The neutron capture cross section of Be-9 for stellar energies was measured via the activation technique using the Karlsruhe Van de Graaff accelerator in combination with accelerator mass spectrometry at the Vienna Environmental Research Accelerator. To characterize the energy region of interest for astrophysical applications, activations were performed in a quasistellar neutron spectrum of kT = 25 keV and for a spectrum at E-n = 473 +/- 53 keV. Despite the very small cross section, the method used provided the required sensitivity for obtaining fairly accurate results of 10.4 +/- 0.6 and 8.4 +/- 1.0 mu b, respectively. With these data it was possible to constrain the cross section shape up to the first resonances at 622 and 812 keV, thus allowing for the determination of Maxwellian-averaged cross sections at thermal energies between kT = 5 and 100 keV. In addition, we report a new experimental cross section value at thermal energy of sigma(th) = 8.31 +/- 0.52 mb.

Accelerator mass spectrometry (AMS) represents a complementary approach for precise measurements of neutron capture cross sections, e.g., for nuclear astrophysics. This technique, completely independent of previous experimental methods, was applied for the measurement of the Fe-54(n,gamma) Fe-55 reaction. Following a series of irradiations with neutrons from cold and thermal to keV energies, the produced long-lived Fe-55 nuclei (t(1/2) = 2.744+-0.009) yr) were analyzed at the Vienna Environmental Research Accelerator. A reproducibility of about 1% could be achieved for the detection of Fe-55, yielding cross-section uncertainties of less than 3%. Thus, this method produces new and precise data that can serve as anchor points for time-of-flight experiments. We report significantly improved neutron capture cross sections at thermal energy (sigma(th) = 2.30 +/- 0.07 b) as well as for a quasi-Maxwellian spectrum of kT = 25 keV (sigma = 30.3 +/- 1.2 mb) and for E-n = 481 +/- 53 keV (sigma = 6.01 +/- 0.23 mb). The new experimental cross sections have been used to deduce improved Maxwellian-averaged cross sections in the temperature regime of the common s-process scenarios. The astrophysical impact is discussed by using stellar models for low-mass asymptotic giant branch stars.

Accelerator mass spectrometry (AMS) represents a complementary approach for precise measurements of neutron capture cross sections, e.g., for nuclear astrophysics. This technique, completely independent of previous experimental methods, was applied for the measurement of the $^{54}\mathrm{Fe}(n,\ensuremath{\gamma})^{55}\mathrm{Fe}$ reaction. Following a series of irradiations with neutrons from cold and thermal to keV energies, the produced long-lived $^{55}\mathrm{Fe}$ nuclei (${t}_{1/2}=2.744+\ensuremath{-}0.009)$ yr) were analyzed at the Vienna Environmental Research Accelerator. A reproducibility of about 1% could be achieved for the detection of $^{55}\mathrm{Fe}$, yielding cross-section uncertainties of less than 3%. Thus, this method produces new and precise data that can serve as anchor points for time-of-flight experiments. We report significantly improved neutron capture cross sections at thermal energy (${\ensuremath{\sigma}}_{\mathrm{th}}=2.30\ifmmode\pm\else\textpm\fi{}0.07$ b) as well as for a quasi-Maxwellian spectrum of $kT=25$ keV ($\ensuremath{\sigma}=30.3\ifmmode\pm\else\textpm\fi{}1.2$ mb) and for ${E}_{n}=481\ifmmode\pm\else\textpm\fi{}53$ keV ($\ensuremath{\sigma}=6.01\ifmmode\pm\else\textpm\fi{}0.23$ mb). The new experimental cross sections have been used to deduce improved Maxwellian-averaged cross sections in the temperature regime of the common $s$-process scenarios. The astrophysical impact is discussed by using stellar models for low-mass asymptotic giant branch stars.

