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The lifetimes of the $ 2^+_1$ , the $ 2^+_2$ and the $ 3^-_1$ states of Po have been measured in the Pb(C,Be)Po transfer reaction by the Doppler-shift attenuation method. The result for the lifetime of the $ 2^+_1$ state is about three times shorter than the adopted value. However, the new value still does not allow for a consistent description of the properties of the yrast $ 2^+_1$ , $ 4^+_1$ , $ 6^+_1$ , and $ 8^+_1$ states of Po in the framework of nuclear shell models. Quasi-particle Phonon Model (QPM) calculations also cannot overcome this problem thus indicating the existence of a peculiarity which is neglected in both theoretical approaches. [ABSTRACT FROM AUTHOR]

One-phonon states of heavy vibrational nuclei with mixed proton-neutron symmetry have recently been made accessible by the technique of projectile Coulomb excitation. Gamma-rays following Coulomb excitation of the nuclides 136,138Ce and 134Xe have been measured with the Gammasphere-array run in singles-mode. M1, E2, and E3 transition matrix elements from low-spin states were measured relative to the known values. The strength distributions up to about serve for identifying the main fragment of the one-quadrupole phonon mixed-symmetry state. The data are presented and their significance is discussed. [Copyright &y& Elsevier]

The evolution of the one-quadrupole phonon mixed-symmetry state is important for understanding the role of the quadrupole proton-neutron interaction in the valence shell. To study the evolution of the 21,ms+ mixed-symmetry state in the N = 78 isotones above Z = 50, a Coulomb excitation measurement was performed to identify the 21,ms+ state in 136Ce by measuring absolute transition strengths. The 21,ms+ state was found to be predominantly concentrated in the 24+ state of this nucleus. The simple picture of shell stabilization given to account for the fragmentation of the strength observed in 138Ce does not seem to apply to 136Ce. [ABSTRACT FROM AUTHOR]

The 2+2 state in 132Te is identified as the one-phonon mixed-symmetry state in a projectile Coulomb excitation experiment presenting a firm example of a mixed-symmetry state in unstable, neutron-rich nuclei. The results of shell-model calculations based on the low-momentum interaction VlOW_k are in good agreement with experiment demonstrating the ability of the effective shell-model interaction to produce states of mixed-symmetry character. [ABSTRACT FROM AUTHOR]

Abstract: An inverse kinematics Coulomb excitation experiment was performed to obtain absolute E2 and M1 transition strengths in 134Xe. The measured transition strengths indicate that the state of 134Xe is the dominant fragment of the one-phonon mixed-symmetry state. Comparing the energy of the mixed-symmetry state in 134Xe to that of the levels in the isotonic chain indicates that the separation in energy between the fully-symmetric state and the level increases as a function of the number of proton pairs outside the shell closure. This behavior can be understood as resulting from the mixing of the basic components of a two-fluid quantum system. A phenomenological fit based on this concept was performed. It provides the first experimental estimate of the strength of the proton–neutron quadrupole interaction derived from nuclear collective states with symmetric and antisymmetric nature. [Copyright &y& Elsevier]

One-phonon states of vibrational nuclei with mixed proton–neutron symmetry have been observed throughout the nuclear chart besides the mass A ≈ 200 region. Very recently, it has been proposed that the 2 2 + state of 212 Po is of isovector nature. This nucleus has two valence protons and two valence neutrons outside the doubly-magic 208 Pb nucleus. The stable isotope 204 Hg, featuring two valence-proton and valence-neutron holes, with respect to 208 Pb, is the particle-hole mirror of 212 Po. In order to compare the properties of low-lying isovector excitations in these particle-hole mirror nuclei, we have studied 204 Hg by using the projectile Coulomb-excitation technique. The measured absolute B ( M 1 ; 2 2 + → 2 1 + ) strength of 0.20 ( 2 ) μ N 2 indicates that the 2 2 + level of 204 Hg is at least the main fragment of the 2 1 , m s + state. For the first time in this mass region, both lowest-lying, one-quadrupole phonon excitations are established together with the complete set of their decay strengths. This allows for a microscopic description of their structures, achieved in the framework of the Quasi-particle Phonon Model. [ABSTRACT FROM AUTHOR]

The decay properties of the Pygmy Dipole Resonance (PDR) have been investigated in the semi-magic N = 82 nucleus 140 Ce using a novel combination of nuclear resonance fluorescence and γ – γ coincidence techniques. Branching ratios for transitions to low-lying excited states are determined in a direct and model-independent way both for individual excited states and for excitation energy intervals. Comparison of the experimental results to microscopic calculations in the quasi-particle phonon model exhibits an excellent agreement, supporting the observation that the Pygmy Dipole Resonance couples to the ground state as well as to low-lying excited states. A 10 % mixing of the PDR and the [ 2 1 + × PDR ] is extracted. [ABSTRACT FROM AUTHOR]

We present an extensive experimental study of the recently predicted pygmy quadrupole resonance (PQR) in Sn isotopes, where complementary probes were used. In this study, ( α , α ′ γ ) and ( γ , γ ′ ) experiments were performed on 124 Sn. In both reactions, J π = 2 + states below an excitation energy of 5 MeV were populated. The E 2 strength integrated over the full transition densities could be extracted from the ( γ , γ ′ ) experiment, while the ( α , α ′ γ ) experiment at the chosen kinematics strongly favors the excitation of surface modes because of the strong α -particle absorption in the nuclear interior. The excitation of such modes is in accordance with the quadrupole-type oscillation of the neutron skin predicted by a microscopic approach based on self-consistent density functional theory and the quasiparticle-phonon model (QPM). The newly determined γ -decay branching ratios hint at a non-statistical character of the E 2 strength, as it has also been recently pointed out for the case of the pygmy dipole resonance (PDR). This allows us to distinguish between PQR-type and multiphonon excitations and, consequently, supports the recent first experimental indications of a PQR in 124 Sn. [ABSTRACT FROM AUTHOR]

