Evolution of the γ\gamma-ray strength function in neodymium isotopes

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.Comment: 14 pages and 10 figure

Effects of cardiac resynchronisation therapy in patients with heart failure having a narrow QRS Complex enrolled in PROSPECT

International audienceINTRODUCTION: Current guidelines recommend cardiac resynchronisation therapy (CRT) in patients with severe symptomatic heart failure, depressed left ventricular (LV) systolic function and a wide QRS complex (>or=120 ms). However, patients with heart failure having a narrow QRS complex might also benefit from CRT. DESIGN SETTING PATIENTS INTERVENTIONS: During the Predictors of Response to Cardiac Resynchronisation Therapy (PROSPECT) trial, 41 patients were enrolled in a 'narrow' QRS sub-study. These patients had a QRS complex <130 ms, but documented evidence of mechanical dyssynchrony by any of seven pre-defined echocardiographic measures. RESULTS: After 6 months of CRT, 26 (63.4%) patients showed improvement according to the Clinical Composite Score, 4 (9.8%) remained unchanged and 11 (26.8%) worsened. In patients with paired data, the 6-min walking distance increased from 334+/-118 m to 382+/-128 m, (p=0.003) and quality-of-life score improved from 44.2+/-19.7 to 26.8+/-20.2 (p<0.0001). Furthermore, there was a significant decrease in LV end-systolic diameter (from 59+/-9 to 55+/-12 mm, p=0.002) and in LV end-diastolic diameter (from 67+/-9 to 63+/-11 mm, p=0.007). CONCLUSION: The results suggest that CRT may have a beneficial effect in heart failure patients with a narrow QRS complex and mechanical dyssynchrony as assessed by echocardiography. The majority of patients improved on clinical symptoms, and there was an evident reduction in LV diameters. Larger studies are needed to clearly define selection criteria for CRT in patients with a narrow QRS complex

Excitation energy dependence of prompt fission γ\gamma-ray emission from 241Pu∗^{241}\mathrm{Pu}{}^{*}

International audiencePrompt fission γ rays (PFGs) resulting from the Pu240(d,pf) reaction have been measured as a function of fissioning nucleus excitation energy Ex at the Oslo Cyclotron Laboratory. We study the average total PFG multiplicity per fission, the average total PFG energy released per fission, and the average PFG energy. No significant changes in these characteristics are observed over the range 5.75<Ex<8.25 MeV. The physical implications of this result are discussed. The experimental results are compared to simulations conducted using the computational fission model FREYA. We find that FREYA reproduces the experimental PFG characteristics within 8% deviation across the Ex range studied. Previous excitation energy-dependent PFG measurements conducted below the second-chance fission threshold have large uncertainties, but are generally in agreement with our results within a 2σ confidence interval. However, both a published parametrization of the PFG energy dependence and the most recent PFG evaluation included in ENDF/B-VIII.0 were found to poorly describe the PFG excitation-energy dependence observed in this and previous experiments

The study of prompt fission <i>γ</i> rays at the Oslo Cyclotron Laboratory

International audienceThe study of prompt fission γ rays (PFGs) is crucial for understanding the energy and angular momentum distribution in fission, and over the last decade there has been an revived interest in this aspect of fission. We present the new experimental setup at the Oslo Cyclotron Laboratory for detecting PFGs resulting from charged particle-induced fission. Additionally, PFGs from the reaction 240 Pu(d,pf) were measured in April 2018, and the fission gated proton-γ coincidence spectrum is shown. In order to explore the dependence of the PFG emission on the excitation energy and angular momentum of the compound nucleus, we plan several experiments where charged particle reactions are used to induce fission in various plutonium isotopes. The final results will be compared to predictions made by the Fission Reaction Event Yield Algorithm (FREYA) in an upcoming publication, to benchmark the current modelling of both the PFGs and the fission process

Comprehensive Test of the Brink-Axel Hypothesis in the Energy Region of the Pygmy Dipole Resonance

The validity of the Brink-Axel hypothesis, which is especially important for numerous astrophysical calculations, is addressed for 116,120,124Sn below the neutron separation energy by means of three independent experimental methods. The gamma-ray strength functions (GSFs) extracted from primary gamma-decay spectra following charged-particle reactions with the Oslo method and with the shape method demonstrate excellent agreement with those deduced from forward-angle inelastic proton scattering at relativistic beam energies. In addition, the GSFs are shown to be independent of excitation energies and spins of the initial and final states. The results provide a critical test of the generalized Brink-Axel hypothesis in heavy nuclei, demonstrating its applicability in the energy region of the pygmy dipole resonance

