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The experimental gamma-ray strength functions (gamma-SFs) of 142,144-151Nd have been studied for gamma-ray energies up to the neutron separation energy. The results represent a unique set of gamma-SFs for an isotopic chain with increasing nuclear deformation. The data reveal how the low-energy enhancement, the scissors mode and the pygmy dipole resonance evolve with nuclear deformation and mass number. The data indicate that the mechanisms behind the low-energy enhancement and the scissors mode are decoupled from each other., 14 pages and 10 figures

Understanding the evolution of level densities in the crossover from spherical to well-deformed nuclei has been a long-standing problem in nuclear physics. We measure nuclear level densities for a chain of neodymium isotopes Nd-142,Nd-144-151 which exhibit such a crossover. These results represent the most complete data set of nuclear level densities to date for an isotopic chain between neutron shell-closure and towards mid-shell. We observe a strong increase of the level densities along the chain with an overall increase by a factor of approximate to 150at an excitation energy of 6 MeV and saturation around mass 150. Level densities calculated by the shell model Monte Carlo (SMMC) are in excellent agreement with these experimental results. Based on our experimental and theoretical findings, we offer an explanation of the observed mass dependence of the level densities in terms of the intrinsic single-particle level density and the collective enhancement. (C) 2021 The Author(s). Published by Elsevier B.V.

The lifetimes of 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 $\gamma$ rays were detected with the EXOGAM array in correlation with the fission fragments identified in mass and atomic number in the VAMOS++ spectrometer. Our result shows very small B(E2;2$_1^+$ $\rightarrow$ 0$_1^+$) 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 coexistence of two additional deformed shapes in this nucleus along with the spherical ground state.

Nuclear level densities and $��$-ray strength functions have been extracted for $^{59, 60}\rm{Ni}$, using the Oslo method on data sets from the $^{60}$Ni($^{3}$He,$^{3}$He$^{\prime}��$)$^{60}$Ni and $^{60}$Ni($^{3}$He,$����$)$^{59}$Ni reactions. Above the neutron separation energy, S$_n$, we have measured the $��$-ray strength functions for $^{61}$Ni and $^{60}$Ni in photoneutron experiments. The low-energy part of the $^{59,60}$Ni $��$-ray strength functions show an increase for decreasing $��$ energies. The experimental $��$-ray strength functions are compared with $M1$ $��$-ray strength functions calculated within the shell model. The $E1$ $��$-ray strength function of $^{60}$Ni has been calculated using the QTBA framework. The QTBA calculations describe the data above $E_��\approx$ 7 MeV, while the shell-model calculations agree qualitatively with the low energy part of the $��$-ray strength function. Hence, we give a plausible explanation of the observed shape of the $��$-decay strength.

The lifetimes of the first excited 2+, 4+, and 6+ states in Zr98 were measured with the recoil-distance Doppler shift method in an experiment performed at GANIL. Excited states in Zr98 were populated using the fission reaction between a 6.2 MeV/u U238 beam and a Be9 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;21+→01+) value in Zr98, 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. © 2018 American Physical Society.

© 2017 American Physical Society. 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 U(d,pf)233 reaction for excitation energies between 4.8 and 10 MeV, and for the Pu(d,pf)239 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.

Nuclear level densities and $��$-ray strength functions of $^{56,57}$Fe have been extracted from proton-$��$ coincidences. A low-energy enhancement in the $��$-ray strength functions up to a factor of 30 over common theoretical E1 models is confirmed. Angular distributions of the low-energy enhancement in $^{57}$Fe indicate its dipole nature, in agreement with findings for $^{56}$Fe. The high statistics and the excellent energy resolution of the large-volume LaBr$_{3}$(Ce) detectors allowed for a thorough analysis of $��$ strength as function of excitation energy. Taking into account the presence of strong Porter-Thomas fluctuations, there is no indication of any significant excitation-energy dependence in the $��$-ray strength function, in support of the generalized Brink-Axel hypothesis., 24 pages, 17 figures, accepted for publication in J. Phys. G: Nucl. Phys; Special Issue

