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18 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Nuclear Data Sheets for A=85

Evaluated experimental data are presented for 13 known nuclides of mass 85 (Zn, Ga, Ge, As, Se, Br, Kr, Rb, Sr, Y, Zr, Nb, Mo). Since the previous 1990 evaluation of A=85, {sup 85}Zn, {sup 85}Ga, {sup 85}Ge and {sup 85}nuclides are newly added here. Excited state data for {sup 85}Se, {sup 85}Zr have become available from radioactive decay and in–beam γ–ray studies. New and improved high–spin data are available for {sup 85}Br, {sup 85}Kr, {sup 85}Rb, {sup 85}Y, {sup 85}Nb and {sup 85}Mo. New direct and precise measurement of atomic masses of {sup 85}Ge, {sup 85}As, {sup 85}Se, {sup 85}Br, {sup 85}Rb, {sup 85}Zr, {sup 85}Nb and {sup 85}Mo have greatly improved the landscape of β decay–Q values and separation energies in this mass region. In spite of extensive experimental work on the isobaric nuclei of this mass chain several deficiencies remain. No excited states are known in {sup 85}Zn, {sup 85}Ga, {sup 85}As. Only a few excited state are assigned in {sup 85}Ge from {sup 85}Ga β– decay. From radioactivity studies, the decay schemes of {sup 85}Zn and {sup 85}Mo are not known, and those for {sup 85}Ga, {sup 85}Ge, {sup 85}As and 10.9–s isomer of {sup 85}Zr are incomplete. Level lifetimes are not known for excited states in {sup 85}Se, {sup 85}Br, {sup 85}Nb and {sup 85}Mo. The {sup 85}Tc nuclide has not been detected in fragmentation experiments at GANIL, alluding to its unbound nature for proton emission. The {sup 85}Kr, {sup 85}Rb, {sup 85}Sr, and {sup 85}Y nuclides remain the most extensively studied from many different reactions and decays. The evaluation of A=85 nuclides has been done after a span of 23 years, thus includes an extensive amount of new data for almost each nuclide. This work supersedes the data for A=85 nuclides presented in earlier full NDS publication by J. Tepel in 1980Te04 and a later one published in an update mode by H. Sievers in 1991Si01.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2026

FIB-ToF-SIMS characterization of irradiated U-10Zr

Post-irradiation examination (PIE) is critical for the performance assessment and qualification of nuclear fuels. Secondary ion mass spectrometry (SIMS) is a powerful materials characterization technique that allows for elemental and isotopic mapping with a depth resolution greater than EDS and EPMA. However, it has not yet been applied to PIE of metallic nuclear fuel. Here, in this work, we characterize an fast neutron spectrum irradiated U-10Zr fuel sample using a time-of-flight SIMS (ToF-SIMS) system connected to a FIB/SEM system, which allows for flexible sample analysis compared to a dedicated ToF-SIMS instrument. Analysis of the resulting hyperspectral micrograph data was aided by the development of an unsupervised machine learning (ML) algorithm that iterates on existing methods to segment the 3D micrographic datasets based on the similarity of mass spectra. The results showed that the FIB-ToF-SIMS instrument was potentially capable of spatially resolving closed fission gas bubbles in 3D by continued ion sputtering of the analyzed volume. Additionally, the ML algorithm proved useful in revealing the chemical segregation of light fission products (those with an atomic mass between approximately 85–105 amu, such as ruthenium and rhodium) plus matrix zirconium, heavy fission products (those with an atomic mass between approximately 135–150 amu, such as the lanthanides) and uranium. Future studies are planned to conduct FIB-ToF-SIMS analysis on more irradiated U-Zr samples to study the constituent redistribution.

