Two-phonon Octupole Excitations Research Articles

Search for two-phonon octupole excitations in 146Gd

The low-spin structure of the nearly spherical nucleus 146Gd was studied using the 144Sm(4He, 2n) fusion-evaporation reaction. High-statistics \(\gamma - \gamma\) coincidence measurements were performed at iThemba LABS with 7× 109 \( \gamma\)-\( \gamma\) coincidence events recorded. Gated \( \gamma\)-ray energy spectra show evidence for the \( 6^{+}_{2} \rightarrow 3^{-}_{1} \rightarrow 0^{+}_{1}\) cascade of E3 transitions in agreement with recent findings by Caballero and co-workers, but with a smaller branching ratio of \( I_{\gamma} = 4.7(10)\) for the \(6^{+}_{2} \rightarrow 3^{-}_{1}\) 1905.1 keV \( \gamma\) ray. Although these findings may support octupole vibrations in spherical nuclei, sophisticated beyond mean-field calculations including angular-momentum projection are required to interpret in an appropriate way the available data due to the failure of the rotational model assumptions in this nucleus.

Investigation of 238U in the (n, n′γ) reaction

An irradiation of 238U with a beam of reactor fast neutrons permits revealing about 550 gamma transitions associated with the respective (n, n′γ) reaction and with fission fragments. The use of all known data on gamma radiation from fission fragments makes it possible to identify gamma transitions belonging to 238U with a high probability. The scheme of levels and gamma transitions is composed for 238U. New levels (including those of spin-parity Jπ = 0+) at excitation energies below 2 MeV are proposed. The low-lying levels in the rotational band for two-phonon octupole excitations are determined. It is found that a hybrid state is formed upon the crossing of this band and the band based on two-quasiparticle excitations. This hybrid state must involve excitations of both types. A small value of the rotational-band parameter in the isomer of energy 2559.0(4) keV is explained by the contribution to this state from two-quasiparticle configurations belonging to the 1k17/2 subshell. The same circumstance may also be responsible for an enhanced yield of ternary fission for this isomer.

Two-phonon octupole excitation inGd146

Based on experimental evidence from the $^{144}\mathrm{Sm}$($\ensuremath{\alpha},2n$) reaction, the $3484.7$-keV ${6}^{+}$ state in $^{146}\mathrm{Gd}$ is identified as the highest-spin member of the ${3}^{\ensuremath{-}}\ensuremath{\bigotimes}{3}^{\ensuremath{-}}$ two-phonon octupole quartet. A previously unknown $\ensuremath{\gamma}$ line of $1905.8$ keV and $E3$ character feeding the ${3}^{\ensuremath{-}}$ octupole state has been observed. These results represent the first observation of a ${6}^{+}\ensuremath{\rightarrow}{3}^{\ensuremath{-}}\ensuremath{\rightarrow}{0}^{+}$ cascade of two $E3$ transitions in an even-even nucleus and provide strong support for the interpretation of the ${6}^{+}$ state as a two-phonon octupole excitation.

Conversion-electron study of 0+ excitations in 208Pb

The 208Pb(p,p′e−) reaction has been studied at a proton energy of 17.3 MeV. Singles data show E0 transitions from the 0+2 and 0+3 states at 4868 and 5241 keV, respectively, to the ground state. Proton–electron gated spectra indicate that few electrons from the 0+2 state are in coincidence with backscattered protons, while those from the 0+3 state are somewhat more abundant. From the singles spectra, an upper limit for the E3(0+3 → 3−1)/E0(0+3 → 0+1) branching ratio of less than four has been measured. This observation is in agreement with previous predictions for a two-phonon octupole excitation.

Two-phonon octupole excitations in and the role of E1 transitions in their decays

In recent work, candidates for two-phonon octupole excitations in have been identified based on the observation of a cascade of E3 decays and on the rates of de-exciting E1 transitions. To gain greater understanding of E1 transition rates in this region, , a nucleus with a neutron hole coupled to the octupole vibrational core of , has been examined. The status of the search for the two-phonon octupole quartet in is presented, and outstanding questions are highlighted.

Candidates for two-phonon octupole excitations in208Pb

Following the identification of the lowest-spin member of the two-phonon octupole quartet in ${}^{208}\mathrm{Pb},$ the additional members of the multiplet have been sought with the ${(n,n}^{\ensuremath{'}}\ensuremath{\gamma})$ reaction. Based on energy arguments and $E1$ transition probabilities determined from Doppler-shift lifetime measurements, the 5286- and 5216-keV states are suggested as candidates for the ${2}^{+}$ and ${4}^{+}$ two-phonon octupole excitations in ${}^{208}\mathrm{Pb}.$ The difficulties of identifying the ${6}^{+}$ member of this quartet are examined.

Two-phonon octupole excitation in 208Pb.

