Nuclear levels of 228Th populated in the decay of 228Pa

Abstract

, — The decay of 228Pa to the levels of 228Th has been extensively studied by various techniques. Single spectra of γ-and K X-rays have been measured with Si(Li) and Ge(Li) detectors. A set of two Ge(Li) detectors has been used to study γ-y and γ-K X-ray coincidences. Measurements of γ-spectra in coincidence with the selected internal-conversion lines have been carried out using a Ge(Li) detector and a six-gap magnetic β-spectrometer. Finally, spectra of internal-conversion electrons have been studied in a β-spectrometer which uses a Si(Li) detector placed together with a system of diaphragms in a homogeneous magnetic field. The decay scheme has been constructed including 39 levels of 228Th. It accounts for 111 of 160 transitions ascribed to the 228Pa activity. The electron-capture decay energy has been determined to be 2 103 /+16 /-12 keV. The strength distribution for the electron-capture feeding of the 228Th levels is analysed in terms of nuclear models. The Coriolis mixing of four octupole bands (K = 0, 1, 2 and 3) is studied in some detail. The coupling matrix elements deduced from the experiment are compared with the results of the microscopic-model calculations