Investigation of theKπ=8−isomer in132

Abstract

The decay of the K ϭ8 Ϫ isomer in 132 Ce with an excitation energy of 2340.2 keV has been investigated using the 120 Sn(16 O,4n) 132 Ce reaction. A half-life of 9.4Ϯ0.3 ms was determined. Two new decay paths have been found in the deexcitation of this isomer. The hindrance factors for the E1, M 2, and E3 transitions deexciting the isomer have been determined. The decay properties of the 8 Ϫ isomers in the Nϭ74 isotones are discussed. A band mixing mechanism involving the ground state and s band seems to be responsible for the behavior of the reduced hindrance factors of the E1 transitions deexciting the K ϭ8 Ϫ isomers in these isotones. A K mixing, characteristic of the axially asymmetric nuclei, may account for the reduced hindrance factors of the E3 transitions to the 5 ␥ ϩ states in 130 Ba and 132 Ce.

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“…Comparison of measured values with theoretically predicted conversion coefficients (α K (M 2) = 0.00737, α L+M +... (M 2) = 0.00125, α K (E3) = 0.00441, α L+M +... (E3) = 0.00086) [13] leads to the conclusion that this transition has a 34% E3 + 66%M 2 mixed character with a mixing parameter δ 2 (E3/M 2) = 0.5 (4). The values of conversion coefficients for the second transition of interest (462 keV, 8 − → 5 + ) were determined as α K = 0.022 (6) and α L+M +... = 0.008 (2). Both numbers are in agreement with the theoretical values for E3 transitions (α K (theor.)…”

“…One of possible reasons of the phenomenon is the Coriolis interaction, which is responsible for mixing of states with different K values [1,2,3,4,5]. However, the non-axial deformation may cause the same experimental effects.…”

“…This isomeric state was interpreted as a two quasi-neutron 7/2 + [404] ⊗ 9/2 − [514] configuration [2,3]. The 8 − isomers arising from such configuration are known in all even-even N = 74 isotones with atomic number Z = 54 ÷ 64 [2,3,4] and their half-lives vary by six orders of magnitude, from nanoseconds (Xe) to milliseconds (Ce, Ba) [6]. Decay modes of these isomers change from one nucleus to another, but in several cases ( 130 Ba, 132 Ce, 134 Nd, 136 Sm, 138 Gd) decay branches of E1, E3/M 2 transitions leading directly to the band with K = 0 were observed.…”

Shape-driving effects in the triaxial nucleus,Xe128

“…Figure 11 shows the f ν value measured for the transition from the K π = 8 − state to the 8 + member of the yrast band in comparison with those for other N = 74 isotones. The plot includes the recently measured value for 132 Ce [32]. The data point for 128 Xe indicates a big decrease in the f ν value between 130 Ba and 128 Xe.…”

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“…The violation of the K selection rule for electromagnetic transitions in nuclei is not yet well understood [1,2,3]. The K-isomer of interest, described by I π = K π = 8 − appears in nuclei with the mass numbers around 130.…”