11 - NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2022

Capability to Process and Characterize Uranium-Zirconium (U-Zr) and Uranium-Zirconium-Plutonium (U-Zr-Pu) Alloys

This work investigated co-reduction of anhydrous compounds of uranium, zirconium, and plutonium to produce uranium rich ternary nuclear fuel alloys. Metallothermic co-reduction is a novel method to produce all-metal nuclear fuels. Metal fuels offer thermal, fissility, compatibility, and security benefits over oxide fuels. Alloys of uranium–10% zirconium with plutonium contents of 0%, 2.5%, 5%, and 10% were produced, with yields of 60–85% of theoretical values in a traditional calciothermic bomb reduction apparatus. Microstructural analysis indicated transformation of uranium phase from gamma to beta and then alpha, in alternating lamellar plates typical of alpha phase uranium and delta phase uranium-zirconium, with zirconium and carbides at prior grain boundaries. Some of the analyses were inconclusive in their results and therefore require additional testing. Successful ternary co-reduction would simplify production of homogeneous feedstock and thereby streamline manufacture of homogeneous ternary metallic fuel.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2026

Electrode chemistry impact on retention performance of ferroelectric hafnium zirconium oxide (Hf 0.5 Zr 0.5 O 2−x ) capacitors

Polarization retention of 10 nm thick ferroelectric hafnium zirconium oxide (Hf 0.5 Zr 0.5 O 2−x , HZO) capacitors with W and TaN electrodes is investigated over temperatures ranging from 85 to 150 °C. Same state and opposite state polarization margins for devices with W electrodes show minimal retention loss after 10 5 at 150 °C. The devices capped with TaN electrodes show excellent same state retention, but the opposite state polarization margin in the TaN-electrode devices displays 40% retention loss at 150 °C after 10 5 s. The TaN-capped devices exhibit a more pronounced imprint, which is attributed to an increased oxygen vacancy content (compared to W-capped devices). The increased oxygen vacancy content in the TaN-capped devices is supported by photoluminescence and leakage current measurements. In addition, TaN-capped devices have chemically diffuse electrode–HZO interfaces; more abrupt interfaces are present in the W-capped devices. The presence of interfacial phases in the TaN-capped devices may lead to larger depolarization fields due to reduced charge screening. The results from this study provide further evidence that for HZO ferroelectric devices the electrode can significantly impact polarization retention behavior due to differences in oxygen vacancy concentration and formation of non-ferroelectric interfacial layers.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Corrosion test of LWR cladding tubes in high temperature seawater

In the case of cooling system failure in spent fuel pools (SFP) due to station black out and/or loss of coolant accidents, a pool water level must be kept high enough to avoid overheat of spent fuels. Seawater is one of the candidate coolant materials in such emergencies. In the present work, we have studied the corrosion behavior of zirconium alloys in seawater. Tube samples of Zircaloy-2 and -4 claddings were immersed in artificial seawater (3, 10 and 20 wt.%) at high temperature (80 - 85 Celsius degrees). Boric acid was added to the seawater at a corrosion test of Zircaloy-4. Two types of specimens were prepared according to the simulation of circumferential tensile stress. One was a plugged tube specimen, which was pressurized to around fuel rod pressure in a spent fuel pool by argon gas. Another specimen was of C-ring shape, where large circumferential tensile stress was partially induced at the surface. A notch was longitudinally grooved to increase the local tensile stress on some of the C-ring specimens. The specimen surface was observed by a scanning electron microscopy (SEM) after the corrosion test. After the 200 day long corrosion test for the plugged tube specimens without boric acid, Mg(OH){sub 2} deposits were observed on the surface of Zircaloy-2 and -4 specimens whereas Mg(OH){sub 2} was not observed with boric acid because of probably a lower pH. Neither pitting corrosion nor stress corrosion cracking (SCC) was observed on the surface of all plugged tube specimens. Only in the C-ring specimens in 20 wt.% seawater without boric acid and in 3 and 10 wt.% seawater with boric acid, small cracks were observed at the notch. The results of the present work show that Zircaloy-2 and -4 have a high tolerance towards corrosion and SCC in high temperature seawater.

21 SPECIFIC NUCLEAR REACTORS AND ASSOCIATED PLANTS↗
DOE OSTI2024

Exploring laser-material interactions of zirconium carbide under additive manufacturing conditions