Nuclear level densities and ?-ray strength functions in 120,124Sn isotopes: Impact of Porter-Thomas fluctuations

Nuclear level densities (NLDs) and gamma -ray strength functions (GSFs) of 120,124Sn have been extracted with the Oslo method from proton-gamma coincidences in the (p, p'gamma ) reaction. The functional forms of the GSFs and NLDs have been further constrained with the Shape method by studying primary gamma -transitions to the ground and first excited states. The NLDs demonstrate good agreement with the NLDs of 116,118,122Sn isotopes measured previously. Moreover, the extracted partial NLD of 1- levels in 124Sn is shown to be in fair agreement with those deduced from spectra of relativistic Coulomb excitation in forward-angle inelastic proton scattering. The experimental NLDs have been applied to estimate the magnitude of the Porter-Thomas (PT) fluctuations. Within the PT fluctuations, we conclude that the GSFs for both isotopes can be considered to be independent of initial and final excitation energies, in accordance with the generalized Brink-Axel hypothesis. Particularly large fluctuations observed in the Shape-method GSFs present a considerable contribution to the uncertainty of the method and may be one of the reasons for deviations from the Oslo-method strength at low gamma -ray energies and low values of the NLD (below eta 1 x 103-2 x 103 MeV-1)

Study of N=50N=50 gap evolution around Z=32Z=32: new structure information for 82{}^{82}Ge

International audienceMedium spin states of light N = 50 isotones have been populated using fast neutron-induced fission of ${}^{232}$Th. Online prompt $\gamma$ spectroscopy has been performed using the hybrid $\gamma$ spectrometer $\nu \text {-}$Ball coupled to the LICORNE directional neutron source at the ALTO facility of IJCLab. Medium spin states of the neutron-rich nucleus ${}^{82}$Ge have been investigated using $\gamma$-$\gamma$ and $\gamma$-$\gamma$-$\gamma$ coincidence data to exploit the resolving power of $\nu \text {-}$Ball. Two new transitions were assigned to this nucleus and a new level was placed in the level scheme. We tentatively assigned to this new state a ($7^{+}$) spin-parity, which is interpreted as a new $N=50$ core breaking state. This provides further insight into the energy evolution of the $N=50$ shell gap toward ${}^{78}$Ni

Prompt and delayed γ\gamma spectroscopy of neutron-rich 94^{94}Kr and observation of a new isomer

Prompt and delayed γ-ray spectroscopy of the neutron-rich 94Kr was performed, as part of the fission campaign at the ALTO facility of the IPN Orsay, using the fast-neutron-induced fission reaction 238U(n,f) in combination with the ν-Ball array, a novel hybrid γ spectrometer for energy and lifetime measurements. Several new yrast and nonyrast transitions were observed for the first time, extending the previously known level scheme. Additionally, we report on the observation of a new short-lived isomer at 3444 keV with a half-life of 32(3) ns. The analysis of the Nilsson orbitals obtained from Gogny cranked Hartree-Fock-Bogoliubov calculations suggests a (9−) spin and an oblate deformation for this isomer corresponding to a two-quasineutron state, indicating an isomeric structure very similar to that of the neighboring isotones 96Sr and 92S

Angular momentum generation in nuclear fission

International audienceWhen a heavy atomic nucleus splits (fssion), the resulting fragments are observed to emerge spinning1 ; this phenomenon has been a mystery in nuclear physics for over 40 years2,3 . The internal generation of typically six or seven units of angular momentum in each fragment is particularly puzzling for systems that start with zero, or almost zero, spin. There are currently no experimental observations that enable decisive discrimination between the many competing theories for the mechanism that generates the angular momentum4–12. Nevertheless, the consensus is that excitation of collective vibrational modes generates the intrinsic spin before the nucleus splits (pre-scission). Here we show that there is no signifcant correlation between the spins of the fragment partners, which leads us to conclude that angular momentum in fssion is actually generated after the nucleus splits (post-scission). We present comprehensive data showing that the average spin is strongly mass-dependent, varying in saw-tooth distributions. We observe no notable dependence of fragment spin on the mass or charge of the partner nucleus, confrming the uncorrelated post-scission nature of the spin mechanism. To explain these observations, we propose that the collective motion of nucleons in the ruptured neck of the fssioning system generates two independent torques, analogous to the snapping of an elastic band. A parameterization based on occupation of angular momentum states according to statistical theory describes the full range of experimental data well. This insight into the role of spin in nuclear fssion is not only important for the fundamental understanding and theoretical description of fssion, but also has consequences for the γ-ray heating problem in nuclear reactors13,14, for the study of the structure of neutron-rich isotopes15,16, and for the synthesis and stability of super-heavy elements17,18