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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

In-Beam γ -Ray Spectroscopy of Mg 34 , 36 , 38 : Merging the N = 20 and N = 28 Shell Quenching

Neutron-rich N = 22, 24, 26 magnesium isotopes were studied in this paper via in-beam γ-ray spectroscopy at the RIKEN Radioactive Isotope Beam Factory following secondary fragmentation reactions on a carbon target at ≈ 200 MeV /nucleon. In the one- and two-proton removal channels from 39 Al and 40 Si beams, two distinct γ-ray transitions were observed in 38 Mg, while in the one-proton removal reaction from 37 Al a new transition was observed in addition to the known 2 + 1 → 0 + g.s. decay. From the experimental systematics and comparison to theoretical predictions it is concluded that the transitions belong to the 2 + 1 → 0 + g.s. and 4 + 1 → 2 + 1 decays in 36 Mg and 38 Mg, respectively. For 34 Mg, previously reported 2 + 1 and 4 + 1 level energies were remeasured. Finally, the deduced E(4 + 1 )/E(2 + 1 ) ratios for 34,36,38 Mg of 3.14(5), 3.07(5), and 3.07(5) are almost identical and suggest the emergence of a large area of deformation extending from the N = 20 to the N = 28 shell quenching.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2016

Nuclear Data Sheets for A = 26

Evaluated spectroscopic data and level schemes from radioactive decay and nuclear reaction studies are presented for {sup 26}O, {sup 26}F, {sup 26}Ne, {sup 26}Na, {sup 26}Mg, {sup 26}Al, {sup 26}Si, {sup 26}P, and {sup 26}S. This evaluation for A = 26 supersedes the earlier one by P. M. Endt (1998En04) and updates for some nuclides in ENSDF. Highlights of this evaluation are the following: This evaluation includes search results for {sup 26}S nuclide and its proton–decay mode (2011Fo08). An isomeric state (2.2 ms) in {sup 26}F has been discovered by 2013Le03. The state is proposed at 643.4 keV 1 from γ–ray measurements. Internal-transition and beta-decay branches for the state are also determined. New excited levels in {sup 26}Ne have been identified from {sup 26}F β{sup −} decay (2.2 ms). For some {sup 26}Si resonance states conflicting spin-parity assignments exist in the literature. These are identified by footnotes. 2015Do07 ({sup 3}He,nγ) propose the first 0+ state above proton separation energy at an excitation energy of 5890 keV and suggested for additional independent measurements to confirm or refute the existence of 5946 keV 4. 2016Ch09 consider 5946 keV level as a distinct excited state in their reanalysis of the literature data with possible spin-parity assignment of 0+ or 4+ This evaluation also includes discovery of an isomeric state, at 164.1 keV 1, in {sup 26}P by 2014NiZZ.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2011

RAPID TIMESCALES FOR MAGMA OCEAN CRYSTALLIZATION ON THE HOWARDITE-EUCRITE-DIOGENITE PARENT BODY

Asteroid 4 Vesta has long been postulated as the source for the howardite-eucrite-diogenite (HED) achondrite meteorites. Here we show that Al-free diogenite meteorites record variability in the mass-independent abundance of {sup 26}Mg ({sup 26}Mg*) that is correlated with their mineral chemistry. This suggests that these meteorites captured the Mg-isotopic evolution of a large-scale differentiating magma body with increasing {sup 27}Al/{sup 24}Mg during the lifespan of the short-lived {sup 26}Al nuclide (t {sub 1/2} {approx} 730,000 yr). Thus, diogenites and eucrites represent crystallization products of a large-scale magma ocean associated with the differentiation and magmatic evolution of the HED parent body. The {sup 26}Mg* composition of the most primitive diogenites requires onset of the magma ocean crystallization within 0.6{sup -0.4} {sub +0.5} Myr of solar system formation. Moreover, {sup 26}Mg* variations among diogenites and eucrites imply that near complete solidification of the HED parent body occurred within the following 2-3 Myr. Thermal models predict that such rapid cooling and magma ocean crystallization could only occur on small asteroids (<100 km), implying that 4 Vesta is not the source of the HED meteorites.