The detection of long-lived radionuclides through ultra-sensitive single atom counting via accelerator mass spectrometry (AMS) offers opportunities for precise measurements of neutron capture cross sections, e.g. for nuclear astrophysics. The technique represents a truly complementary approach, completely independent of previous experimental methods. The potential of this technique is highlighted at the example of the $^{54}$Fe($n, \gamma$)$^{55}$Fe reaction. Following a series of irradiations with neutrons from cold and thermal to keV energies, the produced long-lived $^{55}$Fe nuclei ($t_{1/2}=2.744(9)$ yr) were analyzed at the Vienna Environmental Research Accelerator (VERA). A reproducibility of about 1% could be achieved for the detection of $^{55}$Fe, yielding cross section uncertainties of less than 3%. Thus, the new data can serve as anchor points to time-of-flight experiments. We report significantly improved neutron capture cross sections at thermal energy ($\sigma_{th}=2.30\pm0.07$ b) as well as for a quasi-Maxwellian spectrum of $kT=25$ keV ($\sigma=30.3\pm1.2$ mb) and for $E_n=481\pm53$ keV ($\sigma= 6.01\pm0.23$ mb). The new experimental cross sections have been used to deduce improved Maxwellian average cross sections in the temperature regime of the common $s$-process scenarios. The astrophysical impact is discussed using stellar models for low-mass AGB stars.

We have performed the first direct lifetime measurement of the 2(1)(+) state in Zn-74. The neutron-rich 74Zn beam was produced by in-flight fragmentation of Ge-76 at the Grand Accelerateur National d'Ions Lourds and separated with the LISE spectrometer. The lifetime of the 2(1)(+) state was measured by the recoil-distance Doppler-shift method with the Cologne plunger device combined with the EXOGAM detectors. The lifetime of the 2(1)(+) state in Zn-74 was determined to be 27.0(24) ps, which corresponds to a reduced transition probability B(E2; 2(1)(+) -> 0(+)) = 370(33) e(2)fm(4).

T. Moller,1,* N. Pietralla,1 G. Rainovski,1,2 T. Ahn,1,3,† C. Bauer,1 M. P. Carpenter,4 L. Coquard,1 R. V. F. Janssens,4 J. Leske,1 C. J. Lister,4 E. A. McCutchan,4 O. Moller,1 D. Seweryniak,4 and S. Zhu4 1Institut fur Kernphysik, Technische Universitat Darmstadt, 64289 Darmstadt, Germany 2Faculty of Physics, St. Kliment Ohridski University of Sofia, 1164 Sofia, Bulgaria 3Wright Nuclear Structure Laboratory, Yale University, New Heaven, Connecticut 06520, USA 4Physics Division, Argonne National Laboratory, Argonne, Illinois 60439, USA (Received 29 May 2012; revised manuscript received 2 August 2012; published 17 September 2012)

The one-quadrupole phonon excitation of mixed symmetry, the 2(1,ms)(+) state, is a fundamental building block of nuclear structure. This article gives a summary of our recent experimental research on this excitation mode in the A = 90 and A = 130 mass regions.

We report here the first direct lifetime measurement of the 2(1)(+) state in Zn-72,Zn-74. The neutron-rich beam was produced by in-flight fragmentation of Ge-76 at the Grand Accelerateur National d'Ions Lourds and separated with the LISE spectrometer. The 2(1)(+) state was excited by inelastic scattering and knock-out reaction on a CD2 target and its lifetime was measured by the recoil-distance Doppler-shift method with the Koln plunger device combined with the EXOGAM detectors. The lifetimes of the 2(1)(+) states in Zn-72,Zn-74 were determined to be 17.9(18) and 27.0(24) ps, which correspond to reduced transition probabilities B(E2; 2(1)(+)-> 0(+)) = 385(39) and 370(33) e(2) fm(4), respectively. These values support the idea of a systematic maximum of collectivity at N = 42 for Zn, Ge, and Se nuclei. In addition, the available systematics in the neighboring nuclei point toward a transition from a spherical oscillator at N = 40 to complete gamma-softness at N = 42.

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 2(1,ms)(+) mixed-symmetry state in the N = 78 isotones above Z = 50, a Coulomb excitation measurement was performed to identify the 2(1,ms)(+) state in Ce-136 by measuring absolute transition strengths. The 2(1,ms)(+) state was found to be predominantly concentrated in the 2(4)(+) state of this nucleus. The simple picture of shell stabilization given to account for the fragmentation of the strength observed in Ce-138 does not seem to apply to Ce-136.