Zirconium carbide (ZrC) is an ultra-high temperature ceramic with a melting temperature above 3000°C and a broad range of high temperature applications. Given the high melting and sintering temperatures of pure ZrC, producing near-net shape and fully dense parts remains challenging with conventional techniques. In this study, we investigate the fundamental laser-material interactions of ZrC under laser powder bed fusion (LPBF) additive manufacturing (AM) conditions. Normalized enthalpy, a scaling law term that is used in welding and AM literature for detailing laser-material interactions in metallic alloys, was calculated to determine the predictive capabilities of melt pool features in ZrC. Further, the melt pool quality of laser irradiated ZrC was used to compare LPBF relevant laser parameter combinations of laser power, scan speed, and beam diameter. Laser build parameters that resulted in desirable melt pool morphologies were applied to the fabrication of ZrC coupons using LPBF AM. A custom LPBF system was used to determine hatch spacing and layer height parameters that resulted in a fabricated sample with a density of 85% as measured by Archimedes and a Vicker's microhardness of 20.9 ± 1.9 GPa. This investigation reveals the laser-material interactions of ZrC under AM relevant conditions and is the first step towards LPBF fabrication of ZrC parts.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Crystalline ZrTiO{sub 4} gated p-metal–oxide–semiconductor field effect transistors with sub-nm equivalent oxide thickness featuring good electrical characteristics and reliability

ZrTiO{sub 4} crystallized in orthorhombic (o-) phase was stacked with an amorphous Yb{sub 2}O{sub 3} interfacial layer as the gate dielectric for Si-based p-MOSFETs. With thermal annealing after gate electrode, the gate stack with equivalent oxide thickness (EOT) of 0.82 nm achieves high dielectric quality by showing a low interface trap density (D{sub it}) of 2.75 × 10{sup 11 }cm{sup −2}eV{sup −1} near the midgap and low oxide traps. Crystallization of ZrTiO{sub 4} and post metal annealing are also proven to introduce very limited amount of metal induced gap states or interfacial dipole. The p-MOSFETs exhibit good sub-threshold swing of 75 mV/dec which is ascribed to the low D{sub it} value and small EOT. Owing to the Y{sub 2}O{sub 3} interfacial layer and smooth interface with Si substrate that, respectively, suppress phonon and surface roughness scattering, the p-MOSFETs also display high hole mobility of 49 cm{sup 2}/V-s at 1 MV/cm. In addition, I{sub on}/I{sub off} ratio larger than 10{sup 6} is also observed. From the reliability evaluation by negative bias temperature instability test, after stressing with an electric field of −10 MV/cm at 85 °C for 1000 s, satisfactory threshold voltage shift of 12 mV and sub-threshold swing degradation of 3% were obtained. With these promising characteristics, the Yb{sub 2}O{sub 3}/o-ZrTiO{sub 4} gate stack holds the great potential for next-generation electronics.

36 MATERIALS SCIENCE↗
DOE OSTI2023

A series of cation-modified robust zirconium-based metal–organic frameworks for carbon dioxide capture

Metal–organic frameworks (MOFs) represent one of the most promising porous solids for possible use as sorbents to control and reduce the greenhouse gas emission. Studies have shown that open metal sites (OMS) interact strongly with carbon dioxide and thus, serve as efficient binding sites for CO 2 capture. However, many OMS-bearing MOFs are lack of framework stability and often have high regeneration temperature. To seek ways to solve the stability issue, we designed a series of isoreticular MOFs, Zr-tcpb-COOM (M = alkali/alkaline earth metal), by exchange of protons with metal ions on Zr-tcpb-COOH via post-synthetic modification (PSM). The pristine MOF (Zr-tcpb-COOH) has a very robust framework. The PSM process does not deteriorate the framework stability but creates metal binding sites that form strong bonds with carbon dioxide. The results show that at low CO 2 pressure, the uptake amount is enhanced considerably using Zr-tcpb-COOM and is in trend of increasing atomic number (Li + < Na + < K + < Ca 2+ ). High adsorption selectivity (CO 2 /N 2 IAST selectivity (15 : 85) = 539.5) is also achieved for CO 2 over N 2 at room temperature. This approach offers a feasible method to improve CO 2 capture capacity, especially at low concentrations.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Effect of annealing temperature on optical and electrical properties of ZrO{sub 2}−SnO{sub 2} based nanocomposite thin films

