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Background: The chain of Sm isotopes exhibits a wide range of nuclear shapes and collective behavior. While the onset of deformation for N>82 has been well studied both experimentally and theoretically, fundamental data is lacking for some Sm isotopes with N<82. Purpose: Electromagnetic transition rates represent a sensitive test of theoretical nuclear structure models. Lifetime measurements are furthermore complementary to Coulomb excitation experiments, and the two methods together can give access to spectroscopic quadrupole moments. Method: The lifetime of the 21+ state in 140Sm was measured with the recoil-distance Doppler shift technique using the reaction 124Te(20Ne, 4n)140Sm at 82 MeV. Theoretical calculations were performed based on a mapped collective Hamiltonian in five quadrupole coordinates (5DCH) and the Gogny D1S interaction. Results: The lifetime of the 21+ state in 140Sm was found to be 9.1(6) ps, corresponding to a B(E2;21+→0+1) value of 51(4) Weisskopf units. The theoretical calculations are in very good agreement with the experimental result. Conclusions: The B(E2;21+→0+1) value for 140Sm fits smoothly into the systematic trend for the chain of Sm isotopes. The new beyond-mean field calculations are able to correctly describe the onset of collectivity in the Sm isotopes below the N=82 shell closure for the first time. [ABSTRACT FROM AUTHOR]

The γ-ray strength function in the quasicontinuum has been measured for 231-233Th, 232,233Pa, and 237-239U using the Oslo method. All eight nuclei show a pronounced increase in γ strength at ωSR ≈ 2.4 MeV, which is interpreted as the low-energy M1 scissors resonance (SR). The total strength is found to be BSR=9-11μ²N when integrated over the 1-4 MeV γ-energy region. The SR displays a double-hump structure that is theoretically not understood. Our results are compared with data from (γ, γ') experiments and theoretical sum-rule estimates for a nuclear rigid-body moment of inertia. [ABSTRACT FROM AUTHOR]

Background: Neutron-rich nuclei with protons in the fp shell show an onset of collectivity around N = 40. Spectroscopic information is required to understand the underlying mechanism and to determine the relevant terms of the nucleon-nucleon interaction that are responsible for the evolution of the shell structure in this mass region. Methods: We report on the lifetime measurement of the first 2+ and 4+ states in 70,72,74Zn and the first 6+ state in 72Zn using the recoil distance Doppler shift method. The experiment was carried out at the INFN Laboratory of Legnaro with the AGATA demonstrator, first phase of the Advanced Gamma Tracking Array of highly segmented, high-purity germanium detectors coupled to the PRISMA magnetic spectrometer. The excited states of the nuclei of interest were populated in the deep inelastic scattering of a 76Ge beam impinging on a 238U target. Results: The maximum of collectivity along the chain of Zn isotopes is observed for 72Zn at N = 42. An unexpectedly long lifetime of 20+1855 ps was measured for the 4+ state in 74Zn. Conclusions: Our results lead to small values of the B(E2;4+1→ 2+1)/B(E2;2+1→0+1) ratio for 72,74Zn, suggesting a significant noncollective contribution to these excitations. These experimental results are not reproduced by state-of-the-art microscopic models and call for lifetime measurements beyond the first 2+ state in heavy zinc and nickel isotopes. [ABSTRACT FROM AUTHOR]

