Abstract
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 summaryProgram Title:OMpy (Midtbø et al., 2020)CPC Library link to program files:https://doi.org/10.17632/jbthtbm9bd.1Code Ocean Capsule:https://doi.org/10.24433/CO.6094094.v1Licensing provisions: GPLv3Programming language: Python, CythonNature 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.
Highlights
Paper IV Restricted spin-range correction in the Oslo method: The example of nuclear level density and γ-ray strength function from 239Pu(d, pγ)240Pu This article expands on Paper III and we develop new solution to correct for the low spin transfer of the sub-Coulomb barrier (d, p) reaction
We explored a limit of the Oslo method and tested an assumption that is essential for the first-generation method and the decomposition into the level density and γ-ray strength function
As the apparent nuclear level density (NLD) and γ -ray strength function (γ SF) extracted with the Oslo method from synthetic and experimental coincidences data suffer the same distortions, we can identify a consistent set of NLD and γ SF from those simulations that lead to an apparent NLD and γ SF that match the results from the experimentally obtained coincidences
Summary
Muss ich mir den subatomaren Raum vorstellen als etwas grosses, ruhiges, dunkles, in das man hinabsteigen kann?Do I have to imagine subatomic space as something huge and silent and dark that you can climb down into?Peter Fischli und David Weiss Findet mich das Glück? (Will happiness find me?)Almost 40 years after the first application of what has become known as the Oslo method [1,2,3,4] we have to ask ourselves: Is there still anything new to discover? The honest answer has to be that we do not know – otherwise it would not be a discovery – but there are still many important questions to pursue.The nucleus is a complex quantum mechanical object, but it has been shown that many aspects are well described by average statistical quantities. The Oslo method is an experimental technique that has been at the forefront of the determination of the level density and γ-ray strength function of the atomic nucleus. Nuclear level densities and γ-ray strength functions are essential inputs to the calculation of neutron capture cross-sections, which are a measure for the probability that a nucleus absorbs a neutron and subsequently emits γ-rays. This region is often referred to as the quasicontinuum and represents an excitation-energy range where In addition to their key role in describing fundamental nuclear properties, both the level density and the γray strength function are vital components for calculating cross sections and reaction rates for explaining the nucleosynthesis of heavy elements in astrophysics [2, 3]. The level density ρ of the backshifted Fermi gas (BSFG) is given by [24]
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