The Oslo method comprises a set of analysis techniques designed to extract nuclear level density and average γ-decay strength function from a set of excitation-energy tagged γ-ray spectra. Here we ...present a new software implementation of the entire Oslo method, called OMpy. We provide a summary of the theoretical basis and derive the essential equations used in the Oslo method. In addition to the functionality of the original analysis code, the new implementation includes novel components such as a rigorous method to propagate uncertainties throughout all steps of the Oslo method using a Monte Carlo approach. The resulting level density and γ-ray strength function have to be normalized to auxiliary data. The normalization is performed simultaneously for both quantities, thus preserving all correlations. The software is verified by the analysis of a synthetic spectrum and compared to the results of the previous implementation, the oslo-method-software.
Program Title:OMpy (Midtbø et al., 2020)
CPC Library link to program files:https://doi.org/10.17632/jbthtbm9bd.1
Code Ocean Capsule:https://doi.org/10.24433/CO.6094094.v1
Licensing provisions: GPLv3
Programming language: Python, Cython
Nature of problem: Extraction of the nuclear level density and average γ-ray strength function from a set of excitation-energy tagged γ-ray spectra including the quantification of uncertainties and correlations of the results.
Solution method: The level density and γ-ray strength function can be obtained simultaneously using a set of analysis techniques called the Oslo method. To propagate the uncertainty from the counting statistics, we analyze an ensemble of perturbed spectra, which are created based on the experimental input. One obtains a set of level densities and γ-ray strength functions for each realization from a fit process. The fitting metric (χ2) is degenerate, but the degeneracy is removed by a simultaneous normalization of the level density and γ-ray strength function to external data, such that all correlations are preserved.
There have been several modifications to facilitate a modular program flow and to enhance accuracy, reproducibility and transparency of the results. The main revisions in OMpy are that it (i) uses an ensemble based uncertainty quantification throughout whole method, (ii) the fitting is based on well-tested external libraries, (iii) corrections for the normalization procedure have been introduced, (iv) the code base is auto-documented with Sphinx and automatically tested.
Nuclear level densities (NLDs) and γ -ray strength functions (GSFs)of 120;124 Sn have been extracted with the Oslo method and additionally constrained with the novel Shape method. The GSFs were found ...to be in excellent agreement with the strengths from the inelastic relativistic proton scattering experiment. This comparison suggests the validity of the Brink-Axel hypothesisused as one of the key assumptions in astrophysical calculations. The extracted NLDs and GSFs were further used as experimental inputs to constrain the Maxwellian-averaged cross sections (MACS) for the radiative neutron capture process 119;123 Sn ( n, γ ) 120;124 Sn using the the nuclear reaction code TALYS.
The nuclear level densities (NLDs) and γ-ray strength functions (GSFs) of 120,124Sn were extracted with the Oslo method, compared and combined with other experimental data to be used as inputs in the ...TALYS reaction code to constrain the Maxwellian-averaged cross sections (MACS) for the 119,123Sn(n, γ)120,124Sn reactions. For 120Sn, the MACS are in relatively good agreement with the JINA REACLIB, BRUSLIB libraries, while no clear agreement was found for 124Sn. In addition, the pygmy dipole resonance (PDR) was found to contribute with up to ≈ 3 − 8% of the total MACS in these nuclei.
The cascading 3.21 and 4.44 MeV electric quadrupole transitions have been observed from the Hoyle state at 7.65 MeV excitation energy in C12, excited by the C12(p,p′) reaction at 10.7 MeV proton ...energy. From the proton-γ−γ triple coincidence data, a value of Γrad/Γ=6.2(6)×10−4 was obtained for the radiative branching ratio. Using our results, together with ΓπE0/Γ from Eriksen et al. Phys. Rev. C 102, 024320 (2020) and the currently adopted Γπ(E0) values, the radiative width of the Hoyle state is determined as Γrad=5.1(6)×10−3 eV. This value is about 34% higher than the currently adopted value and will impact models of stellar evolution and nucleosynthesis.
We have made a thorough study of the low-energy behavior of the γ -ray strength function within the framework of the shell model. We have performed large-scale calculations spanning isotopic and ...isotonic chains over several mass regions, considering 283 nuclei in total, with the purpose of studying the systematic behavior of the low-energy enhancement (LEE) for M1 transitions. There are clear trends in the calculations: From being nearly absent in the lowest mass region, the LEE becomes steeper and more pronounced as the mass number increases, and for a given mass region it further increases toward shell closures. Moreover, the LEE is found to be steeper in regions near doubly magic nuclei where proton particles couple to neutron holes. These trends enable us to consolidate several previous works on the LEE into a single, consistent concept. We compare the inferred trends to the available experimental data from the Oslo method and find support for the systematic behavior. Lastly, we have compared the calculations to strength functions compiled from discrete, experimental lifetimes and find excellent agreement; the discrete data are consistent with an LEE and indicate that the slope varies as function of mass number.
The γ-ray energy response of the Oslo Scintillator Array OSCAR Zeiser, F.; Tveten, G.M.; Bello Garrote, F.L. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
01/2021, Letnik:
985
Journal Article
Recenzirano
Odprti dostop
The new Oslo Scintillator Array (OSCAR) has been commissioned at the Oslo Cyclotron Laboratory (OCL). It consists of 30 large volume (⌀ 3.5 × 8 inches) LaBr3(Ce) detectors that are used for γ-ray ...spectroscopy. The response functions for incident γ rays up to 20 MeV are simulated with Geant4. In addition, the resolution, and the total and full-energy peak efficiencies are extracted. The results are in very good agreement with measurements from calibration sources and experimentally obtained mono-energetic in-beam γ-ray spectra.
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 γ -ray strength functions (GSFs) extracted from primary γ -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.
The β-decay half-lives of 55 neutron-rich nuclei 134−139Sn, 134−142Sb, 137−144Te, 140−146I, 142−148Xe, 145−151Cs, 148−153Ba, and 151−155La were measured at the Radioactive Isotope Beam Factory ...employing the projectile fission fragments of 238U. The nuclear level structure, which relates to deformation, has a large effect on the half-lives. The impact of newly measured half-lives on modeling the astrophysical origin of the heavy elements is studied in the context of r process nucleosynthesis. For a wide variety of astrophysical conditions, including those in which fission recycling occurs, the half-lives have an important local impact on the second (A ≈ 130) peak.
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 142,144−151Nd 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 ≈150 at 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.
The nuclear level densities (NLDs) and γ-ray strength functions (GSFs) of
120,124
Sn were extracted with the Oslo method, compared and combined with other experimental data to be used as inputs in ...the TALYS reaction code to constrain the Maxwellian-averaged cross sections (MACS) for the
119,123
Sn(
n
, γ)
120,124
Sn reactions. For
120
Sn, the MACS are in relatively good agreement with the JINA REACLIB, BRUSLIB libraries, while no clear agreement was found for
124
Sn. In addition, the pygmy dipole resonance (PDR) was found to contribute with up to ≈ 3 − 8% of the total MACS in these nuclei.
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