58 GEOSCIENCES↗
DOE OSTI2011

Proton dominance in the 2{sub 2}{sup +}{yields}0{sub 1}{sup +} transition of N=Z{+-}2 nuclei around {sup 28}Si

E2 transitions in {sup 30}Si are investigated in relation to intrinsic deformations based on a method of antisymmetrized molecular dynamics. By comparing E2 transition strengths in the mirror nuclei {sup 30}Si and {sup 30}S, transition matrix amplitudes M{sub p} and M{sub n} for protons and neutrons are discussed in mirror analysis. Particular attention is paid to the M{sub n}/M{sub p} ratio in the transition from the 2{sub 2}{sup +} state to the 0{sub 1}{sup +} state. The M{sub n}/M{sub p} ratio in {sup 26}Mg and {sup 26}Si is also investigated. It is found that the proton dominance in the transition 2{sub 2}{sup +}{yields}0{sub 1}{sup +} in {sup 30}Si and {sup 26}Si originates in the oblate trend of the Z=14 proton structure.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2015

Toward accurate thermochemistry of the {sup 24}MgH, {sup 25}MgH, and {sup 26}MgH molecules at elevated temperatures: Corrections due to unbound states

The total partition functions Q(T) and their first two moments Q{sup ′}(T) and Q{sup ″}(T), together with the isobaric heat capacities C{sub p}(T), are computed a priori for three major MgH isotopologues on the temperature range of T = 100–3000 K using the recent highly accurate potential energy curve, spin-rotation, and non-adiabatic correction functions of Henderson et al. [J. Phys. Chem. A 117, 13373 (2013)]. Nuclear motion computations are carried out on the ground electronic state to determine the (ro)vibrational energy levels and the scattering phase shifts. The effect of resonance states is found to be significant above about 1000 K and it increases with temperature. Even very short-lived states, due to their relatively large number, have significant contributions to Q(T) at elevated temperatures. The contribution of scattering states is around one fourth of that of resonance states but opposite in sign. Uncertainty estimates are given for the possible error sources, suggesting that all computed thermochemical properties have an accuracy better than 0.005% up to 1200 K. Between 1200 and 2500 K, the uncertainties can rise to around 0.1%, while between 2500 K and 3000 K, a further increase to 0.5% might be observed for Q{sup ″}(T) and C{sub p}(T), principally due to the neglect of excited electronic states. The accurate thermochemical data determined are presented in the supplementary material for the three isotopologues of {sup 24}MgH, {sup 25}MgH, and {sup 26}MgH at 1 K increments. These data, which differ significantly from older standard data, should prove useful for astronomical models incorporating thermodynamic properties of these species.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2011

Identification of analog states in the T=1/2 A=27 mirror system from low excitation energies to the region of hydrogen burning in the {sup 26}Al{sup g,m}(p,{gamma}){sup 27}Si reactions

The reactions {sup 26}Al{sup g}(p, {gamma}){sup 27}Si and {sup 26}Al{sup m}(p, {gamma}){sup 27}Si are important for influencing the galactic abundance of the cosmic {gamma}-ray emitter {sup 26}Al{sup g} and for the excess abundance of {sup 26}Mg found in presolar grains, respectively. Precise excitation energies and spin assignments of states from the ground state to the region of astrophysical interest in {sup 27}Si, including the identification and pairing of key astrophysical resonances with analog states in the mirror nucleus {sup 27}Al, are reported using {gamma} rays observed in the {sup 12}C + {sup 16}O fusion reaction. The detailed evolution of Coulomb energy differences between the states in {sup 27}Si and {sup 27}Al is explored, including the region above the astrophysical reaction thresholds.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2015

Inferred initial {sup 26}Al/{sup 27}Al ratios in presolar stardust grains from supernovae are higher than previously estimated