Experimental evidence for identification of the 5241-keV ${0}^{+}$ state in ${}^{208}$Pb as the lowest-spin member of the ${3}^{\ensuremath{-}}\ensuremath{\bigotimes}{3}^{\ensuremath{-}}$ quartet is reported. This state, observed previously in neutron transfer reactions and inelastic proton scattering, has been populated with the inelastic neutron scattering reaction. Its decay to the ${3}^{\ensuremath{-}}$ octupole phonon in ${}^{208}$Pb is confirmed by $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ coincidence measurements, and the spin-0 assignment is substantiated by the population cross section. The cascade of two E3 transitions thus established is an expected signature of a two-phonon octupole excitation.

One- and two-phonon octupole states in nuclei near 146Gd

Recent in-beam and other experiments have identified one- and two-phonon octupole excitations in nuclei with up to three valence particles outside 146Gd. It is shown that the observed deviations from harmonic vibration in complex multiparticle × octupole spectra as well as the anharmonicities of the two-phonon octupole states in 147Gd and 148Gd are quantitatively related to the one-particle × phonon anharmonicities observed in 147Tb and 147Gd.

E0 decays of 0+ states in 146Gd: Search for two-phonon octupole excitations.

Conversion-electron measurements following the $^{144}(\mathit{\ensuremath{\alpha}}$,2n) and $^{144}\mathrm{He}$,n) reactions reveal high-energy E0 transitions at 3020, 3485, and 3639 keV. The lowest of these deexcites the known two-neutron pairing vibrational state in $^{146}\mathrm{Gd}$. It is not clear whether either of the others can be associated with the ${0}^{+}$ member of the ${3}^{\mathrm{\ensuremath{-}}}$\ifmmode\times\else\texttimes\fi{}${3}^{\mathrm{\ensuremath{-}}}$ two-phonon octupole multiplet.

Single- and Double-Octupole Excitations inGd148

In the two-valence-neutron nucleus $^{148}\mathrm{Gd}$, in-beam $^{148}\mathrm{Sm}(\ensuremath{\alpha},4n)$ and ($^{3}\mathrm{He}$,$3n$) measurements have identified nine ${\ensuremath{\nu}}^{2}\ifmmode\times\else\texttimes\fi{}\mathrm{octupole}$ levels with $3<I<14$. The measured half-life of the ${(\ensuremath{\nu}{f}_{\frac{7}{2}}^{2}\ifmmode\times\else\texttimes\fi{}{3}^{\ensuremath{-}})}_{{9}^{\ensuremath{-}}}$ state gives $B(E3,{9}^{\ensuremath{-}}\ensuremath{\rightarrow}{6}^{+})=55(6){B}_{\mathrm{W}}$. A high-lying ${12}^{+}$ level, which decays by $E3$ to the ${9}^{\ensuremath{-}}$ state, as well as a ${14}^{+}$ level are shown to be two-phonon octupole excitations of stretched ${\ensuremath{\nu}}^{2}\ifmmode\times\else\texttimes\fi{}{3}^{\ensuremath{-}}\ifmmode\times\else\texttimes\fi{}{3}^{\ensuremath{-}}$ character. All these results can be quantitatively derived from the experimental data for the coupling of one neutron to the core octupole observed in $^{147}\mathrm{Gd}$.

In-beam studies of the shell model nuclei around146Gd

We will discuss multiparticle yrast states around146Gd and analyse their properties in terms of the empirical shell model. Furthermore, we will consider the first known example of a nuclear two-phonon octupole excitation which was recently identified in147Gd. Finally, we will present results on a collective ΔI=2 ground band level sequence in the spherical six-valence-particle nucleus152Dy.

Search for two-octupole-phonon states in 208Pb

A search for two-phonon octupole vibrational states in 208Pb was carried out using the 207Pb(n th, γ) reaction at the High Flux Reactor of the Institut Laue-Langevin. The conversion electron spectrum was scanned with the spectrometer BILL with a sensitivity of 20 μb in the 4–6 MeV interval. Coincidence and singles γ-ray measurements were performed at the end of the thermal neutron beam guide H22-F reaching a sensitivity of 80 μb for energies above 0.511 MeV. In addition the singles spectrum was investigated with the pair spectrometer at the tangential beam facility. Levels have been observed at (energies in keV and parenthesis for tentative level identification and spin-parity assignments): 2614.4, 3 −; 3997, (3 −); 4084.7, 2 +; 4253, (2 −, 3 −); 4704, (2 −, 3 −); (4882.0), (0 +); (4904.9), (0 +); 4935, 2 +; (6343), (2 +) and the 0 −, 1 − capture state at 7367.9 keV. The 4882.0 keV level probably corresponds to the known two-neutron pairing vibration. Possible candidates for two-phonon octupole excitations are the (0 +)(4904.9) and 2 + 4935 keV states. The nuclear field theory formalism is used to calculate branching ratios and energies for states in 208Pb under several assumptions. The most inimportant result of the comparison of theory and experiment is that the experimental properties of the 2 + 4935 keV state favour the interpretation of this state as a two-phonon octupole vibration rather than as a one-phonon non-collective quadrupole state.