Excited states in l67Lu were populated in the l23Sb(48Ca,4n) reaction at 203 MeV and decay γ rays measured using the Gammasphere spectrometer array. Two triaxial strongly deformed bands were identified previously and interpreted as zero- and one-phonon wobbling excitations. As a result of more extensive band search, the level scheme has been considerably extended to include ten new rotational bands and some 630 γ-ray transitions. A number of interband linking transitions were revealed, so that all but two bands could be connected with each other. Configurations are proposed for all new bands based on measured observables, with the help of cranked shell model calculations. A γ-ray sequence, previously suggested as a triaxial strongly deformed band based on quasiparticle excitations coexisting with the wobbling excitation in the triaxial potential well, has now been determined to be a signature partner of a coupled band, associated with a normal deformed five-quasiparticle configuration. The possibility of two new bands being associated with triaxial deformation is discussed. [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 previously accepted spin values of the 991 and 1599 keV levels in 140Sm have been revised and established as 2+ and 0(+), respectively. The γ-γ angular correlation method was used to determine the new spin values. The excited low-spin levels in 140Sm were populated in the 140Eu→ 140Sm and 140Gd 140Eu→ 140Sm decays. The 140Gd and 140Eu nuclei were produced in the 112Cd + 32S reaction at a beam energy of 155 MeV. [ABSTRACT FROM AUTHOR]

The average electromagnetic dipole response of levels in the quasicontinuum of 197,198Au has been measured using (³He,³He') and (d,p) reactions. The extracted y-ray strength functions have been normalized according to three model assumptions for the nuclear spin distribution. An enhancement in the energy region Eγ = 3.0-6.5 MeV is observed for both isotopes. The EI component of such excess of strength is studied in detail for 198Au and is interpreted as the pygmy dipole resonance with an energy centroid of 5.9(1) MeV and exhausts about 1% of the total integrated strength. The pygmy dipole resonance is shown to have a significant impact on the calculated 197Au(n,γ)198Au cross section. [ABSTRACT FROM AUTHOR]

The nuclear structure of the odd-mass 69Zn and 71Zn isotopes, with one hole or one particle outside the N=40 subshell closure, has been investigated by measuring the lifetime of excited states using the recoil distance Doppler-shift method. The nuclei of interest were produced in a deep-inelastic reaction in inverse kinematics with a 238U beam impinging on a 70Zn target. Single-particle and collective states were found to coexist at low energy in 69Zn and 71Zn. [ABSTRACT FROM AUTHOR]

The level structure of the neutron-rich 69Cu,71Cu, and 73Cu isotopes has been investigated by means of multinucleon transfer reactions. The experiment was performed at Laboratori Nazionali di Legnaro using the AGATA Demonstrator array coupled to the PRISMA magnetic spectrometer. Lifetimes of excited states in Cu nuclei were measured with the recoil-distance Doppler-shift method. The resulting electromagnetic matrix elements for transitions from excited states in 69,71,73Cu nuclei are used to assess the collective or single-particle character of these states. The results are compared with predictions of large-scale shell-model calculations, giving further insight into the evolution of the proton pf shell as neutrons fill the 1g9/2 orbital. [ABSTRACT FROM AUTHOR]

The nuclear level densities of 194-196Pt and 197,198Au below the neutron separation energy have been measured using transfer and scattering reactions. All the level density distributions follow the constant-temperature description. Each group of isotopes is characterized by the same temperature above the energy threshold corresponding to the breaking of the first Cooper pair. A constant entropy excess ΔS = 1.9kB and 1.1kB is observed in 195Pt and 198Au with respect to 196Pt and 197Au, respectively, giving information on the available single-particle level space for the last unpaired valence neutron. The breaking of nucleon Cooper pairs is revealed by sequential peaks in the microcanonical caloric curve. [ABSTRACT FROM AUTHOR]

The recoil-β tagging technique was used to identify transitions associated with the decay of the 2+ and, tentatively, the 4+ excited states in 74Sr. Combining these results with published data for the A = 74 isobars, triplet energy differences (TEDs) have been extracted, the heaviest case for which these values have been evaluated. State-of-the-art shell-model calculations using the JUN45 interaction and incorporating a J = 0 isospin nonconserving (INC) interaction with an isotensor strength of 100 keV can reproduce the trend in the TED data, with particularly good agreement for the 2+ state. This agreement for the TED data taken together with the fact that agreement has also been shown between shell-model calculations with the same strength of INC interaction in the f7/2 shell and recently for A = 66 strongly suggests that such an interaction exists throughout the nuclear chart and cannot have a strong dependence on details of nuclear structure such as which nuclear orbitals are occupied. It also supports the hypothesis that only a J = 0 component of the INC interaction need be included to explain the observed TEDs. [ABSTRACT FROM AUTHOR]