Background: Identification of proton-neutron mixed-symmetric one-quadrupole phonon excitations (the 2+ms states) of atomic nuclei provides information on the isovector part of the residual nucleon-nucleon interaction. It was predicted that the 2+ms state of particular nuclei close to the U(5) limit of the interacting boson model, in particular 140Ba, should be considerably populated by α-transfer reactions [C. E. Alonso et al., Phys. Rev. C 78, 017301 (2008)]. Purpose: We aim at the identification of the 2+ms mixed-symmetry state (MSS) of radioactive 140Ba and investigate its population by the a-transfer reaction as a suitable tool to selectively populate MSSs and as a potential new signature for its mixed-symmetric character. Method: A γ-ray spectroscopy experiment was performed in inverse kinematics in order to populate the 2+ms state of 140Ba by α-transfer from anatC target on 136Xe beam ions. The population of the candidate for the 2+ms state of 140Ba was measured relative to the population of the 2+I state. Results: The candidate for the 2+ms state of 140Ba was populated by a transfer three times weaker than predicted. Another 2+ms state that can be ruled out as the MSS was in turn as strongly populated by the a transfer as predicted for the MSS. Conclusions: The relative population of 2+ states by α-transfer cannot serve as a new signature for MSSs, since other 2+ states are also strongly populated. Nevertheless, the substantial population of the MSS candidate of 140Ba by a transfer qualifies this type of reaction as suitable tool to excite MSSs and study their electromagnetic decay properties. [ABSTRACT FROM AUTHOR]

The low-lying dipole strength of the N = 28 closed-shell nucleus 52Cr was studied with nuclear resonance fluorescence up to 9.9 MeV, using bremsstrahlung at the superconducting Darmstadt linear electron accelerator S-DALINAC. Twenty-eight spin-1 states were observed between 5.0 and 9.5 MeV excitation energy, 14 of which for the first time. Both electric dipole excitations (E1, around 8 MeV) and magnetic dipole excitations (A/1, around 9 MeV) were detected. Microscopic calculations within the quasiparticle-phonon nuclear model were performed and show good agreement with experimental results. The structure of E1 and M1 excitations, respectively, is discussed. [ABSTRACT FROM AUTHOR]

The low-lying electric dipole (E1) strength in the semimagic nucleus 136Xe has been measured, which finalizes the systematic survey to investigate the so-called pygmy dipole resonance (PDR) in all stable even N=82 isotones with the method of nuclear resonance fluorescence using real photons in the entrance channel. In all cases, a fragmented resonance-like structure of E1 strength is observed in the energy region 5-8 MeV. An analysis of the fragmentation of the strength reveals that the degree of fragmentation decreases toward the proton-deficient isotones, while the total integrated strength increases, indicating a dependence of the total strength on the neutron-to-proton ratio. The experimental results are compared to microscopic calculations within the quasiparticle phonon model. The calculation includes complex configurations of up to three phonons and is able to reproduce also the fragmentation of the E1 strength, which allows us to draw conclusions on the damping of the PDR. Calculations and experimental data are in good agreement on the degree of fragmentation and also on the integrated strength if the sensitivity limit of the experiments is taken into account. [ABSTRACT FROM AUTHOR]

High-resolution inelastic electron scattering experiments at the S-DALINAC and proton scattering experiments at iThemba LABS permit a thorough test of the nature of proposed one- and two-phonon symmetric and mixed-symmetric 2+ states of the nucleus 94Mo. The combined analysis reveals the one-phonon content of the mixed-symmetry state and its isovector character suggested by microscopic calculations. The purity of two-phonon 2+ states is extracted. [Copyright &y& Elsevier]

Background: The B(E2) transition strength to the 22+ state in 94Zr was initially reported to be larger by a factor of 1.63 than the one to the 2+ state from lifetime measurements with the Doppler-shift attenuation method using the (n,n'y) reaction [Elhami et al., Phys. Rev. C 75, 011301(R) (2007)]. This surprising behavior was recently revised in a new measurement by the same group using the same experimental technique leading to a ratio below unity as expected in vibrational nuclei. Purpose: The goal is an independent determination of the ratio of B(E2) strengths for the transitions to the 21,2+ states of 94Zr with inelastic electron scattering. Method: The relative population of the 21,2+ states in the (e,e') reaction was measured at the S-DALINAC in a momentum transfer range q = 0.17-0.51 fm-1 and analyzed in plane-wave Born approximation with the method described by Scheikh Obeid et al. [Phys. Rev. C 87, 014337 (2013)]. Results: The extracted B(E2) strength ratio of 0.789(43) between the excitation of the 21+ and 22+ states of 94Zr is consistent with but more precise than the latest (n,n'y) experiment. Using the B(E2) transition strength to the first excited state from the literature a value of 3.9(9) Weisskopf units is deduced for the B(E2; 22+➔01+)transition. Conclusions: The electron scattering result independently confirms the latest interpretation of the different (n,n'y) results for the transition to the 22+ state in 94Zr. [ABSTRACT FROM AUTHOR]