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Lifetimes of excited states in 99Y,101Y,101Nb,103Nb, and 105Nb were measured in an experiment using the recoil distance Doppler shift method at GANIL (Grand Accélérateur National d'Ions Lourds). The neutron-rich nuclei were produced in fission reactions between a 238U beam and a 9Be target. Prompt γ rays were measured with the EXOGAM array and correlated with fission fragments that were identified in mass and atomic number with the VAMOS++ spectrometer. The measured lifetimes, together with branching ratios, provide B(M1) and B(E2) values for the strongly coupled rotational bands built on the [422]5/2+ ground state in the Y and Nb nuclei with neutron number N≥60. The comparison of the experimental results with triaxial particle-rotor calculations provides information about the evolution of the nuclear shape in this mass region. [ABSTRACT FROM AUTHOR]

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 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 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]

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]

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]

The γ-ray strength function of 56Fe has been measured from proton-γ coincidences for excitation energies up to ≈ MeV. The low-energy enhancement in the γ-ray strength function, which was first discovered in the (³He, αγ)56Fe reaction, is confirmed with the (p, p′γ)56Fe experiment reported here. Angular distributions of the γ rays give for the first time evidence that the enhancement is dominated by dipole transitions. [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 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]

The orbital Ml scissors resonance has been measured for the first time in the quasicontinuum of actinides. Particle--y coincidences are recorded with deuteron and 3He-induced reactions on 232Th. The residual nuclei 23'.232.233τη an(j 232,233 pa show an unexpectedly strong integrated strength of Bm = 11-15µ2 in the Ey = 1.0-3.5 MeV region. The increased y-decay probability in actinides due to scissors resonance is important for cross-section calculations for future fuel cycles of fast nuclear reactors and may also have an impact on stellar nucleosynthesis. [ABSTRACT FROM AUTHOR]

Nuclear level densities (NLDs) and γ-ray strength functions (γSFs) have been extracted from particle-γ coincidences of the 92Zr(p,p'γ)92Zr and 92Zr(p,dγ)91Zr reactions using the Oslo method. The new 91,92Zr γSF data, combined with photonuclear cross sections, cover the whole energy range from Eγ ≈ 1.5 MeV up to the giant dipole resonance at Eγ ≈ 17 MeV. The wide-range γSF data display structures at Eγ ≈ 9.5 MeV, compatible with a superposition of the spin-flip M1 resonance and a pygmy E1 resonance. Furthermore, the γ SF shows a minimum at Eγ ≈ 2-3 MeV and an increase at lower γ-ray energies. The experimentally constrained NLDs and γSFs are shown to reproduce known (n,γ) and Maxwellian-averaged cross sections for 91,92Zr using the talys reaction code, thus serving as a benchmark for this indirect method of estimating (n,γ) cross sections for Zr isotopes. [ABSTRACT FROM AUTHOR]

Prompt-fission γ rays are responsible for approximately 5% of the total energy released in fission, and therefore important to understand when modeling nuclear reactors. In this work we present prompt γ-ray emission characteristics in fission as a function of the nuclear excitation energy of the fissioning system. Emitted γ-ray spectra were measured, and γ-ray multiplicities and average and total γ energies per fission were determined for the 233U(d,pf) reaction for excitation energies between 4.8 and 10 MeV, and for the 239Pu(d,pf) reaction between 4.5 and 9 MeV. The spectral characteristics show no significant change as a function of excitation energy above the fission barrier, despite the fact that an extra ~5 MeV of energy is potentially available in the excited fragments for γ decay. The measured results are compared with model calculations made for prompt γ-ray emission with the fission model code gef. Further comparison with previously obtained results from thermal neutron induced fission is made to characterize possible differences arising from using the surrogate (d,p) reaction. [ABSTRACT FROM AUTHOR]

The nuclear level densities and γ-ray strength functions of 138,139,140La were measured using the 139La(³He, α), 139La(³He,' ³He), and 139La(d, p) reactions. The particle-γ coincidences were recorded with the silicon particle telescope (SiRi) and NaI(Tl) (CACTUS) arrays. In the context of these experimental results, the low-energy enhancement in the A ~ 140 region is discussed. The 137,138,139La(n, γ) cross sections were calculated at s- and p-process temperatures using the experimentally measured nuclear level densities and γ-ray strength functions. Good agreement is found between 139La (n, γ) calculated cross sections and previous measurements. [ABSTRACT FROM AUTHOR]