We performed an in-depth exploration of the Al–Mg system for presolar graphite, SiC, and Si{sub 3}N{sub 4} grains found to contain large excesses of {sup 26}Mg, indicative of the initial presence of live {sup 26}Al. Ninety of the more than 450 presolar grains processed in this study contain well-correlated δ{sup 26}Mg/{sup 24}Mg and {sup 27}Al/{sup 24}Mg ratios, derived from Nano-scale Secondary Ion Mass Spectrometer depth profiles, whose isochron-like regression lines yield inferred initial {sup 26}Al/{sup 27}Al ratios that, on average, are ∼1.5–2 times larger than the ratios previously reported for the grains. The majority of presolar graphite and SiC grains are heavily affected by Al contamination, resulting in large negative δ{sup 26}Mg/{sup 24}Mg intercepts of the isochron lines. Al contamination is potentially due to etching of the grains’ surfaces and subsequent capture of dissolved Al during the acid dissolution of their meteorite host rocks. From the isochron fits, the magnitude of Al contamination was quantified for each grain. The amount of Al contamination on each grain was found to be random and independent of grain size, following a uniform distribution with an upper bound at 59% contamination. The Al contamination causes conventional whole-grain estimates to underpredict the initial {sup 26}Al/{sup 27}Al ratios. The presolar grains with the highest {sup 26}Al/{sup 27}Al ratios are from Type II supernovae whose isochron-derived initial {sup 26}Al/{sup 27}Al ratios greatly exceed those predicted in the He/C and He/N zones of SN models.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

ISOCHRONS IN PRESOLAR GRAPHITE GRAINS FROM ORGUEIL

Primitive meteorites contain tiny dust grains that condensed in stellar outflows and explosions. These stardust grains can be extracted from their host meteorites and studied in detail in the laboratory. We investigated depth profiles of the Al-Mg, Ca-K, and Ti-Ca isotopic systems obtained during NanoSIMS isotopic analysis of presolar graphite grains from the CI carbonaceous meteorite Orgueil. Large {sup 26}Al/{sup 27}Al, {sup 41}Ca/{sup 40}Ca, and {sup 44}Ti/{sup 48}Ti ratios, inferred from {sup 26}Mg, {sup 41}K, and {sup 44}Ca excesses from the decay of the short-lived radioisotopes {sup 26}Al, {sup 41}Ca, and {sup 44}Ti, indicate a supernova (SN) origin. From the depth distribution of the radiogenic isotopes and the stable isotopes of their parent elements we constructed isochron-type correlation plots. The plots indicate quantitative retention of radiogenic {sup 26}Mg, {sup 41}K, and {sup 44}Ca in most grains. Deviations from straight lines in the Al-Mg and Ca-K plots can be explained by contamination with {sup 27}Al and isotopically normal Ca, respectively. For the Ti-Ca system in some grains, the lack of parent-daughter correlation indicates either redistribution of radiogenic {sup 44}Ca or heterogeneity in the initial {sup 44}Ti/{sup 48}Ti ratio. We also obtained Si isotopic depth profiles in three graphite grains with large {sup 29}Si and {sup 30}Si excesses, for which a SN origin has been proposed. In two grains no Si-rich subgrains are observed; in the third grain with an apparent Si-rich subgrain the anomalous Si isotopic ratios in the subgrain are the same as in the rest of the graphite host. Our studies show that by measuring depth profiles, information on presolar grains can be obtained that cannot be obtained by whole-grain analysis.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Impact of the uncertainty in α-captures on {sup 22}Ne on the weak s-process in massive stars

Massive stars at solar metallicity contribute to the production of heavy elements with atomic masses between A = 60 and A = 90 via the so-called weak s-process (which takes place during core He and shell C burning phases). Furthermore, recent studies have shown that rotation boosts the s-process production in massive stars at low metallicities, with a production that may reach the barium neutron-magic peak. These results are very sensitive to neutron source and neutron poison reaction rates. For the weak s-process, the main neutron source is the reaction {sup 22}Ne(α,n){sup 25}Mg, which is in competition with {sup 22}Ne(α,γ){sup 26}Mg. The uncertainty of both rates strongly affects the nucleosynthesis predictions from stellar model calculations. In this study, we investigate the impact of the uncertainty in α-captures on {sup 22}Ne on the s-process nucleosynthesis in massive stars both at solar and at very low metallicity. For this purpose, we post-process, with the Nugrid mppnp code, non-rotating and rotating evolutionary models 25M{sub ⊙} stars at two different metallicities: Z = Z{sub ⊙} and Z = 10{sup −5}Z{sub ⊙}, respectively. Our results show that uncertainty of {sup 22}Ne(α,n){sup 25}Mg and {sup 22}Ne(α,γ){sup 26}Mg rates have a significant impact on the final elemental production especially for metal poor rotating models. Beside uncertainties in the neutron source reactions, for fast rotating massive stars at low metallicity we revisit the impact of the neutron poisoning effect by the reaction chain {sup 16}O(n,γ){sup 17}O(α,γ){sup 21}Ne, in competition with the {sup 17}O(α,n){sup 20}Ne, recycling the neutrons captured by {sup 16}O.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2016