Background: An unexpected enhancement in the γ-ray strength function, as compared to the low-energy tail of the giant dipole resonance (GDR), has been observed for Sc, Ti, V, Fe, and Mo isotopes for Eγ < 4 MeV. This enhancement was not observed in subsequent analyses on Sn isotopes, but a pygmy dipole resonance (PDR) centered at Eγ ≈ 8 MeV was however detected. The γ-ray strength functions measured for Cd isotopes exhibit both features over the range of isotopes, with the low-energy enhancement decreasing and PDR strength increasing as a function of neutron number. This suggests a transitional region for the onset of low-energy enhancement, and also that the PDR strength depends on the number of neutrons. Purpose: The γ-ray strength functions of 105-108Pd have been measured in order to further explore the proposed transitional region. Method: Experimental data were obtained at the Oslo Cyclotron Laboratory by using the charged particle reactions (³He,³He'γ) and (³He,αγ) on 106,108Pd target foils. Particle-γ coincidence measurements provided information on initial excitation energies and the corresponding γ-ray spectra, which were used to extract the level densities and γ-ray strength functions according to the Oslo method. Results: The γ-ray strength functions indicate a sudden increase in magnitude for Eγ > 4 MeV, which is interpreted as a PDR centered at Eγ ≈ 8 MeV. An enhanced γ-ray strength at low energies is also observed for 105Pd, which is the lightest isotope measured in this work. Conclusions: A PDR is clearly identified in the γ-ray strength functions of 105-108Pd, and a low-energy enhancement is observed for Pd105. Further, the results correspond and agree very well with the observations from the Cd isotopes, and support the suggested transitional region for the onset of low-energy enhancement with decreasing mass number. The neutron number dependency of the PDR strength is also evident. [ABSTRACT FROM AUTHOR]

Particle-γ coincidences from the 89Y(p,p'γ)89Y and 89Y(d,pγ)90Y reactions were utilized to obtain γ-ray spectra as function of excitation energy. The Oslo method was used to extract the level density from the particle-γ coincidence matrices. The impact of the N = 50 shell closure on the level densities is discussed within the framework of a combinatorial quasiparticle model. [ABSTRACT FROM AUTHOR]

The level density and y-ray strength function in the quasicontinuum of 238Np have been measured using the Oslo method. The level-density function follows closely the constant-temperature level-density formula and reaches 43 × 106 levels per MeV at Sn = 5.488 MeV of excitation energy. The γ-ray strength function displays a two-humped resonance at low-energy as also seen in previous investigations of Th, Pa, and U isotopes. The structure is interpreted as the scissors resonance and has an average centroid of ωSR = 2.26(5) MeV and a total strength of BSR = 10.8(12)μ²N, which is in excellent agreement with sum-rule estimates. The scissors resonance is shown to have an impact on the 237Np(n.y)238Np cross section. [ABSTRACT FROM AUTHOR]

Proton double-differential cross sections from 59Co(α, p)62Ni, 57Fe(α, p)60Co, 56Fe(7Li, p)62Ni, and 55Mn(6Li, p)60Co reactions have been measured with 21-MeV α and 15-MeV lithium beams. Cross sections have been compared against calculations with the EMPIRE reaction code. Different input level density models have been tested. It was found that the Gilbert and Cameron [A. Gilbert and A. G W. Cameron, Can. J. Phys. 43, 1446 (1965)] level density model is best to reproduce experimental data. Level densities and spin cutoff parameters for 62Ni and 60Co above the excitation energy range of discrete levels (in continuum) have been obtained with a Monte Carlo technique. Excitation energy dependencies were found to be inconsistent with the Fermi-gas model. [ABSTRACT FROM AUTHOR]

