Bulletin of the American Physical Society
2020 Fall Meeting of the APS Division of Nuclear Physics
Volume 65, Number 12
Thursday–Sunday, October 29–November 1 2020; Time Zone: Central Time, USA
Session FD: Nuclear Theory II: Structure and Reactions |
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Chair: Amy Lovell, LANL |
Friday, October 30, 2020 2:00PM - 2:12PM |
FD.00001: Benchmarking Methods for Obtaining Scattering Observables from Bound-State Techniques Mamoon Sharaf, James Vary, Andrey Shirokov Motivated by the need for extracting scattering observables from many-body bound-state methods we benchmark several approaches for calculating the scattering phase shifts $\delta_l(E)$ using various model potentials. We also compute resonance energies, widths, and scattering cross sections. In particular, we compare the SS-HORSE and the variable phase methods with the approach suggested by Victor D. Efros in Phys. Rev. C 99, 034620 (2019). We investigate the selection of optimal bases for accurate computation of phase shifts using bound state techniques. We analyze the applicability of those methods to realistic many-body problems. [Preview Abstract] |
Friday, October 30, 2020 2:12PM - 2:24PM |
FD.00002: Short-range correlation physics from operator evolution Anthony Tropiano, Dick Furnstahl, Scott Bogner In analyzing scattering observables, there is scale and scheme dependence in the factorization of structure and reaction components. The similarity renormalization group (SRG) is well-suited to analyze these components by applying SRG transformations to wave functions and the corresponding operators to evolve to low resolution, tuning the scale with the SRG decoupling parameter. Under two different SRG schemes, we evolve a high-resolution nucleon-nucleon interaction to low resolution, decoupling the low- and high-energy physics. We show that short-range correlation (SRC) physics is shifted from the wave functions to consistently evolved operators. We demonstrate that a high-resolution description of SRC physics such as the ratio of proton-neutron over proton-proton pairs in nuclei at various relative momenta can be equivalently described with simple calculations at low resolution. [Preview Abstract] |
Friday, October 30, 2020 2:24PM - 2:36PM |
FD.00003: Cross Sections for Neutron Induced Reactions from Surrogate Measurements: Revisiting the Weisskopf-Ewing Approximation Oliver Gorton, Jutta Escher With increased use of surrogate nuclear reactions as an indirect method to determine compound-nuclear reaction cross sections, it is important to adjudicate the theoretical methods involved. Previous work has shown that a common approximation method has varying accuracy depending on the exit channel of the reaction. The Weisskopf-Ewing approximation, which is based on the assumption that the decay of the compound nucleus does not depend on whether it is produced via a surrogate reaction or via fusion of the projectile with the target of interest, greatly simplifies the extraction of the desired cross section. For surrogate measurements of neutron-capture cross sections, the results can differ significantly from the true cross section, while for fission the results are comparable. In this work, the prospects for determining one- and two-neutron emission cross sections are investigated. A nuclear reaction model is used to simulate quantities which are typically measured in a surrogate experiment. These simulated values are then used to assess the validity of the Weisskopf-Ewing approximation. The expected accuracy of this approximation method, if applied to fast-neutron induced reactions, is discussed and the limitations are illustrated. [Preview Abstract] |
Friday, October 30, 2020 2:36PM - 2:48PM |
FD.00004: Improving Inelastic Scattering Descriptions: Reaction Theory for Deformed Targets with the QRPA Emanuel Chimanski, Walid Younes, Jutta Escher Inelastic scattering is widely used to determine nuclear structure properties, but also provide indirect information on nuclear reaction cross sections. To understand the origin of heavy elements one requires knowledge of neutron capture cross sections for many exotic isotopes. These data are difficult to be obtained experimentally, requiring theoretical supplementation to the existing measurements, or even provide them if necessary. However, current inelastic scattering calculations rely on simplified models that are limited in precision and predictability. Standard approaches assume spherical targets and make statistical assumptions that are often difficult to justify. Many nuclei of interest are deformed and the associated degrees of freedom increase the complexity of the calculations. To improve the predictive power of nuclear reaction calculations, we are combining a state-of-the-art nuclear structure approach with a modern reaction description. Specifically, we are extending the transition density formalism to reactions with deformed targets. The excited states are taken in the deformed QRPA and angular momentum is restored. We will present preliminary results for representative deformed system. Our objective is to obtain transition potentials between different states. [Preview Abstract] |
Friday, October 30, 2020 2:48PM - 3:00PM |