Flash Kα radiography of laser-driven solid sphere compression for fast ignition

Time-resolved compression of a laser-driven solid deuterated plastic sphere with a cone was measured with flash Kα x-ray radiography. A spherically converging shockwave launched by nanosecond GEKKO XII beams was used for compression while a flash of 4.51 keV Ti Kα x-ray backlighter was produced by a high-intensity, picosecond laser LFEX (Laser for Fast ignition EXperiment) near peak compression for radiography. Areal densities of the compressed core were inferred from two-dimensional backlit x-ray images recorded with a narrow-band spherical crystal imager. The maximum areal density in the experiment was estimated to be 87 ± 26 mg/cm{sup 2}. The temporal evolution of the experimental and simulated areal densities with a 2-D radiation-hydrodynamics code is in good agreement.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2012

Fusion-fission and quasifission in the reactions with heavy ions leading to the formation of Hs

Mass and energy distributions of binary reaction products obtained in the reactions {sup 22}Ne+{sup 249}Cf,{sup 26}Mg+{sup 248}Cm,{sup 36}S+{sup 238}U and {sup 58}Fe+{sup 208}Pb leading to Hs isotopes have been measured. At energies below the Coulomb barrier the bimodal fission of Hs*, formed in the reaction {sup 26}Mg+{sup 248}Cm, is observed. In the reaction {sup 36}S+{sup 238}U the considerable part of the symmetric fragments arises from the quasifission process. At energies above the Coulomb barrier the symmetric fragments originate mainly from fusion-fission process for both reactions with Mg and S ions. In the case of the {sup 58}Fe+{sup 208}Pb reaction the quasifission process dominates at all measured energies. The pre- and post-scission neutron multiplicities as a function of the fragment mass have been obtained for the reactions studied.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2013

THE MAGNESIUM ISOTOPOLOGUES OF MgH IN THE A {sup 2}{Pi}-X {sup 2}{Sigma}{sup +} SYSTEM

Using laboratory hollow cathode spectra we have identified lines of the less common magnesium isotopologues of MgH, {sup 25}MgH and {sup 26}MgH, in the A {sup 2}{Pi}-X {sup 2}{Sigma}{sup +} system. Based on the previous analysis of {sup 24}MgH, molecular lines have been measured and molecular constants derived for {sup 25}MgH and {sup 26}MgH. Term values and linelists, in both wavenumber and wavelength units, are presented. The A {sup 2}{Pi}-X {sup 2}{Sigma}{sup +} system of MgH is important for measuring the magnesium isotope ratios in stars. Examples of analysis using the new linelists to derive the Mg isotope ratio in a metal poor dwarf and giant are shown.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

An interval of high salinity in ancient Gale crater lake on Mars

Precipitated minerals, including salts, are primary tracers of atmospheric conditions and water chemistry in lake basins. Ongoing in situ exploration by the Curiosity rover of Hesperian (around 3.3–3.7 Gyr old) sedimentary rocks within Gale crater on Mars has revealed clay-bearing fluvio-lacustrine deposits with sporadic occurrences of sulfate minerals, primarily as late-stage diagenetic veins and concretions. In this work, we report bulk enrichments, disseminated in the bedrock, of 30–50 wt% calcium sulfate intermittently over about 150 m of stratigraphy, and of 26–36 wt% hydrated magnesium sulfate within a thinner section of strata. We use geochemical analysis, primarily from the ChemCam laser-induced breakdown spectrometer, combined with results from other rover instruments, to characterize the enrichments and their lithology. The deposits are consistent with early diagenetic, pre-compaction salt precipitation from brines concentrated by evaporation, including magnesium sulfate-rich brines from extreme evaporative concentration. This saline interval represents a substantial hydrological perturbation of the lake basin, which may reflect variations in Mars’ obliquity and orbital parameters. Our findings support stepwise changes in Martian climate during the Hesperian, leading to more arid and sulfate-dominated environments as previously inferred from orbital observations.

