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At least 865 records · Page 48

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DOE OSTI2021

National Opacity Program: Analysis of Opacity-relevant X-ray Emission Spectra (Milestone Report ID 7121)

We have produced high energy density iron plasmas at temperatures above 1 keV and electron densities exceeding 1023 cm -3 (~ 1 g/cm 3 ) using the Orion laser at the Atomic Weapons Establishment. These plasmas were created by irradiating 50 µm diameter layered targets with frequency doubled (λ = 527 nm), 1 ps laser pulses focused to a 100 µm diameter producing an irradiance of ~ 2 x 10 18 W/cm 2 . The buried layer targets consist of 160 nm iron sulfide (FeS), 60 nm potassium chloride (KCl), and 15 nm carbon. The combined layers are tamped on both sides with 3 µm of parylene-N. The x-ray emission from the plasma was measured using two time-resolved, and four time-integrated Bragg crystal spectrometers, as well as one time-integrated imaging system. One time-resolved x-ray spectrometer measured emission from L-shell transitions in highly charged iron, the other from K-shell transitions in helium-like S 14+ and hydrogen-like S 15+ . The time-integrated spectrometers are intensity-calibrated and measured emission from K-shell transitions in sulfur, potassium, chlorine, and both K-shell and L-shell transitions in iron. The density and temperature of the plasma were determined by modeling the x-ray spectra using different spectral and hydrodynamic modeling packages. A brief overview of the uncertainties associated with the measurements and models are presented. We also give an overview of our 1-D HYDRA-DCA radiation-hydrodynamics model and improvements for future work. Our results aid in assessing experimental uncertainties associated with plasma uniformity and with x-ray emission employed as diagnostics in opacity experiments at temperatures and densities not achievable elsewhere and represent a significant step in creating and diagnosing plasmas near LTE. These results are summarized as part of the completion requirements for milestone 7121

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2021

5.5 Hybrid Additively Manufactured Tooling for Large Composite Structures

This program sought to lower tooling costs for tightly toleranced, large composite aerostructures by using large format additive manufacturing (LFAM). LFAM tooling has the potential to lower tooling costs, but the large and unpredictable thermal expansion of polymer-based tooling did not previously allow for the production of large structures made with high performance resins requiring 350 deg F cures. Geometric tolerance issues and demolding concerns continued to arise. The proposed approach for this work was to print a thin shell via additive manufacturing that would be used as a scaffold for a conventional tooling prepreg. The prepreg would be laminated and cured onto both sides of the shell, creating a tool with a sandwich construction. The cured prepreg would then be machined and serve as the tooling surface. In addition to providing a clean, pit-free surface, the prepreg stiffness and low thermal expansion would physically restrain the AM shell and prevent it from expanding during thermal cycling, thus preventing any geometric mismatch between the nominal and actual part dimensions. A stair-step approach was taken to prove out this approach. Coupon level testing was done to ensure a good bond was possible between the AM shell and the tooling prepreg. These samples were also used to determine the amount of prepreg needed to achieve the desired coefficient of thermal expansion (CTE). The project team than progressed to a subscale tool measuring two feet in length. The geometry used was a C-section spar from Airbus’ Wing of Tomorrow program. A hybrid tool, using the AM shell and tooling prepreg, was made, and a composite part was fabricated with the tool. Dimensional scans of the tool and part indicated that the desired CTE was achieved with the tool. A second tooling approach was taken to reduce risk. In this approach, a subscale tool of the same geometry as previously mentioned was created, but the tool was entirely constructed of the tooling prepreg. AM was used to print a “master” that was then used to fabricate the tool by creating a splash off of the master using tooling prepreg. The innovation was that the master was not machined, but rather, was used right off the printer after a small amount of surface preparation. Removing this machining step was thought to make this approach economically viable when compared to conventional master fabrication. As before, the tool was used to fabricate a part and tested for dimensional accuracy. For the conclusion of the program, a ten-foot tool was created using the hybrid tool approach. The geometry also came from the Airbus C-section spar previously mentioned. Some difficulty arose in accurately predicting the geometry of the tool after laminating the tooling prepreg, resulting in multiple material additions and a tool geometry change. A part was fabricated in an autoclave using the tool. A cost savings of almost 50% was achieved on the subscale tool in both the hybrid and master-style tool, but the savings dropped considerably as the tool length increased in the full-scale tool. Given the less than expected cost savings and added complexity of the tool construction, it is suggested that the master-style tool is best for immediate commercialization for larger tools (although it may be appropriate for smaller, table-top size tools). Since its construction is very similar to conventionally made composite tooling, there already exists a broad supply chain for building the tool, and project partner Additive Engineering Solutions (AES) is well acquainted with the design and build of the AM master needed.