One of the greatest questions for modern physics to address is how elements heavier than iron are created in extreme astrophysical environments. A particularly challenging part of that question is the creation of the so-called p-nuclei, which are believed to be mainly produced in some types of supernovae. The lack of needed nuclear data presents an obstacle in nailing down the precise site and astrophysical conditions. In this work, we present for the first time measurements on the nuclear level density and average γ strength function of 92Mo. State-of-the-art p-process calculations systematically underestimate the observed solar abundance of this isotope. Our data provide stringent constraints on the 91Nb(p,γ)92Mo reaction rate, which is the last unmeasured reaction in the nucleosynthesis puzzle of 92Mo. Based on our results, we conclude that the 92Mo abundance anomaly is not due to the nuclear physics input to astrophysical model calculations. [ABSTRACT FROM AUTHOR]

We investigated the 238U(d,p) reaction as a surrogate for the n+238U reaction. For this purpose we measured for the first time the γ-decay and fission probabilities of 239U* simultaneously and compared them to the corresponding neutron-induced data. We present the details of the procedure to infer the decay probabilities, as well as a thorough uncertainty analysis, including parameter correlations. Calculations based on the continuum-discretized coupled-channels method and the distorted-wave Born approximation (DWBA) were used to correct our data from detected protons originating from elastic and inelastic deuteron breakup. In the region where fission and γ emission compete, the corrected fission probability is in agreement with neutron-induced data, whereas the γ-decay probability is much higher than the neutron-induced data. We have performed calculations of the decay probabilities with the statistical model and of the average angular momentum populated in the 238U(d,p) reaction with the DWBA to interpret these results. [ABSTRACT FROM AUTHOR]

The γ-ray strength functions and level densities of 73,74Ge have been extracted up to the neutron-separation energy Sn from particle-γ coincidence data using the Oslo method. Moreover, the γ-ray strength function of 74Ge above Sn has been determined from photoneutron measurements; hence these two experiments cover the range of Eγ≈1-13 MeV for 74Ge. The obtained data show that both 73,74Ge display an increase in strength at low γ energies. The experimental γ-ray strength functions are compared with M1 strength functions deduced from average B(M1) values calculated within the shell model for a large number of transitions. The observed low-energy enhancements in 73,74Ge are adopted in the calculations of the 72,73Ge(n,γ) cross sections, where there are no direct experimental data. Calculated reaction rates for more neutron-rich germanium isotopes are shown to be strongly dependent on the presence of the low-energy enhancement. [ABSTRACT FROM AUTHOR]

The nuclear level density and the γ-ray strength function have been extracted for 89Y by using the Oslo method on 89Y(p,p'γ)89Y coincidence data. The γ-ray strength function displays a low-energy enhancement consistent with previous observations in this mass region (93-98Mo). Shell-model calculations support the conclusion that the observed enhancement is due to strong, low-energy M1 transitions at high excitation energies. The data were further used as input for calculations of the 88Sr(p,γ)89Y and 88Y(n,γ)89Y cross sections with the talys reaction code. Comparison with cross-section data, where available, as well as with values from the BRUSLIB library, shows a satisfying agreement. [ABSTRACT FROM AUTHOR]

The lifetimes of the first excited 2^{+}, 4^{+}, and 6^{+} states in ^{98}Zr were measured with the recoil-distance Doppler shift method in an experiment performed at GANIL. Excited states in ^{98}Zr were populated using the fission reaction between a 6.2  MeV/u ^{238}U beam and a ^{9}Be target. The γ rays were detected with the EXOGAM array in correlation with the fission fragments identified by mass and atomic number in the VAMOS++ spectrometer. Our result shows a very small B(E2;2&#95;{1}^{+}→0&#95;{1}^{+}) value in ^{98}Zr, thereby confirming the very sudden onset of collectivity at N=60. The experimental results are compared to large-scale Monte Carlo shell model and beyond-mean-field calculations. The present results indicate the coexistence of two additional deformed shapes in this nucleus along with the spherical ground state.

The γ-ray strength function of 56Fe has been measured from proton-γ coincidences for excitation energies up to ≈11  MeV. The low-energy enhancement in the γ-ray strength function, which was first discovered in the (3He,αγ)56Fe reaction, is confirmed with the (p,p'γ)56Fe experiment reported here. Angular distributions of the γ rays give for the first time evidence that the enhancement is dominated by dipole transitions.