58 GEOSCIENCES↗
DOE OSTI2012

Resonance neutron-capture cross sections of stable magnesium isotopes and their astrophysical implications

We have measured the neutron capture cross sections of the stable magnesium isotopes {sup 24,25,26}Mg in the energy range of interest to the s process using the neutron time-of-flight facility n{_}TOF at CERN. Capture events from a natural metal sample and from samples enriched in {sup 25}Mg and {sup 26}Mg were recorded using the total energy method based on C{sub 6}{sup 2}H{sub 6} detectors. Neutron resonance parameters were extracted by a simultaneous resonance shape analysis of the present capture data and existing transmission data on a natural isotopic sample. Maxwellian-averaged capture cross sections for the three isotopes were calculated up to thermal energies of 100 keV and their impact on s-process analyses was investigated. At 30 keV the new values of the stellar cross section for {sup 24}Mg, {sup 25}Mg, and {sup 26}Mg are 3.8 {+-} 0.2 mb, 4.1 {+-} 0.6 mb, and 0.14 {+-} 0.01 mb, respectively.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

MAGNESIUM ISOTOPE EVIDENCE FOR SINGLE STAGE FORMATION OF CB CHONDRULES BY COLLIDING PLANETESIMALS

Chondrules are igneous spherical objects preserved in chondritic meteorites and believed to have formed during transient heating events in the solar protoplanetary disk. Chondrules present in the metal-rich CB chondrites show unusual chemical and petrologic features not observed in other chondrite groups, implying a markedly distinct formation mechanism. Here, we report high-precision Mg-isotope data for 10 skeletal olivine chondrules from the Hammadah al Hamra 237 (HH237) chondrite to probe the formation history of CB chondrules. The {sup 27}Al/{sup 24}Mg ratios of individual chondrules are positively correlated to their stable Mg-isotope composition (μ{sup 25}Mg), indicating that the correlated variability was imparted by a volatility-controlled process (evaporation/condensation). The mass-independent {sup 26}Mg composition (μ{sup 26}Mg*) of chondrules is consistent with single stage formation from an initially homogeneous magnesium reservoir if the observed μ{sup 25}Mg variability was generated by non-ideal Rayleigh-type evaporative fractionation characterized by a β value of 0.5142, in agreement with experimental work. The magnitude of the mass-dependent fractionation (∼300 ppm) is significantly lower than that suggested by the increase in {sup 27}Al/{sup 24}Mg values, indicating substantial suppression of isotopic fractionation during evaporative loss of Mg, possibly due to evaporation at high Mg partial pressure. Thus, the Mg-isotope data of skeletal chondrules from HH237 are consistent with their origin as melts produced in the impact-generated plume of colliding planetesimals. The inferred μ{sup 26}Mg* value of –3.87 ± 0.93 ppm for the CB parent body is significantly lower than the bulk solar system value of 4.5 ± 1.1 ppm inferred from CI chondrites, suggesting that CB chondrites accreted material comprising an early formed {sup 26}Al-free component.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2013

MAGNESIUM ISOTOPE RATIOS IN {omega} CENTAURI RED GIANTS

We have used the high-resolution observations obtained at the Anglo-Australian Telescope with Ultra-High Resolution Facility (R {approx} 100,000) and at Gemini-S with b-HROS (R {approx} 150,000) to determine magnesium isotope ratios for seven {omega} Cen red giants that cover a range in iron abundance from [Fe/H] = -1.78 to -0.78 dex, and for two red giants in M4 (NGC 6121). The {omega} Cen stars sample both the ''primordial'' (i.e., O-rich, Na- and Al-poor) and the ''extreme'' (O-depleted, Na- and Al-rich) populations in the cluster. The primordial population stars in both {omega} Cen and M4 show ({sup 25}Mg, {sup 26}Mg)/{sup 24}Mg isotopic ratios that are consistent with those found for the primordial population in other globular clusters with similar [Fe/H] values. The isotopic ratios for the {omega} Cen extreme stars are also consistent with those for extreme population stars in other clusters. The results for the extreme population stars studied indicate that the {sup 26}Mg/{sup 24}Mg ratio is highest at intermediate metallicities ([Fe/H] < -1.4 dex), and for the highest [Al/Fe] values. Further, the relative abundance of {sup 26}Mg in the extreme population stars is notably higher than that of {sup 25}Mg, in contrast to model predictions. The {sup 25}Mg/{sup 24}Mg isotopic ratio in fact does not show any obvious dependence on either [Fe/H] or [Al/Fe] nor, intriguingly, any obvious difference between the primordial and extreme population stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2012

