Grain Thickness Research Articles

Thick Amorphous Complexion Formation and Extreme Thermal Stability in Ternary Nanocrystalline Cu-Zr-Hf Alloys

Building on the recent discovery of tough nanocrystalline Cu-Zr alloys with amorphous intergranular films, this paper investigates ternary nanocrystalline Cu-Zr-Hf alloys with a focus on understanding how alloy composition affects the formation of disordered complexions. Binary Cu-Zr and Cu-Hf alloys with similar initial grain sizes were also fabricated for comparison. The thermal stability of the nanocrystalline alloys was evaluated by annealing at 950 °C (>95% of the solidus temperatures), followed by detailed characterization of the grain boundary structure. All of the ternary alloys exhibited exceptional thermal stability comparable to that of the binary Cu-Zr alloy, and remained nanocrystalline even after two weeks of annealing at this extremely high temperature. Despite carbide formation and growth in these alloys during milling and annealing, the thermal stability of the ternary alloys is mainly attributed to the formation of thick amorphous intergranular films at high temperatures. Our results show that ternary alloy compositions have thicker boundary films compared to the binary alloys with similar global dopant concentrations. While it is not required for amorphous complexion formation, this work shows that having at least three elements present at the interface can lead to thicker grain boundary films, which is expected to maximize the previously reported toughening effect.

Coercivity enhancement of hot-pressed magnet prepared by HDDR Nd-Fe-B powders using Pr-Cu eutectic alloys diffusion

The effect on the Nd-rich phase distribution of the ingot for hydrogenation-disproportionation-desorption-recombination (HDDR) powders was studied. Coercivity of HDDR powders was elevated to 1.64 T by crushing the ingot into particles and annealing at 800 °C for 18 h. HDDR processed Nd-Fe-B powders blended with Pr-Cu powders and annealed at 580 °C for 3 h were hot-pressed to obtain fully dense magnets. The microstructure evolution in hot-pressed HDDR magnets proved the impact of Pr-Cu diffusion on coercivity that the coercivity of hot-pressed magnets was improved to 1.55 T significantly. Backscatter electron microscopy and transmission electron microscopy observations revealed that the variation in coercivity owing to the diffusion of Pr-Cu along the grain boundary with heat treatment. L-TEM images illustrated the thicker grain boundary layer can pin the motion of the domain wall more effectively, resulting in a higher coercivity in HP magnets.

Correlation between the microstructure and permeability stability of ferrite materials

Permeability stability is very important for ferrite materials used in modern electronic devices. In this study, different contents of Bi2O3–B2O3–SiO2–ZnO (BBSZ) glass were added to NiCuZn ferrite materials to obtain samples with different average grain sizes and grain boundary thicknesses. On the premise of obtaining the same permeability, this study determined that ferrite sample with a large average grain size and thick grain boundary width presented better permeability stability characteristics, including frequency stability, temperature stability, and DC-bias-superposition stability, than the sample with a small average grain size and thin grain boundary. This study subsequently established a model and theoretically demonstrated why the sample with a larger average grain size and thicker grain boundary width possessed superior permeability stability characteristics. This investigation provides a good guide for developing superior stable ferrite materials by improving microstructure characteristics.

Identification of QTLs for rice grain size using a novel set of chromosomal segment substitution lines derived from Yamadanishiki in the genetic background of Koshihikari.

Grain size is important for brewing-rice cultivars, but the genetic basis for this trait is still unclear. This paper aims to identify QTLs for grain size using novel chromosomal segment substitution lines (CSSLs) harboring chromosomal segments from Yamadanishiki, an excellent sake-brewing rice, in the genetic background of Koshihikari, a cooking cultivar. We developed a set of 49 CSSLs. Grain length (GL), grain width (GWh), grain thickness (GT), 100-grain weight (GWt) and days to heading (DTH) were evaluated, and a CSSL-QTL analysis was conducted. Eighteen QTLs for grain size and DTH were identified. Seven (qGL11, qGWh5, qGWh10, qGWt6-2, qGWt10-2, qDTH3, and qDTH6) that were detected in F2 and recombinant inbred lines (RILs) from Koshihikari/Yamadanishiki were validated, suggesting that they are important for large grain size and heading date in Yamadanishiki. Additionally, QTL reanalysis for GWt showed that qGWt10-2 was only detected in early-flowering RILs, while qGWt5 (in the same region as qGWh5) was only detected in late-flowering RILs, suggesting that these QTLs show different responses to the environment. Our study revealed that grain size in the Yamadanishiki cultivar is determined by a complex genetic mechanism. These findings could be useful for the breeding of both cooking and brewing rice.