36 MATERIALS SCIENCE↗

DOE OSTI2020

High-Temperature Particle Heat Exchanger for sCO2 Power Cycles [Award 30342]

This report describes the design, development, and testing of a prototype 100 kWt particle-to-supercritical CO 2 (sCO2) heat exchanger. An analytic hierarchy process was implemented to compare and evaluate alternative heat-exchanger designs (fluidized bed, shell-and-plate moving packed bed, and shell-and-tube moving packed bed) that could meet the high pressure (≥ 20 MPa) and high temperature (≥ 700 °C) operational requirements associated with sCO2 power cycles. Cost, heat-transfer coefficient, structural reliability, manufacturability, parasitics and heat losses, scalability, compatibility, erosion and corrosion, transient operation, and inspection ease were considered in the evaluation. A 100 kWt shell-and-plate design was selected for construction and integration with Sandia’s falling particle receiver system that heats the particles using concentrated sunlight. Sandia worked with industry to design and construct the moving packed-bed shell-and-plate heat exchanger. Tests were performed to evaluate its performance using both electrical heating and concentrated sunlight to heat the particles. Overall heat transfer coefficients at off-design conditions (reduced operating temperatures and only three stainless steel banks in the counter-crossflow heat exchanger) were measured to be approximately ~25 - 70 W/m 2 -K, significantly lower than simulated values of >100 W/m 2 -K. Tests using the falling particle receiver to heat the particles with concentrated sunlight yielded overall heat transfer coefficients of ~35 – 80 W/m 2 -K with four banks (including a nickel-alloy bank above the three stainless steel banks). The overall heat transfer coefficient was observed to decrease with increasing particle inlet temperatures, which contrasted the results of simulations that showed an increase in heat transfer coefficient with temperature due to increased effective particle-bed thermal conductivity from radiation. The likely cause of the discrepancy was particle-flow maldistributions and funnel flow within the heat exchanger caused by internal ledges and cross-bracing, which could have been exacerbated by increased particle-wall friction at higher temperatures. Additional heat loss at higher temperatures may also contribute to a lower overall heat-transfer coefficient. Design challenges including pressure drop, particle and sCO2 flow maldistribution, and reduced heat transfer coefficient are discussed with approaches for mitigation in future designs. Lessons learned regarding instrumentation, performance characterization, and operation of particle components and sCO2 flow loops are also discussed. Finally, a 200 MWt commercial-scale shell-and-plate heat-exchanger design based on the concepts investigated in this report is proposed.

14 SOLAR ENERGY↗

DOE OSTI2022

“Complete Arc development for Double/Multi-Shell target designs" (Level 2 Milestone Report)

The goal of the Double Shell platform is to determine the efficacy of the shell platforms to provide a robust burn platform. One critical aspect of this is to understand the physics associated with the inner shell. In this report, we will discuss the motivation, development and results from the experimental platform to study the physics of the inner shell of a double (or multi-) shell capsule.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2024

Independent Fuel Property Effects of Fuel Volatility on Low Temperature Heat Release and Fuel Autoignition (Final Report)