Transparent nanocomposite ZrO{sub 2}−SnO{sub 2} thin films were prepared by sol-gel dip-coating technique. Films were annealed at 500°C, 800°C and 1200°C respectively. X-ray diffraction(XRD) spectra showed a mixture of three phases: tetragonal ZrO{sub 2} and SnO{sub 2} and orthorhombic ZrSnO{sub 4}. The grain size of all the three phases' increased with annealing temperature. An average transmittance greater than 85%(in UV-Visible region) is observed for all the films. The band gap for the films decreased from 4.79 eV to 4.62 eV with increase in annealing temperature from 500 to 1200 °C. The electrical resistivity increased with increase in annealing temperature. Such composite ZrO{sub 2}−SnO{sub 2} films can be used in many applications and in optoelectronic devices.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Critical behavior of the ferromagnetic-paramagnetic phase transition in Fe{sub 90−x}Ni{sub x}Zr{sub 10} alloy ribbons

This work presents a detailed study on the critical behavior of the ferromagnetic-paramagnetic (FM-PM) phase transition in Fe{sub 90−x}Ni{sub x}Zr{sub 10} (x = 0 and 5) alloy ribbons. Basing on field dependences of magnetization (M-H), M{sup 2} versus H/M plots prove the alloys exhibiting a second-order magnetic phase transition. To investigate the nature of the FM-PM phase transition at T{sub C} = 245 and 306 K for x = 0 and 5, respectively, we performed a critical-exponent study. The values of critical components β, γ, and δ determined by using the modified Arrott plots, Kouvel-Fisher (KF), and critical isotherm analyses agree with each other. For x = 0, the critical parameters β = 0.365 ± 0.013 and γ = 1.615 ± 0.033 are obtained by modified Arrott plots while β = 0.368 ± 0.008 and γ = 1.612 ± 0.016 are obtained by the KF method. These values are close to those expected for the 3D-Heisenberg model, revealing short-range FM interactions in Fe{sub 90}Zr{sub 10}. Meanwhile, for x = 5, the values of the critical parameters β = 0.423 ± 0.008 and γ = 1.325 ± 0.036 are obtained by modified Arrott plots, and β = 0.425 ± 0.006 and γ = 1.323 ± 0.012 are obtained by the KF method. The falling of the β value in between the values of the mean-field theory (β = 0.5) and the 3D-Heisenberg model (β = 0.365) indicates an existence of FM short-range order and magnetic inhomogeneity in Fe{sub 85}Ni{sub 5}Zr{sub 10}. With a partial replacement of Ni for Fe in Fe{sub 90−x}Ni{sub x}Zr{sub 10}, the value of the critical exponent β trends to shift towards that of the mean-field theory. Such the result proves the presence of Ni favors establishing FM long-range order. The nature of this phenomenon is carefully discussed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2018

High Strength, Utilizable Ductility and Electrical Conductivity in Cold Rolled Sheets of Cu-Cr-Zr-Ti Alloy

The microstructure and properties of Cu-0.5Cr-0.03Zr-0.04Ti (wt.%) alloy subjected to cold rolling and aging were investigated using hardness and electrical conductivity measurement, tensile testing, and transmission electron microscopy. Plates subjected to 85% reduction in thickness showed significant improvement in the strength compared to solution treated and aged condition. By proper selection of aging temperature, ductility could be significantly improved without reduction in the strength. Aging of the cold rolled sheet at 420 °C for 1 h yielded an ultimate tensile strength and yield strength of 540 and 460 MPa, respectively. On the other hand, total and uniform elongation was 16 and 12%, respectively, with an electrical conductivity of 65 %IACS. The combination of properties achieved after cold rolling and aging is attributed to the higher dislocation density, ultrafine grains and nano-sized chromium precipitates.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Waste Management Approach for a Potential Future U.S. Closed Fuel Cycle - 16233