アルミニウム微量添加が酸化亜鉛（ZnO）バリスタ素子特性に与える影響

It is well known that aluminum doping is very effective for improving the Voltage-Current (V-I) characteristics of ZnO varistor elements comprising Bismuth oxide and Antimony oxide. Although it is established that the presence of aluminum lowers the resistance of ZnO grains by valence control, in this study, it is proposed that the homogeneity of micro structure of ZnO elements is more effective for improvement of V-I characteristics. The relationship between electrical characteristics and microstructure at different aluminum doping levels and sintering temperatures is shown with V-I characteristics, SEM observation and capacitance measurement of ZnO elements. As the effect of Al doping, it was found that moderation of ZnO grain growth causes the homogeneity of microstructure of ZnO varistors. In detail, reduction of the volume ratio of high resistive parts like thicker grain boundaries and multiple point between ZnO grains leads to excellent non-ohmic V-I characteristics. In addition, it is suggested that the tunnel effect by thinner grain boundaries is important as the mechanisms of improvement of V-I characteristics.

Mapping of QTLs Controlling Grain Shape and Populations Construction Derived from Related Residual Heterozygous Lines in Rice

<p>Grain shape is usually characterized by grain length (GL), grain width (GW), grain thickness (GT) and length to width ratio (LWR), and controlled by quantitative trait locus (QTL). In this paper, QTL analysis was performed using an F<sub>2</sub> population and an F<sub>8</sub> recombinant inbred line (RIL) population from a cross Xiang743/Katy. A total of 38 QTLs for grain shape were detected and eight of them were repeatedly identified in both populations. Seven for GL, five for GW, five for GT, and eight for LWR were detected in F<sub>2 </sub>population, explaining totally phenotypic variance of 94.51%, 61.52%, 54.33% and 91.84%, respectively. Five for GL, three for GW, and five for LWR were detected in RILpopulation, explaining totally phenotypic variance of 39.83%, 37.52% and 36.71%, respectively. Many QTLs were located in similar intervals, contributing to complicated trait correlation. A few QTLs were mapped in intervals coincided with previously cloned genes associated with grain size. Two residual heterozygous lines (RHLs) were selected out on the basis of newly identified loci, populations derived from RHLs were constructed for fine mapping QTLs associated with grain shape.</p>

QTL mapping of grain appearance quality traits and grain weight using a recombinant inbred population in rice (Oryza sativa L.)

Grain appearance quality traits, measured as grain length (GL), grain width (GW), length to width ratio (LWR), grain thickness (GT) and the percentage of grain with chalkiness (PGWC), as well as 1 000-grain weight (TGW), are very important factors that contribute to rice grain quality and yield. To detect quantitative trait loci (QTLs) affecting these traits, we developed a set of recombinant inbred lines (RILs) derived from Gang46B (G46B) and K1075, a G46B introgression line with lower PGWC. Based on a linkage map containing 33 simple sequence repeat (SSR) markers, a total of 15 additive QTLs governing six measured traits were identified on 4 chromosomes across two environments. Of these, the five major QTLs which controlled GW, LWR, GT, PGWC, and TGW, each explaining up to 44.30, 55.29, 62.30, 30.94, and 28.78% of the variation, respectively, were found in the same interval of RM18004–RM18068 on chromosome 5. The G46B alleles contributed to the increase in GW, GT and PGWC at all loci, as well as the increase in TGW at its major QTL locus. Significant interactions between additive QTL and the environment were found at most loci, in which the largest, accounting for 15.06% of variation, was observed between qPGWC-5 and the environment. A total of 15 epistasis QTLs were detected for all the traits, and GL, GW and PGWC had significant epistasis QTLs based on environment interactions with minor effects. These results are valuable for future map-based cloning of the QTLs and the collaborative improvement of G46B in grain appearance quality and yield.

Genetic Diversity and Elite Allele Mining for Grain Traits in Rice (Oryza sativa L.) by Association Mapping.

Mining elite alleles for grain size and weight is of importance for the improvement of cultivated rice and selection for market demand. In this study, association mapping for grain traits was performed on a selected sample of 628 rice cultivars using 262 SSRs. Grain traits were evaluated by grain length (GL), grain width (GW), grain thickness (GT), grain length to width ratio (GL/GW), and 1000-grain weight (TGW) in 2013 and 2014. Our result showed abundant phenotypic and genetic diversities found in the studied population. In total, 2953 alleles were detected with an average of 11.3 alleles per locus. The population was divided into seven subpopulations and the levels of linkage disequilibrium (LD) ranged from 34 to 84 cM. Genome-wide association mapping detected 10 marker trait association (MTAs) loci for GL, 1MTAs locus for GW, 7 MTAs loci for GT, 3 MTAs loci for GL/GW, and 1 MTAs locus for TGW. Twenty-nine, 2, 10, 5, and 3 elite alleles were found for the GL, GW, GT, GL/GW, and TGW, respectively. Optimal cross designs were predicted for improving the target traits. The accessions containing elite alleles for grain traits mined in this study could be used for breeding rice cultivars and cloning the candidate genes.