This Cooperative Research and Development Agreement (CRADA) project between Argonne National Laboratory (ANL), Oak Ridge National Laboratory (ORNL), and Shell Global Solutions (Shell) was initiated as part of a Directed Funding Opportunity (DFO) call for proposals from the Co-Optimization of Fuels and Engines (Co-Optima) initiative. Shell had observed that volatile fuels suppress low temperature heat release (LTHR) more than expected based on conventional gasoline autoignition metrics: research octane number (RON) and motor octane number (MON). The role of LTHR contributes to autoignition phenomena for both boosted spark ignition (BSI) and advanced compression ignition (ACI) combustion modes. ACI combustion modes are applicable to large engines in the hard-to-electrify applications such as off-road, rail, and marine. Thus, having a reliable understanding of autoignition phenomena, including being able to accurately account for the effects of fuel volatility, is particularly important as new synthetic and bio-fuel compositions are considered.This CRADA project aimed to test the hypothesis that the decreased LTHR is due to preferential evaporation of multicomponent fuels when using direct injection (DI) fueling technology, creating composition and reactivity stratification. A custom set of fuels was designed and blended to test this hypothesis by Shell, with experimental engine studies at ORNL and engine combustion modeling by ANL. However, the initial experimental findings did not show the expected effect of fuel volatility suppressing LTHR. Instead, the LTHR propensity observed was independent of the fuel volatility. Due to the unexpected experimental result, the remainder of the experimental effort was redirected to study the effect of fuel volatility on emissions under spark-ignited cold-start conditions. However, as with the LTHR experiments, the cold start effort did not show a meaningful effect of fuel volatility on cold start emissions. Meanwhile, improved engine CFD models have been developed for both LTHR and cold start operations for the Shell fuels with different volatilities. While the simulation efforts were not pursued further due to the insignificant effects of fuel volatility as shown in experiments, the models developed can be easily retooled for off-road, rail, and marine applications.

09 BIOMASS FUELS↗

DOE OSTI2026

Characterization of ceramic fuel powder packing fractions to support INFLUX

Triply periodic minimal surface (TPMS)-based structures show marked potential in novel nuclear reactor fuel designs, as their high surface area-to-volume ratio increases the efficiency of heat transfer out of the fuel, enabling safer, more innovative reactor designs. This milestone report addresses the role of dUO 2 powder processing route on the fill behavior of TPMS-based cladding shells to understand and advance the feasibility of manufacturing TPMS-based nuclear fuel forms. dUO 2 powder was processed through either a dry granulation route, varying consolidation pressure, or through milling, varying milling time, milling method and milled size distribution. The lowest tapped bulk densities (TBD), but best powder flowabilities, were obtained when testing unprocessed dUO 2 powder which was prone to self-agglomeration and formed low-density spheroids. The highest TBD and lowest flowabilities were obtained when using powder produced by hammer-milling dUO 2 powder to pass through a 200-mesh sieve, which led to particles with angular morphologies. Powder produced by dry granulation exhibited TBD that varied according to the consolidation pressure used to form the initial pellets and exhibited improved flowabilities when compared to hammer-milled material. Because of the large span of granule sizes formed as well as the irregular shape associated with the granules, a packing fraction of 0.69 was achieved, exceeding the analytical solution for random close packing of mono-sized spheres. TPMS polymer shells were loaded with unprocessed, granulated, and hammer-milled dUO 2 powders, and their qualitative packing behaviors were analyzed using x-ray computed tomography (xCT). TBDs calculated after loading TPMS polymer shells were 10-20% lower when compared to tapped bulk density measurements taken in a glass graduated cylinder, indicating a non-trivial impact on the tapped bulk density of either the TPMS channel size, TPMS channel surface material, powder cohesiveness, or a combination of the two parameters. A metallic zircaloy-4 TPMS shell will be loaded with hammer-milled dUO 2 powder upon receipt of the shell from Oak Ridge National Laboratory (ORNL) and shipped to Idaho National Labs (INL) for subsequent hot isostatic pressing (HIP) densification experiments.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗

DOE OSTI2022

Atypical Carboxysome Loci: JEEPs or Junk?

Carboxysomes, responsible for a substantial fraction of CO 2 fixation on Earth, are proteinaceous microcompartments found in many autotrophic members of domain Bacteria, primarily from the phyla Proteobacteria and Cyanobacteria. Carboxysomes facilitate CO 2 fixation by the Calvin-Benson-Bassham (CBB) cycle, particularly under conditions where the CO 2 concentration is variable or low, or O 2 is abundant. These microcompartments are composed of an icosahedral shell containing the enzymes ribulose 1,5-carboxylase/oxygenase (RubisCO) and carbonic anhydrase. They function as part of a CO 2 concentrating mechanism, in which cells accumulate HCO 3 - in the cytoplasm via active transport, HCO 3 - enters the carboxysomes through pores in the carboxysomal shell proteins, and carboxysomal carbonic anhydrase facilitates the conversion of HCO 3 - to CO 2 , which RubisCO fixes. Two forms of carboxysomes have been described: α-carboxysomes and β-carboxysomes, which arose independently from ancestral microcompartments. The α-carboxysomes present in Proteobacteria and some Cyanobacteria have shells comprised of four types of proteins [CsoS1 hexamers, CsoS4 pentamers, CsoS2 assembly proteins, and α-carboxysomal carbonic anhydrase (CsoSCA)], and contain form IA RubisCO (CbbL and CbbS). In the majority of cases, these components are encoded in the genome near each other in a gene locus, and transcribed together as an operon. Interestingly, genome sequencing has revealed some α-carboxysome loci that are missing genes encoding one or more of these components. Some loci lack the genes encoding RubisCO, others lack a gene encoding carbonic anhydrase, some loci are missing shell protein genes, and in some organisms, genes homologous to those encoding the carboxysome-associated carbonic anhydrase are the only carboxysome-related genes present in the genome. Given that RubisCO, assembly factors, carbonic anhydrase, and shell proteins are all essential for carboxysome function, these absences are quite intriguing. In this review, we provide an overview of the most recent studies of the structural components of carboxysomes, describe the genomic context and taxonomic distribution of atypical carboxysome loci, and propose functions for these variants. We suggest that these atypical loci are JEEPs, which have modified functions based on the presence of Just Enough Essential Parts.