The one-quadrupole phonon excitation of mixed symmetry, the 2+ 1,ms state, is a fundamental building block of nuclear structure. This article gives a summary of our recent experimental research on this excitation mode in the A =90 and A =130 mass regions.

The dipole strength distribution of Se-76 has been investigated via photon scattering in the energy region below 9 MeV utilizing bremsstrahlung produced at the S-DALINAC facility at the TU Darmstadt. About 0.20(1)% of the classical E1 sum rule is exhausted by observed J = 1 states of justifiably assumed negative parity. An extrapolation of the GDR below 9 MeV suggests that considerable strength may remain unobserved due to background, finite detector resolution, and fragmentation. The observed strength thus represents a lower limit. Candidates for the 2(1,ms)(+) state and a fragment of the 1(sc)(+) mixed-symmetry states are presented.

Background: Quadrupole moments of excited nuclear states are important observables for geometrically interpreting nuclear structure in terms of deformed shapes, although data are scarce and sometimes ambiguous, in particular, in neutron-rich nuclides.Purpose: A measurement was performed for determining the spectroscopic quadrupole moment of the 2(1)(+) state of Ba-140 in order to clarify the character of quadrupole deformation (prolate or oblate) of the state in its yrast sequence of levels.Method: We have utilized a new combined technique of lifetime measurement at REX-ISOLDE and MINIBALL using the Doppler-shift attenuation method (DSAM) and a reorientation analysis of Coulomb-excitation yields.Results: On the basis of the new lifetime of tau(2(1)(+)) = 10.4(-0.8)(+2.2) ps the electric quadrupole moment was determined to be Q(2(1)(+)) = -0.52(34) eb, indicating a predominant prolate deformation. Conclusions: This finding is in agreement with beyond-mean-field calculations using the Gogny D1S force and with results from the Monte Carlo shell-model approach.

We report here the first direct lifetime measurement of the 2+1 state in 72,74Zn. The neutron-rich beam was produced by in-flight fragmentation of 76Ge at the Grand Acc\'el\'erateur National d'Ions Lourds and separated with the LISE spectrometer. The 2+1 state was excited by inelastic scattering and knock-out reaction on a CD2 target and its lifetime was measured by the recoil-distance Dopplershift method with the K\"oln plunger device combined with the EXOGAM detectors. The lifetimes of the 2+1 states in 72,74Zn were determined to be 17.9(18) and 27.0(24) ps, which correspond to reduced transition probabilities B(E2; 2+1 \rightarrow 0+) = 385(39) and 370(33) e2fm4, respectively. These values support the idea of a systematic maximum of collectivity at N = 42 for Zn, Ge and Se nuclei. In addition, the available systematics in the neighboring nuclei point towards a transition from a spherical oscillator at N = 40 to complete {\gamma}-softness at N = 42.

Low-lying collective states of Sm-154 are studied via the C-12(Sm-154, Sm-154*) Coulomb excitation reaction at 85% of the Coulomb barrier (570 MeV) using the Gammasphere Ge-detector array. Absolute transition strengths are obtained from the Coulomb excitation cross sections deduced from the relative gamma-ray yields. The results include transition strengths for E2 decays of energy levels in the low-spin part of the first excited K-pi = 0(+) rotational band of Sm-154. The agreement of these results with the confined beta-soft rotor model predictions establishes the assignment of this band as a beta band.

Low-lying collective states in Xe-126 have been investigated via the C-12(Xe-126, Xe-126*) projectile Coulomb excitation reaction at 399 MeV. The. decays were detected with the Gammasphere array. Coulomb excitation cross sections relative to the 2(1)(+) state were obtained. Twenty-two absolute E2 transition strengths have been deduced. An sd-interacting boson model (IBM-1) fit agrees well with the new experimental data. This makes a quantitative test of O(6)-symmetry breaking in Xe-126 possible. The measured absolute B(E2) values indicate a preservation of O(5) symmetry, while the O(6) symmetry is broken. The evolution of O(6)-symmetry breaking and of O(5)-symmetry conservation in the Xe-124,Xe-126,Xe-128 isotopic chain is discussed.