The nuclear level density and the $��$-ray strength function have been extracted for $^{89}$Y, using the Oslo Method on $^{89}$Y($p,p' ��$)$^{89}$Y coincidence data. The $��$-ray strength function displays a low-energy enhancement consistent with previous observations in this mass region ($^{93-98}$Mo). Shell-model calculations give support 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 $^{88}$Sr($p,��$)$^{89}$Y and $^{88}$Y($n,��$)$^{89}$Y 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., 12 pages, 10 figures, version accepted for publication in Phys. Rev. C

The $��$-ray strength functions and level densities of $^{73,74}$Ge have been extracted up to the neutron separation energy S$_n$ from particle-$��$ coincidence data using the Oslo method. Moreover, the $��$-ray strength function of $^{74}$Ge above S$_n$ has been determined from photo-neutron measurements, hence these two experiments cover the range of E$_��\approx$ 1-13 MeV for $^{74}$Ge. The obtained data show that both $^{73,74}$Ge display an increase in strength at low $��$ energies. The experimental $��$-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 $^{73,74}$Ge are adopted in the calculations of the $^{72,73}$Ge(n,$��$) 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.

Our measurements of $^{195,196}$Pt $\gamma$-strength functions show a double-humped enhancement in the $E_{\gamma}= 4-8$ MeV region. For the first time, the detailed shape of these resonances is revealed for excitation energies in the quasicontinuum. We demonstrate that the corresponding neutron-capture cross sections and astrophysical reaction rates are increased by up to a factor of 2 when these newly observed pygmy resonances are included. These results lend credence to theoretical predictions of enhanced reaction rates due to such pygmy resonances and hence are important for a better understanding of r-process nucleosynthesis., Comment: 5 pages, 4 figures

The $��$-ray strength function of $^{56}$Fe has been measured from proton-$��$ coincidences for excitation energies up to $\approx 11$ MeV. The low-energy enhancement in the $��$-ray strength function, which was first discovered in the ($^3$He,$����$)$^{56}$Fe reaction, is confirmed with the ($p,p^\prime��$)$^{56}$Fe experiment reported here. Angular distributions of the $��$ rays give for the first time evidence that the enhancement is dominated by dipole transitions., 5 pages, 4 figures. Version accepted for publication in Phys. Rev. Letters

The level densities and $��$-ray strength functions of $^{105,106,111,112}$Cd have been extracted from particle-$��$ coincidence data using the Oslo method. The level densities are in very good agreement with known levels at low excitation energy. The $��$-ray strength functions display no strong enhancement for low $��$ energies. However, more low-energy strength is apparent for $^{105,106}$Cd than for $^{111,112}$Cd. For $��$ energies above $\approx$ 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., 10 pages, 11 figures

Lifetimes of first 4(+) and 6(+) states have been measured in neutron-rich isotopes of Zr, Mo, Ru and Pd using the recoil distance Doppler shift method at GANIL. The nuclei were produced through a fusion-fission reaction in inverse kinematics. The fission fragments were fully identified in the large-acceptance VAMOS spectrometer and gamma -rays were detected in coincidence with the EXOGAM germanium array. Lifetimes of excited states in the range of 1-100 ps were measured with the Cologne plunger. Preliminary lifetime results are presented as well as a discussion on the evolution of the collectivity in this region.

Lifetimes of the low-lying states in Zn isotopes around N = 40 have been measured using the Recoil Distance Doppler Shift method. The nuclei of interest were populated in deep-inelastic reactions in inverse kinematics. The ratios B(E2; 4(+) - GT 2(+))/B(E2; 2(+) - GT 0(+)) measured for the Zn isotopes follow a systematics consistent with a seniority classification. DOI:10.5506/APhysPolB.44.375 47th Zakopane Conference on Nuclear Physics - Extremes of the Nuclear Landscape, Aug 27-Sep 02, 2012, Zakopane, Poland