Intrinsic reflection asymmetry in 225Ra: Additional information from a study of the α-decay scheme of 229Th

Abstract

The level structure of 225Ra has been studied by measuring the radiations associated with the α-decay of 229Th. These measurements include α- and γ-singles spectra, and αγ-, αe −-, and γγ-coincidence measurements. The α-spectra were measured with a high-resolution magnetic spectrometer and Si surface-barrier detectors, the γ-ray spectra with Ge and Ge(Li) semiconductor detectors and the electron spectra with a Si(Li) detector. Chemically purified and isotope-separated sources were used for various experiments. These data provide the energies and intensities for 20 α-branches and for ~87 γ-rays. From this information and previous e − spectrograph measurements, ~81 γ-rays are placed in a level scheme involving 22 excited states below 400 keV. Twenty of these excited states (excluding only tentative levels at 203 and 248 keV) can be assigned as members of four bands having K π = 1 2 + (ground state), 1 2 − (55.1 keV) , 3 2 + (149.9 keV) and 5 2 + (236.7 keV) . It is found necessary to include an additional alternating term ( B 1) in the rotational energy formula to adequately fit the energy spacings within the K = 1 2 bands. This approach yields decoupling parameters of opposite sign for these two bands whose absolute values are closer to each other than previously deduced, which has relevance to their interpretation as being a parity doublet. We propose a band structure for the 3 2 + band at 149.9 keV that is significantly different from earlier proposals. Under this proposal, the energy-level spacings depart substantially from a simple I( I + 1) pattern. No conclusive evidence has been found for the presence of a K π = 5 2 − band which could be the parity-doublet partner of the well established “favored” 5 2 + band at 236.7 keV. According to the predictions of models which include stable octupole-deformed nuclear shapes, the members of such a 5 2 − band will be fed by α-transitions having relatively small hindrance factors; and they should, consequently, have been observed. Although the existence of reflection-asymmetric shapes in 225Ra is an open question at present, the bulk of the available evidence on this nuclide can be accounted for, at least qualitatively, by the hypothesis that it possesses a static octupole-deformed shape. However, the nonobservation of the expected K π = 5 2 − band and the large difference in the α-hindrance factors of the spin- 7 2 members of the K π = 1 2 + and 1 2 − bands represent potential problems for this interpretation which need to be studied further.