59 BASIC BIOLOGICAL SCIENCES↗

DOE OSTI2020

X-ray Spectroscopic Studies of a Solid-Density Germanium Plasma Created by a Free Electron Laser

The generation of solid-density plasmas in a controlled manner using an X-ray free electron laser (XFEL) has opened up the possibility of diagnosing the atomic properties of hot, strongly coupled systems in novel ways. Previous work has concentrated on K-shell emission spectroscopy of low Z (<= 14) elements. Here, we extend these studies to the mid-Z(=32) element Germanium, where the XFEL creates copious L-shell holes, and the plasma conditions are interrogated by recording of the associated L-shell X-ray emission spectra. Given the desirability of generating as uniform a plasma as possible, we present here a study of the effects of the FEL photon energy on the temperatures and electron densities created, and their uniformity in the FEL beam propagation direction. We show that good uniformity can be achieved by tuning the photon energy of the XFEL such that it does not overlap significantly with L-shell to M-shell bound-bound transitions, and lies below the L-edges of the ions formed during the heating process. Reasonable agreement between experiment and simulations is found for the emitted X-ray spectra, demonstrating that for these higher Z elements, the selection of appropriate XFEL parameters is important for achieving uniformity in the plasma conditions.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

DOE OSTI2020

Superconducting air core inductor systems and methods

Provided is a low-weight, high-efficiency inductor design for use with or in electrical power equipment, such as inverters. A toroidal power inductor includes a support structure comprising an outer shell, an inner shell, and one or more coolant channels formed therebetween, a plurality of conductors wrapped around and supported by an exterior surface of the outer shell, and an interior cavity substantially enclosed by the inner shell of the toroidal support structure. The plurality of conductors are configured to provide an inductance for the toroidal power inductor, and the one or more coolant channels are distributed beneath the exterior surface of the outer shell to cool the plurality of conductors. An air-core power inductor may implement the conductors using high-temperature superconducting (HTS) tapes cooled by cryogenic fluid flowing within the coolant channels.

42 ENGINEERING↗

DOE OSTI2020

Technical Performance of Refractory Liners for Molten Chloride Salt Thermal Energy Storage Systems