Particle-y coincidences have been measured to obtain y-ray spectra as a function of excitation energy for 231-233Th and 237-239U The level densities, which were extracted using the Oslo method, show a constant temperature behavior. The isotopes display very similar temperatures in the quasicontinuum, however, the even-odd isotopes reveal a constant entropy increase A S compared to their even-even neighbors. The entropy excess depends on available orbitals for the last unpaired valence neutron of the heated nuclear system. Also, experimental microcanonical temperature and heat capacity have been extracted. Several poles in the heat capacity curve support the idea that an almost continuous melting of Cooper pairs is responsible for the constant-temperature behavior. [ABSTRACT FROM AUTHOR]

The radioactive isotope 107In was studied using sub-barrier Coulomb excitation at the REX-ISOLDE facility at CERN. Two y rays were observed during the experiment, corresponding to the low-lying 11/2+ and 3/2- states. The reduced transition probability of the 11/2+ state was determined with the semiclassical Coulomb excitation code GOSIA2. The result is discussed in comparison to large-scale shell-model calculations, previous unified-model calculations, and earlier Coulomb excitation measurements in the odd-mass In isotopes. [ABSTRACT FROM AUTHOR]

The level densities and γ-ray strength functions of 105,106,111,112Cd have been extracted from particle-γ coincidence data using the Oslo method. The level densities are in very good agreement with known levels at low excitation energy. The γ-ray strength functions display no strong enhancement for low γ energies. However, more low-energy strength is apparent for 105,106Cd than for 111,112Cd. For γ energies above ~4 MeV, there is evidence for some extra strength, similar to what has been previously observed for the Sn isotopes. The origin of this extra strength is unclear; it might be due to E1 and M1 transitions originating from neutron skin oscillations or the spin-flip resonance, respectively. [ABSTRACT FROM AUTHOR]

Over a period of several years we have performed three separate experiments at Lawrence Berkeley National Laboratory's 88-Inch Cyclotron in which 235U (thick target) was Coulomb-excited. The program involved standalone experiments with Gammmasphere and with the 8pi Spectrometer using 136Xe beams at 720 MeV, and a CHICO-Gammasphere experiment with a 40Ca beam at 184 MeV. In addition to extending the known negative-parity bands to high spin, we have assigned levels in some seven positive-parity bands which are in some cases (e.g., [631]l/2, [624]7/2, and [622]5/2) strongly populated by E3 excitation. The CHICO data have been analyzed to extract E2 and E3 matrix elements from the observed yields. Additionally, many M1 matrix elements could be extracted from the γ-ray branching ratios. A number of new features have emerged, including the unexpected attenuation of magnetic transitions between states of the same Nilsson multiplet, the breakdown of Coriolis staggering at high spin, and the effect of E3 collectivity on Coriolis interactions. [ABSTRACT FROM AUTHOR]

In this paper we investigate the rotational band built upon a two-quasiparticle 8- isomeric state of 252No up to spin Iπ = 22-. The excited states of the band were populated with the206Pb(48Ca, 2n) fusion-evaporation reaction. An unambiguous assignment of the structure of the 8- isomer as a 7/2+[624]v ⊗ 9/2-[734]v configuration has been made on the basis of purely experimental data. Comparisons with triaxial self-consistent Hartree-Fock-Bogoliubov calculations using the DIS force and breaking time-reversal as well as z-signature symmetries are performed. These predictions are in agreement with present measurements. Mean-field calculations extended to similar states in 250Fm support the interpretation of the same two-neutron quasiparticle structure as the bandhead in both N = 150 isotones. [ABSTRACT FROM AUTHOR]