The U.S. Department of Energy, Office of Nuclear Energy has chartered an effort to develop technologies to enable safe and cost effective recycle of commercial used nuclear fuel (UNF) in the U.S. Part of this effort includes the evaluation of exiting waste management technologies for effective treatment of wastes in the context of current U.S. regulations and development of waste forms and processes with significant cost and/or performance benefits over those existing. This study summarizes the results of these ongoing efforts with a focus on the highly radioactive primary waste streams. The primary streams considered and the recommended waste forms include: - Tritium separated from either a low volume gas stream or a high volume water stream. The recommended waste form is low-water cement in high integrity containers. - Iodine-129 separated from off-gas streams in aqueous processing. There are a range of potentially suitable waste forms. As a reference case, a glass composite material (GCM) formed by the encapsulation of the silver Mordenite (AgZ) getter material in a low-temperature glass is assumed. A number of alternatives with distinct advantages are also considered including a fused silica waste form with encapsulated nano-sized AgI crystals. - Carbon-14 separated from LWR fuel treatment off-gases and immobilized as a CaCO{sub 3} in a cement waste form. - Krypton-85 separated from LWR and SFR fuel treatment off-gases and stored as a compressed gas. - An aqueous reprocessing high-level waste (HLW) raffinate waste which is immobilized by the vitrification process in one of three forms: a single phase borosilicate glass, a borosilicate based glass ceramic, or a multi-phased titanate ceramic [e.g., synthetic rock (Synroc)]. - An undissolved solids (UDS) fraction from aqueous reprocessing of LWR fuel that is either included in the borosilicate HLW glass or is immobilized in the form of a metal alloy in the case of glass ceramics or titanate ceramics. - Zirconium-based LWR fuel cladding hulls and stainless steel (SS) fuel assembly hardware that are washed and super-compacted for disposal or as an alternative Zr purification and reuse (or disposal as low-level waste, LLW) by reactive gas separations. - Electrochemical process salt HLW which is immobilized in a glass bonded Sodalite waste form known as the ceramic waste form (CWF). - Electrochemical process UDS and SS cladding hulls which are melted into an iron based alloy waste form. Mass and volume estimates for each of the recommended waste forms based on the source terms from a representative flowsheet are reported. (authors)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2016

High charge-discharge performance of Pb{sub 0.98}La{sub 0.02}(Zr{sub 0.35}Sn{sub 0.55}Ti{sub 0.10}){sub 0.995}O{sub 3} antiferroelectric ceramics

The energy storage performance and charge-discharge properties of Pb{sub 0.98}La{sub 0.02}(Zr{sub 0.35}Sn{sub 0.55}Ti{sub 0.10}){sub 0.995}O{sub 3} (PLZST) antiferroelectric ceramics were investigated through directly measuring the hysteresis loops and pulse discharge current-time curves. The energy density only varies 0.2% per degree from 25 °C to 85 °C, and the energy efficiency maintains at about 90%. Furthermore, an approximate calculating model of maximum power density p{sub max} was established for the discharge process. Under a relatively high working electric field (8.2 kV/mm), this ceramics possess a greatly enhanced power density of 18 MW/cm{sup 3}. Moreover, the pulse power properties did not show degradation until 1500 times of charge-discharge cycling. The large released energy density, high energy efficiency, good temperature stability, greatly enhanced power density, and excellent fatigue endurance combined together make this PLZST ceramics an ideal candidate for pulse power applications.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Room temperature creep behavior of Ti–Nb–Ta–Zr–O alloy

The room temperature creep behavior and deformation mechanisms of a Ti–Nb–Ta–Zr–O alloy, which is also called “gum metal”, were investigated with the nanoindentation creep and conventional creep tests. The microstructure was observed with electron backscattered diffraction analysis (EBSD) and transmission electron microscopy (TEM). The results show that the creep stress exponent of the alloy is sensitive to cold deformation history of the alloy. The alloy which was cold swaged by 85% shows high creep resistance and the stress exponent is approximately equal to 1. Microstructural observation shows that creep process of the alloy without cold deformation is controlled by dislocation mechanism. The stress-induced α' martensitic phase transformation also occurs. The EBSD results show that the grain orientation changes after the creep tests, and thus, the creep of the cold-worked alloy is dominated by the shear deformation of giant faults without direct assistance from dislocations. - Highlights: •Nanoindentation was used to investigate room temperature creep behavior of gum metal. •The creep stress exponent of gum metal is sensitive to the cold deformation history. •The creep stress exponent of cold worked gum metal is approximately equal to 1. •The creep of the cold-worked gum metal is governed by the shear deformation of giant faults.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Phase transitions in [001]-oriented morphotropic PbZr{sub 0.52}Ti{sub 0.48}O{sub 3} thin film deposited onto SrTiO{sub 3}-buffered Si substrate