A chloride-based molten-salt system that uses a ternary blend of MgCl2/KCl/NaCl is investigated to provide higher temperature thermal energy storage capability. Despite higher thermal stability, molten chlorides present several unique challenges, including the design of internal refractory-ceramic liners to prevent the corrosion and thermal stress of alloy tank shells. This work discusses issues and potential solutions related to containment of molten chloride salt, specifically the optimization of the refractory material at the molten salt interface (hot face). The down-selected hot face candidate limits permeation of salt through the material and forms a highly stable secondary surface phase in equilibrium with the molten salt. A mortar is created using the corrosion resistant hot face brick. Brick and mortar composite are subjected to mechanical stress/strain analysis, in order to calculate composite material properties and better inform thermomechanical models. The U.S. Department of Energy Generation 3 (DOE Gen3) program seeks to develop higher efficiency CSP plants that can provide cost-competitive, flexible power in the U.S. electric grid. The proposed Gen3 Liquid Pathway CSP plant closely resembles the configuration of current nitrate salt power towers with two-tank storage (Gen2). The differences between Gen2 and Gen3 include the types of compatible materials used in salt storage tank construction. Stainless steel loses strength at Gen3 temperatures, and although nickel superalloys would be capable of withstanding sustained high temperatures, these materials are prohibitively expensive at scale. Uninsulated tank shells also pose a significant risk as common steels are highly susceptible to chemical attack from molten chloride salt. To address these concerns, refractory-ceramic based containment materials are proposed to line the inside of the hot and cold storage tanks. In doing so, stainless or carbon steel shells may be used in construction depending on the level of insulation provided. The composition of the internal liner requires careful consideration to maximize the efficacy of multiple parameters including corrosion resistance, strength at operating temperature, durability, and cost. This is particularly true for the material at the interface with the salt, known as the "hot face", which is responsible for protecting the insulating layers between the tank shell and the hot face brick layer. The molten salt in this system is superheated over 300 °C above its freeze temperature. Therefore, unlike other industrial processes which use refractory-lined vessels, it is not expected that a freeze plane will develop in the hot face. Therefore, the hot face must be designed to withstand chemical corrosion and inhibit permeation of molten salt into the insulating layers. A down selection was performed to identify a hot face candidate best equipped to maintain thermal, mechanical, and chemical integrity when exposed to molten salt over extended periods of time. Long-duration chemical capability experiments were conducted with the down selected hot face refractory fully immersed in molten chloride salt for up to 3000 hours. The average salt penetration does not exceed 100 microns. When extrapolated to 20 and 30 years of continuous exposure, the expected salt penetration depth is approximately 2.0 mm and 2.9 mm, respectively. A magnesium-rich secondary phases develops at the salt/refractory interface. X-ray diffraction identifies the material as forsterite (Mg2SiO4), which is reported to form synthetically in molten chloride salt solutions. These results suggest the selected hot face will adequately inhibit salt permeation. While there is optimism that the hot face brick will inhibit salt permeation, mortar joints are typically the weakest point of a refractory brick liner. From a thermochemical perspective, differing thermal expansion coefficients may result in the mortar and brick to grow independent of each other, creating gaps through which molten salt can penetrate. To address this issue, NREL has developed an in-house mortar composed of the down selected hot face brick that has been shown to be compatible with the salt. Compressive stress/strain analyses have been performed on the brick/ mortar composites to generate stress/strain curves. Modulus of elasticity and Poisson's ratio of the composite may be calculated from the stress/strain curves, in order to provide more representative data to finite element mechanical models for accurate approximation of stress on the tank shell and the amount of thermal expansion expected within the tank liner.

41 EE - Solar Energy Technologies Office (EE-4S)↗

DOE OSTI2020

Method and apparatus for capturing carbon dioxide during combustion of carbon containing fuel

A boiler system having a series of boilers. Each boiler includes a shell having an upstream end, a downstream end, and a hollow interior. The boilers also have an oxidizer inlet entering the hollow interior adjacent the upstream end of the shell and a fuel nozzle positioned adjacent the upstream end of the shell for introducing fuel into the hollow interior of the shell. Each boiler includes a flue duct connected to the shell adjacent the downstream end for transporting flue gas from the hollow interior. Oxygen is delivered to the oxidizer inlet of the first boiler in the series. Flue gas from the immediately preceding boiler in the series is delivered through the oxidizer inlet of each boiler subsequent to the first boiler in the series.

42 ENGINEERING↗

DOE OSTI2020

Nozzle assembly and method for forming nozzle assembly

A nozzle assembly is disclosed, including a CMC nozzle shell, a nozzle spar, and an endwall. The CMC nozzle shell includes a CMC composition and an interior cavity. The nozzle spar is partially disposed within the interior cavity and includes a metallic composition, a cross-sectional conformation, a plurality of spacers protruding from the cross-sectional conformation, the plurality of spacers contacting the CMC nozzle shell, and a spar cap. The endwall includes at least one surface in lateral contact with the spar cap and maintains a lateral orientation of the CMC nozzle shell and the nozzle spar relative to the endwall. The lateral orientation maintains a predetermined throat area of the nozzle assembly. A method for forming the nozzle assembly includes inserting the nozzle spar into the interior cavity, rotating the CMC nozzle shell and the nozzle spar laterally relative to the endwall, and maintaining the lateral orientation.