The neutron-rich nuclei 109Pd and 111Pd were produced as fission fragments following the 30Si + l68Er reaction at 142 MeV. Using the identification based on the coincidences with the complementary fission fragments, the only positive-parity bands observed so far in l09Pd and 111Pd emerged from this work. A band, built on top of the 5/2+ ground state exhibiting Δ = 1 energy-level staggering, was observed in each of these nuclei. Both nuclei of interest, 109Pd and 111Pd, are suggested to lie in the transitional region of Pd isotopes of maximum y softness. The ground states of both nuclei are predicted by total Routhian surface calculations to be extremely y soft with shallow triaxial minima. The first crossing in the new bands is proposed to be attributable to an alignment of h²11/2 neutrons. [ABSTRACT FROM AUTHOR]

A set of B(E2) values for the low-lying excited states in the radioactive isotope 109Sn were deduced from a Coulomb excitation experiment. The 2.87-MeV/w radioactive beam was produced at the REX-ISOLDE facility at CERN and was incident on a secondary 58Ni target. The B(E2) values were determined using the known 2+ ->· 0+ reduced transition probability in58Ni as normalization with the semiclassical Coulomb excitation code GOSIA2. The transition probabilities are compared to shell-model calculations based on a realistic nucleonnucleon interaction and the predictions of a simple core-excitation model. This measurement represents the first determination of multiple B(E2) values in a light Sn nucleus using the Coulomb excitation technique with low-energy radioactive beams. The results provide constraints for the single-neutron states relative to 100Sn and also indicate the importance of both single-neutron and collective excitations in the light Sn isotopes. [ABSTRACT FROM AUTHOR]

Indirect neutron capture (n,γ) cross sections have been extracted for the key thorium cycle nuclei 232Th, 231Pa, and 230Th using the surrogate reaction method. Final nucleus γ-decay probabilities were measured between the neutron binding energy and around 1 MeV above it using the 232Th(d,p)233Th, 232Th(3He,t)232Pa, and 232Th(3He,α)231Threactions in experiments with the CACTUS y-detector array and Silicon Ring charged-particle detectors at the Oslo Cyclotron Laboratory. Because the neutron capture cross section for 232Th is already well known from direct measurements a comparison with these results provides a stringent test of the applicability of the surrogate method in the actinide region for indirect (n,γ) cross-section measurements. In addition, a new technique for correcting measured γ-ray decay probabilities below the neutron emission energy threshold is proposed and used. We find good agreement between indirect and direct (n,γ) cross-section measurements in the range 500 keV-1 MeV, but large discrepancies outside this range. Explanations for the observed differences are proposed. [ABSTRACT FROM AUTHOR]

Primary &ggr;-ray spectra for a wide excitation-energy range have been extracted for 44Ti from particle-y coincidence data of the 46Ti(p,t&ggr;)44Ti reaction. These spectra reveal information on the &ggr;-decay pattern of the nucleus and may be used to extract the level density and radiative strength function applying the Oslo method. Models of the level density and radiative strength function are used as input for cross-section calculations of the 40Ca(&agr;,&ggr;)44Ti reaction. Acceptable models should reproduce data on the 40Ca(&agr;,&ggr;)44Ti reaction cross section as well as the measured primary &ggr;-ray spectra. This is only achieved when a coherent normalization of the slope of the level density and radiative strength function is performed. Thus, the overall shape of the experimental primary &ggr;-ray spectra puts a constraint on the input models for the rate calculations. [ABSTRACT FROM AUTHOR]

High-spin states in 139Nd were investigated using the reaction 96Zr(48Ca,5n) at a beam energy of 195 MeV and γ-ray coincidences were acquired with the Euroball spectrometer. Apart from several dipole bands at medium excitation energy, three quadrupole bands have been observed at high spin. Linking transitions connecting two of the high-spin bands to low-energy states have been observed. Calculations based on the cranked-Nilsson-Strutinsky formalism have been used to assign configurations for the high-spin quadrupole bands. [ABSTRACT FROM AUTHOR]

