Two-phonon octupole excitation inGd146

Abstract

Based on experimental evidence from the 144 Sm(α,2n) reaction, the 3484.7-keV 6 + state in 146 Gd is identified as the highest-spin member of the 3 − ⊗ 3 − two-phonon octupole quartet. In the harmonic approximation for noninteracting phonons, a degenerate two-phonon multiplet should occur at twice the energy of the one-phonon state. For low-energy quadrupole (λ = 2) shape vibrations, multiphonon states have been characterized in many even-even nuclei; however, these structures frequently exhibit large anharmonicities, even in nearly spherical nuclei [1]. The octupole (λ = 3) mode in doubly magic nuclei likely comes much closer to the ideal harmonic oscillator as a result of the smaller vibrational amplitudes and the larger number of particles participating in the motion. Indeed, a two-phonon 3 − ⊗ 3 − quartet (J π = 0 + , 2 + , 4 + , and 6 + ) should occur at twice the energy of the octupole phonon and is expected to decay with B(E3) values which are twice that of the 3 − → 0 + transition [1,2]. Unfortunately, two-phonon excitations of the octupole type have proven difficult to identify.It is not surprising that 208 Pb, with its 3 − first-excited state at 2614 keV and B(E3; 3 − → 0 + ) of 34 W.u.[3], has been the subject of many searches for the two-phonon octupole quartet at about 5.2 MeV, that is, at 2hω 3 [4]. Firm evidence, an observed cascade of two E3 transitions from a 0 + state at 5241 keV, for the lowest-spin member of the two-phonon quartet in 208 Pb has been obtained [5]. In addition, candidates for the 2 + and 4 + members of the quartet at nearly harmonic energies have been proposed, but these identifications are based primarily on energy arguments and E1 transition rates; the signature cascade of E3 transitions was not observed [6]. Transfer reaction and inelastic light-ion scattering studies [7] support these assignments, but no clear candidate for the 6

Search for two-POV states, excited quintuplets and septuplets in88–92Sr and94,96Zr

“…Even though there is some configuration mixing near the one-and two-POV bands in 96 Zr, those states were interpreted reasonably as the two-POV states based on the calculations of the spdf IBM model [5]. Another two-POV 6 + state was identified in 146 Gd [6]. Therefore, the 6 + state of 88 Sr can be understood as a two-POV 6 + state.…”

“…Two-phonon octupole (λ = 3) vibrational (two-POV) states have been proposed in several spherical nuclei [1][2][3][4][5][6][7][8][9][10][11]. In a pure harmonic vibrational limit, two-phonon octupole vibrational (two-POV) states (0 + , 2 + , 4 + , 6 + ) are degenerate and should have twice the energy of the one-phonon octupole vibrational (one-POV) state (3 − ).…”

Lifetime measurements of low-spin negative-parity levels in Gd160

“…The increasing level density with excitation energy and the proximity of the pairing gap complicate the identification of vibrations in deformed nuclei. Two-phonon octupole excitations have been observed in spherical nuclei of the rare-earth region -144 Sm [12], 146 Sm [13], 146 Gd [14], 147 Gd [15], and 148 Gd [16,17] -as has the combined two-phonon quadrupole-octupole vibrational mode (2 + ⊗ 3 − ) in nearly spherical nuclei -e.g., 144 Sm [12], 146 Sm [18], and 148 Gd [13]. A few cases have been suggested as two-phonon double-γ vibrations (K π = 4 + ) in 164 Dy [19], 166 Er [20], 168 Er [21][22][23], and (K π = 0 + ) in 166 Er [24].…”

Search for possible one- and two-phonon octupole vibrational states in 134Sb, 134, 135Te, 135, 136I, 137Xe and 139Ba near 132Sn

“…Two-phonon octupole (λ = 3) vibrational (two-POV) states in several spherical nuclei have been reported [1][2][3][4][5][6][7][8][9][10][11][12]. Those spherical nuclei include 146−149 Gd [1][2][3][4][5], 144,145 Nd [6,7], 144,146,147 Sm [6][7][8], 208 Pb [9,10], 95−98 Zr [11,12] and 88,90,92,93,94,96 Sr [12,13]. One-and two-POV states have never been proposed for the spherical nuclei near the double magic nucleus of 132 Sn.…”