Hafner, Matthew Troy↗

DOE OSTI2021

Air-buoyant structures and vehicles

Air-buoyant structures, and vehicles incorporating air-buoyant structures, are provided. Hollow, air-buoyant structures may include a shell of ultra-low density aerogel material, foam material, or vapor-expanded material that is strong and stiff enough to withstand atmospheric pressure and lightweight enough to achieve buoyancy in air under evacuation. The shell may be reinforced with a suitable reinforcing material, such as helical nanofibers. The air-buoyant structures may also include vacuum pumps and valves operably connected to or integrated with the hollow shell. The vacuum pumps and valves may be configured to pump air out of the hollow shell and allow air back into the hollow shell to control buoyancy.

Beaux, II, Miles Frank↗

DOE OSTI2021

Differential neutron spectrum generator and related methods and systems

A neutron spectrum generator is disclosed herein including a neutron source, a scatterer positioned in a direct path between the neutron source and a neutron detector, and a material shell configured to have at least one non-uniform characteristic selected from the group consisting of a material, a thickness, a length, an angle, a layer, and combinations thereof to generate a specific spectrum at the neutron detector that is different than the spectrum of the neutron source. A related method includes measuring a first response generated by a first material shell of a neutron spectrum generator interacting with a neutron source, replacing the first material shell with a second material shell, measuring a second response generated by a second material shell of a neutron spectrum generator interacting with the neutron source, and determining a total fission response by determining a difference between the first response and the second response.

Scates, Wade↗

DOE OSTI2021

Iron-based nanoparticles and grains

Example nanoparticles may include an iron-based core, and a shell. The shell may include a non-magnetic, anti-ferromagnetic, or ferrimagnetic material. Example alloy compositions may include an iron-based grain, and a grain boundary. The grain boundary may include a non-magnetic, anti-ferromagnetic, or ferrimagnetic material. Example techniques for forming iron-based core-shell nanoparticles may include depositing a shell on an iron-based core. The depositing may include immersing the iron-based core in a salt composition for a predetermined period of time. The depositing may include milling the iron-based core with a salt composition for a predetermined period of time. Example techniques for treating a composition comprising core-shell nanoparticles may include nitriding the composition.

Wang, Jian-Ping↗

DOE OSTI2023

Internal hydroforming method for manufacturing heat pipe wicks

A forming assembly configured to form a wick is disclosed. The forming assembly includes an expandable tube and a forming shell assembly. The expandable tube is hydraulically expandable to an expanded configuration. A wick mesh is configured to be wrapped about the expandable tube. The forming shell assembly includes a first forming shell comprising a first recess defined therein and a second forming shell comprising a second recess defined therein. The first recess and the second recess cooperate to define an outer diameter of the wick. The expandable tube and the wick mesh are positionable between the first recess and the second recess. The expandable tube and the forming shell assembly are configured to deform the wick mesh and form the wick based on the expandable tube hydraulically expanding towards the expanded configuration.

Lojek, III, John↗

DOE OSTI2023

Iron-based nanoparticles and grains

Wang, Jian-Ping↗

DOE OSTI2024

Magnetic dipole γ-ray strength functions in the crossover from spherical to deformed neodymium isotopes

We calculate the magnetic dipole $\gamma$-ray strength functions in a chain of even-mass neodymium isotopes $^{144-152}$Nd in the framework of the configuration-interaction (CI) shell model. We infer the strength function by applying the maximum entropy method (MEM) to the exact imaginary-time response function calculated with the shell-model Monte Carlo (SMMC) method. The success of the MEM depends on the choice of a good strength function as a prior distribution. We investigate two choices for the prior strength function: the static path approximation (SPA) and the quasiparticle random-phase approximation (QRPA). We find that the QRPA is a better approximation at low temperatures (i.e., near the ground state), while the SPA is a better choice at finite temperatures. We identify a low-energy enhancement (LEE) in the MEM deexcitation $M1$ strength functions of the even-mass neodymium isotopes and compare with recent experimental results for the total deexcitation $\gamma$-ray strength functions. The LEE is already seen in the SPA strength function but not in the QRPA strength function, indicating the importance of large-amplitude static fluctuations around the mean field in reproducing the LEE. Our method is currently the only one which can reproduce LEE in heavy open-shell nuclei where conventional CI shell model calculations are prohibited. With the onset of deformation as number of neutrons increases along the chain of neodymium isotopes, we observe that some of the LEE strength transfers to a low-energy excitation, which we interpret as a finite-temperature ``scissors'' mode. Here, we also observe a finite-temperature spin-flip mode.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