LABORATORY ANALYSIS OF PRESOLAR SILICATE STARDUST FROM A NOVA

We report the major element as well as the oxygen, magnesium, and silicon isotope composition of a unique presolar silicate grain found in the fine-grained fraction of the Antarctic CR2 chondrite Graves Nunataks 95229. The grain is characterized by an extremely high {sup 17}O/{sup 16}O ratio (6.3 {+-} 0.2 Multiplication-Sign 10{sup -3}) relative to solar values, whereas its {sup 18}O/{sup 16}O ratio is solar within measurement uncertainty. It also shows enrichments in {sup 25,26}Mg and a significant excess in {sup 30}Si relative to solar system compositions, with {delta}{sup 25}Mg = 79 {+-} 21 per mille , {delta}{sup 26}Mg = 70 {+-} 20 per mille , and {delta}{sup 30}Si = 379 {+-} 92 per mille . This isotopic composition is consistent with an origin in the ejecta of a {approx}1.3-1.4 M{sub Sun} ONe nova with large contributions of material from a main-sequence companion star of roughly solar metallicity. However, many details of the stellar source remain undetermined, owing to the uncertainties of current nova nucleosynthesis models. Auger electron spectroscopic analyses identify O, Mg, Si, and Fe as the grain's major constituents. Its (Mg+Fe)/Si atomic ratios are lower than that of olivine and correspond on average to Fe-Mg-pyroxene. A complex texture and heterogeneous major element distribution within the grain attest to condensation under non-equilibrium conditions, which is consistent with the proposed nova origin.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

An Investigation of the Formation and Line Properties of MgH in 3D Hydrodynamical Model Stellar Atmospheres

Studies of the isotopic composition of magnesium in cool stars have so far relied upon the use of 1D model atmospheres. Since the isotopic ratios derived are based on asymmetries of optical MgH lines, it is important to test the impact from other effects affecting line asymmetries, like stellar convection. Here, we present a theoretical investigation of the effects of including self-consistent modeling of convection. Using spectral syntheses based on 3D hydrodynamical CO{sup 5}BOLD models of dwarfs (4000 K ≲ T {sub eff} ≲ 5160 K, 4.0 ≤ logg ≤ 4.5, −3.0⩽[Fe/H]⩽−1.0) and giants (T {sub eff} ∼ 4000 K, logg = 1.5, −3.0⩽[Fe/H]⩽−1.0), we perform a detailed analysis comparing 3D and 1D syntheses. We describe the impact on the formation and behavior of MgH lines from using 3D models, and perform a qualitative assessment of the systematics introduced by the use of 1D syntheses. Using 3D model atmospheres significantly affect the strength of the MgH lines, especially in dwarfs, with 1D syntheses requiring an abundance correction of up to +0.69 dex, with the largest for our 5000 K models. The corrections are correlated with T {sub eff} and are also affected by the metallicity. The shape of the strong {sup 24}MgH component in the 3D syntheses is poorly reproduced in 1D. This results in 1D syntheses underestimating {sup 25}Mg by up to ∼5 percentage points and overestimating {sup 24}Mg by a similar amount for dwarfs. This discrepancy increases with decreasing metallicity. {sup 26}Mg is recovered relatively well, with the largest difference being ∼2 percentage points. The use of 3D for giants has less impact, due to smaller differences in the atmospheric structure and a better reproduction of the line shape in 1D.

79 ASTRONOMY AND ASTROPHYSICS↗