Spectroscopy of221Th

Abstract

A positive and a negative parity band in 221Zffl have been investigated via the 2~ 3n) ~ 2 ~Th reaction. Strong El transitions con~ete with stretched E2 transitions deexciting levels of alternating parity. The measured averaged <B(EI)/B(E2)>=2.6 9 10 -6 fm -2 is similar to that observed in neighbouring nuclei. The level scheme 1~y be interpreted assuming an octupole defo~ned core.The light nuclei in the Pa-Th region around A-222 have been the subject of recent examinations from an experimental as well as theoretical .point of view. The most striking features are low-lying negative parity states and strong El transitions connecting states of alternating parity. Experimentally, high-spin states were measured mainly for the even-even nuclei 22~[1,2,3,4], z• [5,6] and for the odd nuclei Zl~Ra [7] and 219Ac [8]. The observed level schemes have been discussed in the framework of IBA-calculations [9] , a molecular cluster model [10] or static octupele deformations [I I, 12,13] .In this note we report on the measurements of high-spin states in 22 ~Th and compare the data with calculations allowing for octupole deformed shapes. No excited levels of 22 iTh have been known until now.In order to produce 22 iTh ' the reaction 20 spb ( ~O, 3n) 221That bombarding energies of 84.5, 86 and 87 MeV was used. The 400 nA pulsed beaJn of tile FAn-tandem accelerator at the MPI f~r Kern_Dhysik in Heidelberg has been employed. The target consisted of an enriched 130 bg/cm 2 2~ sandwiched between carbon foils of "30 bg/cm 2. The produced evaporation residues were deflected in an electrostatic separator [ 14 ] and detected in a 2000 mm 2 Si-surface barrier detector. Without this additional coincidence signal y-spectroscopy would be very dJ ffJcult due to large y-background from fission and other reaction channels, y-rayswere measured with 7 Ge (i) -detectors under 90 ~ and 145 ~ with respect to the beam axis, 5 of them were shielded with NaI-or BGO-anticompton crystals, y-recoil and y-y-recoil coincidences were recorded during a beam time of 57 h beam on the target. Fig. Ib shows a typical y-recoil spectrum, figs. Ic-lf projections out of the y-y-recoil matrix, gated on a specific transition. Fig. la shows the intensity ratios under 145 ~ and 90 ~ wit/l respect to the beam axis together with the expected angular distributions for stretched dipole and quadrupole transitions. From this figure the multipolarity of the radJation was determined. The prominent transitions could be assigned to 22 iTh from excitation functions and the observed G-decay chain in the recoil detector. For the 250.9 keV transition the E2 character was determined from a previous conversion e]ectron m~asur~m~nt [I]. This transition is in cascade with all others and is assigned as the transition to the band hoad. The absence of the dipole transitions in the conversion electron spectrum is a strong indication for the El character of these transitions. For every dipole transition two quadrupole transitions are missing in the coincidence spectra and vice-versa. The decay sc...

Excited states in neutron deficient even-even thorium isotopes (218?A?222)

“…Their results are consistent with our data. Such experiments are in progress [49] and the results will be reported elsewhere. It is therefore described under the assumption of a-clustering as well.…”