 39/2ħ and excitation energy 2494 keV. Parity doublets have been observed, forming two “quasimolecular bands”, along with enhanced E1 transitions linking states of opposite parity. γ-branching ratios have been measured and an average value of |D0/Q0| = (0.51±0.06)×10−3fm−1 has been deduced. The results are compared with calculations incorporating odd multipole degrees of freedom in the description of the nuclear shape.

The rotational bands built on the excited 112− levels in 191Au and 193Au were investigated in (α,4n) reactions with in-beam γ-ray and conversion electron spectroscopy. Half-lives of the rotation-aligned levels in these bands were determined from e−e− time spectra, which were measured with a new double-orange spectrometer. In addition, some information on the magnetic moments of the rotation-aligned levels was obtained from integral perturbed angular distribution measurements. Finally, the previously unknown low-energy 312− → 272− transition in 191Au was identified from conversion electron measurements. From these data it is concluded that the rotation-aligned Hg cores have predominantly νvi1322 structure, contrary to the earlier suggestion of a πh1122 core structure.

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 ΔEπ=104 keV. These levels, as well as a higher-lying Kπ=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.