Coulomb excitation of230Th with32S,84Kr and142Nd projectiles

Abstract

The Coulomb excitation measurements for the 23~ nucleus with 328, 84Kr and 142Nd projectiles are presented. The use of different projectiles allowed us to get information in the ground-state band and side bands. The energy spectrum of the ground-state band and of the lowest negative-parity band have been investigated up to the spin value I=24 + and I=19-(21), respectively. Five side bands (K~=0+,2+,2~, 1-,2-) were observed also. The branching ratios for a large number of transitions in the spin region I< 10 for ~= + l and 1<9 for re=-1 are analysed. The full set of experimental data contains information on the mixing of the adiabatic states and on the nuclear response to the electromagnetic field of ;:-radiation. It is shown that the experimental data may be explained taking into account the coupling of the ground-/3-and two 7-bands and also of the K~= 0-, 1-and 2-negative-parity bands. An enhancement of the transitions from the ~-to the/~-band in respect to the transitions from the 7 to the ground band and from the /~-to the ground band is reported. The mixing of the negative-parity bands is found to be typical for the alignment of the octupole-vibrational angular momentum. The strong spin dependence of the intrinsic matrix elements of the electric-dipole operator follows from the branching ratios of inter-and intra-band transitions from the K s=0 states.

Experimental investigation of vibrational excitations in230Th

“…For convenience we list here the intrinsic transition moments used below [7,17,18] (labelled with the numbers given in Table 3 and work and [7,8] [ 19]). We note, however, that Kulessa et al [5] quote negative values for the 0+~0 + and 21 + --*0 + E2 transition moments.…”

“…possible small M1 contributions for the I~I and I-~I+__ 1 transitions have been ignored. The resulting B (E2) ratios are summarized in Table 4 where we have also included the results for these ratios quoted by Kulessa et al [5]. The agreement between the two sets of data is not satisfying.…”

“…The 2 + and 3 + members of the second K = = 2 + band, with the band head at 1010 keV, are well established from the 23~ decay. Higher members up to the 5 + level were proposed in the Coulomb excitation work of Kulessa et al [5]. We propose additional members of this band up to the 8 + level.…”

“…Kulessa et al [5] report in addition to the ),-band ~ground-state band transitions a strong 2+~0+E2 transition. This transition has an energy of 146.5 keV and with the intensities quoted in [5] the 146.5 keV ),-ray should have an intensity of 15 (7) in the 23~ decay, whereas we obtain an upper limit of 0.4 (a similar discrepancy was found earlier [7] for a proposed 5-~3-transition with an energy of l l5keV).…”

“…The present work was triggered by a recent study of 23~ via Coulomb excitation [5] and our investigation of the neighbouring nucleus 228Th [6]. Kulessa et al report on some unusual decay properties of the ),-vibrational and lowest K ~ = 0-bands: the E2 transitions from the ~-to the B-band are found to be strongly enhanced with respect to those to the ground-state band, with B (E2) ratios of ,,~200, and for the E1 transitions from the K" =0-band to the ground-state band a strong dependence on spin is reported.…”