An 85 nm-thick morphotropic PbZr{sub 0.52}Ti{sub 0.48}O{sub 3} (PZT) film grown epitaxially and [001]-oriented onto a SrTiO{sub 3}-buffered Si-wafer is investigated using temperature dependent X-ray diffraction. Two phase transitions, at T{sub rt} ∼ 500 K and T{sub c} ∼ 685 K, are evidenced and are attributed to structural phase transitions from monoclinic-like to tetragonal-like phase and from tetragonal to paraelectric phase, respectively. The stronger upper shift of T{sub rt} value with respect to the bulk one and the weakly affected T{sub c} (T{sub c} bulk ∼ 665 K) are explained assuming misfit strain changes when crossing T{sub rt}. This finding opens new perspectives for piezoelectric PZT films in harsh applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

On the dynamics of the reentrant spin-glass state of the Fe{sub 100−x}Zr{sub x} (x = 9, 10) amorphous alloys

Low temperature magnetization (M) and ac susceptibility (χ{sub ac}) data were used to investigate the reentrant spin-glass state in lengths of melt-spun ribbons of Fe{sub 100−x}Zr{sub x} (x = 9, 10) amorphous alloys. The temperature range investigated was from 2 to 300 K, while the applied magnetic field H was varied in the range of ±85 kOe. The magnetic properties were found to be strongly influenced by the sample composition despite the fact that the amount of Fe varies by 1 at. %. For instance, the Curie temperature (T{sub C}) is reduced from 232.5 K to 213.0 K with decreasing Fe concentration, while M for the lower Fe concentration, measured at the highest applied magnetic field (H = 85 kOe) was nearly double the value for that of the higher; the coercivity in the ferromagnetic regime is reduced by a factor close to five when x is increased from 9 at. % to 10 at. %. The ac susceptibility measured for frequencies f in the range of 10–10{sup 4 }Hz showed a sharp drop in the magnitude of the in-phase contribution (χ{sub ac}{sup ′}) and a peak at the out-of-phase component (χ{sub ac}{sup ″}), which shifts to higher temperatures with increasing values of f. The Voguel-Fulcher law applied to the χ{sub ac}{sup ″} data yielded an activation energy E{sub a}/k{sub B} = 21.3 K (40.1 K), the glassy temperature T{sub G} = 15.5 K (38.2 K), and a relaxation time τ{sub 0}=9.1×10{sup −7} s (8.3×10{sup −7} s), for the sample with x = 10 (x = 9). A plot of χ{sub ac}{sup ″}vsχ{sub ac}{sup ′} for a broad range of T and f yielded a broad maximum near the glassy temperature T{sub G} for both sample concentrations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

s-processing in AGB stars revisited. I. Does the main component constrain the neutron source in the {sup 13}C pocket?

Slow neutron captures at A ≳ 85 are mainly guaranteed by the reaction {sup 13}C(α,n){sup 16}O in asymptotic giant branch (AGB) stars, requiring proton injections from the envelope. These were so far assumed to involve a small mass (≲ 10{sup –3} M {sub ☉}), but models with rotation suggest that in such tiny layers excessive {sup 14}N hampers s-processing. Furthermore, s-element abundances in galaxies require {sup 13}C-rich layers substantially extended in mass (≳ 4 × 10{sup –3} M {sub ☉}). We therefore present new calculations aimed at clarifying those issues and at understanding whether the solar composition helps to constrain the {sup 13}C 'pocket' extension. We show that: (1) mixing 'from bottom to top' (as in magnetic buoyancy or other forced mechanisms) can form a {sup 13}C reservoir substantially larger than assumed so far, covering most of the He-rich layers; (2) on the basis of this idea, stellar models at a fixed metallicity reproduce the main s-component as accurately as before; and (3) they make nuclear contributions from unknown nucleosynthesis processes (LEPP) unnecessary, against common assumptions. These models also avoid problems of mixing at the envelope border and fulfil requirements from C-star luminosities. They yield a large production of nuclei below A = 100, so that {sup 86,} {sup 87}Sr may be fully synthesized by AGB stars, while {sup 88}Sr, {sup 89}Y, and {sup 94}Zr are contributed more efficiently than before. Finally, we suggest tests suitable for providing a final answer regarding the extension of the {sup 13}C pocket.

79 ASTRONOMY AND ASTROPHYSICS↗