Lifetimes of the 9/21- and 3/21- states in 63Co and the 9/21- state in 65Co were measured using the recoil distance Doppler shift and the differential decay curve methods. The nuclei were populated by multinucleon transfer reactions in inverse kinematics. γ rays were measured with the EXOGAM Ge array and the recoiling fragments were fully identified using the large-acceptance VAMOS spectrometer. The E2 transition probabilities from the 3/21- and 9/21- states to the 7/2- ground state could be extracted in 63Co as well as an upper limit for the 9/21-→7/21- B(E2) value in 65Co. The experimental results were compared to large-scale shell-model calculations in the pf and pfg9/2 model spaces, allowing us to draw conclusions on the single-particle or collective nature of the various states. [ABSTRACT FROM AUTHOR]

Spectra of outgoing neutrons and protons from the 6Li + 55Mn reaction and protons from the α + 57Fe reaction have been measured with beams of 15-MeV 6Li ions and 30-MeV α particles. These reactions proceed through the same 61Ni nucleus at the same excitation energy, thus allowing the difference in reaction mechanisms to be studied. It is shown that spectra from the first reaction measured at backward angles are due to emission from a traditional compound nucleus reaction, in which the intermediate nucleus has reached statistical equilibrium; the spectra from the second reaction contain a significant fraction of pre-equilibrium emission at all angles. Level density parameters of the residual nucleus 60Co have been obtained from the first reaction. Both emission spectra and angular distributions have been measured for the second reaction. It was found that the pre-equilibrium component exhibits a forward-peaked angular distribution, as expected, but with a steeper slope than predicted and with an unusual slight rise at angles above 120°. The backward-angle rise is explained qualitatively by the dominance of the multistep compound mechanism at backward angles. [ABSTRACT FROM AUTHOR]

Nuclear level density and γ-ray strength functions of 121-122Sn below the neutron separation energy are extracted with the Oslo method using the (³He,³He′γ) and (³He,αγ) reactions. The level densities of 121-122Sn display steplike structures, interpreted as signatures of neutron pair breaking. An enhancement in both strength functions, compared to standard models for radiative strength, is observed in our measurements for Eγ ≳ 5.2 MeV. This enhancement is compatible with pygmy resonances centered at ≈ 8.4(1) and ≈8.6(2) MeV, respectively, and with integrated strengths corresponding to ≈1.8-5+1% of the classical Thomas-Reiche-Kuhn sum rule. Similar resonances were also seen in 116-119Sn. Experimental neutron-capture cross reactions are well reproduced by our pygmy resonance predictions, while standard strength models are less successful. The evolution as a function of neutron number of the pygmy resonance in 116-l22Sn is described as a clear increase of centroid energy from 8.0(1) to 8.6(2) MeV, but with no observable difference in integrated strengths. [ABSTRACT FROM AUTHOR]

In this work, we have reviewed the Oslo method, which enables the simultaneous extraction of the level density and γ-ray transmission coefficient from a set of particle-γ coincidence data. Possible errors and uncertainties have been investigated. Typical data sets from various mass regions as well as simulated data have been tested against the assumptions behind the data analysis. [ABSTRACT FROM AUTHOR]

Particle-γ coincidences from the 46Ti(p,p'γ)46Ti inelastic scattering reaction with 15-MeV protons are utilized to obtain γ-ray spectra as a function of excitation energy. The rich data set allows analyzsis of the coincidence data with various gates on excitation energy. For many independent data sets, this enables a simultaneous extraction of level density and radiative strength function (RSF). The results are consistent with one common level density. The data seem to exhibit a universal RSF as the deduced RSFs from different excitation energies show only small fluctuations provided that only excitation energies above 3 MeV are taken into account. If transitions to well-separated low-energy levels are included, the deduced RSF may change by a factor of 2-3, which might be expected because of the involved Porter-Thomas fluctuations. [ABSTRACT FROM AUTHOR]