FD.00005: Convergence of ab initio calculated $M1$ observables: The role of symmetry, structure and selection rules Zhou Zhou, Patrick J. Fasano, Mark A. Caprio, Anna E. McCoy, Pieter Maris, James P. Vary In order to test \textit{ab initio} no core shell model predictions against experiment, we must first obtain well converged calculations of observables. In particular, $M1$ observables converge more rapidly than long range (e.g., $E2$) electromagnetic observables. In order to understand how the $M1$ convergence and predicted strengths relate to underlying structure of the nucleus, we study the contributions of the different components of the $M1$ operator. Each of these components is subject to different selection rules on angular momentum (orbital and spin), isospin and $\mathrm{SU}(3)$ quantum numbers. We use the Lanczos decomposition method to determine the dominant $LS$ and $\mathrm{SU}(3)$ contributions to the calculated wave functions and thus understand the relevant selection rules for each transition. We present analysis of calculated $M1$ moments and low-lying transitions in $p$-shell nuclei, obtained with the Daejeon16 interaction. [Preview Abstract] |
Friday, October 30, 2020 3:00PM - 3:12PM |
FD.00006: Using Similarity Renormalization Group Methods to Analyze Optical Potentials Mostofa Hisham, Anthony Tropiano, R.J. Furnstahl Similarity Renormalization Group (SRG) operations evolve Hamiltonians by continuous unitary transformations, driving hard potentials to softer potentials by decoupling high- and low-momentum components. Using a toy model, we examine properties of the optical potential through SRG transformations and we study the effects of commonly used approximation methods on the SRG-evolved potential. Furthermore, we see the prospects for using the SRG to decouple the projectile and target in high energy scattering. [Preview Abstract] |
Friday, October 30, 2020 3:12PM - 3:24PM |
FD.00007: Ab initio Effective Potentials for Nucleon-Nucleus Elastic Scattering on Light Nuclei Matthew Burrows, Robert Baker, Charlotte Elster, Stephen Weppner, Kristina Launey, Pieter Maris, Gabriela Popa Effective interactions (`optical potentials') are needed as input to nuclear reaction calculations. In a multiple scattering expansion for nucleon-nucleus elastic scattering the leading order term requires integrating over nonlocal, translationally invariant one-body densities and off-shell nucleon-nucleon (NN) scattering amplitudes. For consistency the spin of the struck nucleon must be taken into account on the same footing as the spin of the projectile nucleon. In this talk, the first complete nucleon-nucleus {\it ab initio} leading order effective interactions will be used to calculate elastic scattering observables for light nuclei. These potentials are based on NCSM spin-dependent one-body densities together with NN amplitudes derived from the same NN interaction. We will focus on elastic scattering off the Helium isotope chain $^4$He, $^6$He, and $^8$He in the energy regime between 71 and 200 MeV laboratory kinetic energy. [Preview Abstract] |
Friday, October 30, 2020 3:24PM - 3:36PM |
FD.00008: Spin structure of light nuclei related to nucleon-nucleus elastic scattering Robert Baker, Matthew Burrows, Charlotte Elster, Gabriela Popa, Kristina Launey, Pieter Maris, Stephen Weppner We discuss recent work related to the calculation of \textit{ab initio} microscopic effective interactions for elastic nucleon-nucleus scattering. In the framework of the spectator expansion of the multiple scattering series, we can construct a leading-order consistent effective interaction that includes information about the spin of the struck nucleon. Using one-body densities from the no-core shell model, we are able to examine the spin structure of the target $0^+$ nucleus and investigate the effects of the underlying shell structure on the resulting effective interaction as well as physical observables. With a focus on differences along isotopic chains, we present results for light nuclei and discuss the relevant physics. [Preview Abstract] |
Friday, October 30, 2020 3:36PM - 3:48PM |
FD.00009: No-Core Shell Model With Continuum Approach To $\alpha$ Clustering And $\alpha$-induced Reactions. Konstantinos Kravvaris, Sofia Quaglioni, Petr Navratil Providing accurate predictions for reaction cross sections relevant in stellar fusion processes is one of the main goals of ab initio reaction theory. While a lot of successful predictions have already been made in lighter systems, the high many-body complexity encountered in the continuum limit has thus far limited applicability to reactions where the lightest nucleus consists of up to two nucleons. We will outline the basics of the no-core shell model with continuum and present a general method that allows dealing with $\alpha$-induced reactions and the description of $\alpha$ clustering in a computationally efficient manner. [Preview Abstract] |
Friday, October 30, 2020 3:48PM - 4:00PM |
FD.00010: Toward the Next Generation of Optical-Model Potentials Cole Pruitt, Jutta Escher, Mack Atkinson, Wim Dickhoff, Bob Charity, Lee Sobotka Almost 70 years after their debut, phenomenological optical-model potentials (OMPs) remain the standard for theoretical descriptions of low-energy nuclear reactions. A handful of venerable nucleon-nucleus potentials, including Koning-Delaroche and Chapel Hill ‘89, accurately reproduce average scattering observables on stable, near-spherical isotopes up to several hundred MeV. But despite caveats from their creators, these potentials are often pushed beyond their intended limits to make predictions for highly-deformed, highly-asymmetric systems, many of which will be newly accessible in the FRIB era. Are these extrapolations justifiable, or do they yield unreliable predictions? Are there sufficient experimental structure and scattering data to constrain the functional forms of the potential? To address these questions, we have begun to characterize the inherent uncertainty in widely-used OMPs and to study the sensitivity of the potentials’ components to various sectors of experimental data. We compare standard phenomenological potentials with new dispersive and microscopic optical models and formulate recommendations for developing the next generation of OMPs. [Preview Abstract] |
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