QTL mapping of flag leaf traits in common wheat using an integrated high-density SSR and SNP genetic linkage map

Photosynthesis of carbohydrates is the primary source of grain yield in wheat. Photosynthetic organs, especially flag leaves and awns play important roles in wheat growth and development. Genetic analysis of flag leaf posture, size and shape and presence/absence of awns was conducted using a set of 269 recombinant inbred lines (RILs) derived from Yanda1817 × Beinong6. Six agronomic traits comprising flag leaf angle (FLAN), flag leaf width (FLW), flag leaf length (FLL), the ratio of length/width of flag leaf (FLR), flag leaf area (FLA) and presence/absence of awns were evaluated in Shijiazhuang (2011, 2012 and 2013) and Beijing (2012). Using the available high-density single nucleotide polymorphism and simple sequence repeats (SSR) genetic linkage map, a total of 61 putative quantitative trait loci (QTL) for FLAN, FLW, FLL, FLR and FLA were detected on 16 of the 21 wheat chromosomes excluding 1D, 4B, 5D, 6A and 7A, with single QTL in different environments explaining 2.49–42.41 % of the phenotypic variation. Among the identified QTL, 17 were for FLAN, 11 for FLW, seven for FLL, 13 for FLR and 13 for FLA. Twenty-five (41 %) QTL were detected in at least two environments, while four QTL for FLW were detected in all environments. Thirty QTL were associated with higher number of flag leaf traits originated from Yanda1817 alleles, whereas the remaining 31 QTL were derived from Beinong6. In addition, pleiotropic effects were detected for QTL on chromosomes 2D, 3B, 4A, 4D, 5A, 5B, 6B, 6D and 7D that could serve as target regions for fine mapping and marker-assisted selection in wheat breeding programs. Genetic analysis revealed that the presence/absence of awns in the RIL population is controlled by the awn-inhibitor gene B1 linked to SSR marker Xgwm291 on the long arm of chromosome 5A. Our results also suggest that physiological traits FLL, FLW and FLA were significantly and positively correlated to spike length (SL), grain weight per spike and grain number per spike. FLR was significantly and positively related to SL but negatively related to grain width and grain thickness (GT). In addition, the awn trait was strongly and positively correlated to thousand grain weight, grain length and GT.

High-density genetic linkage map construction and QTL mapping of grain shape and size in the wheat population Yanda1817 × Beinong6.

High-density genetic linkage maps are necessary for precisely mapping quantitative trait loci (QTLs) controlling grain shape and size in wheat. By applying the Infinium iSelect 9K SNP assay, we have constructed a high-density genetic linkage map with 269 F 8 recombinant inbred lines (RILs) developed between a Chinese cornerstone wheat breeding parental line Yanda1817 and a high-yielding line Beinong6. The map contains 2431 SNPs and 128 SSR & EST-SSR markers in a total coverage of 3213.2 cM with an average interval of 1.26 cM per marker. Eighty-eight QTLs for thousand-grain weight (TGW), grain length (GL), grain width (GW) and grain thickness (GT) were detected in nine ecological environments (Beijing, Shijiazhuang and Kaifeng) during five years between 2010–2014 by inclusive composite interval mapping (ICIM) (LOD≥2.5). Among which, 17 QTLs for TGW were mapped on chromosomes 1A, 1B, 2A, 2B, 3A, 3B, 3D, 4A, 4D, 5A, 5B and 6B with phenotypic variations ranging from 2.62% to 12.08%. Four stable QTLs for TGW could be detected in five and seven environments, respectively. Thirty-two QTLs for GL were mapped on chromosomes 1B, 1D, 2A, 2B, 2D, 3B, 3D, 4A, 4B, 4D, 5A, 5B, 6B, 7A and 7B, with phenotypic variations ranging from 2.62% to 44.39%. QGl.cau-2A.2 can be detected in all the environments with the largest phenotypic variations, indicating that it is a major and stable QTL. For GW, 12 QTLs were identified with phenotypic variations range from 3.69% to 12.30%. We found 27 QTLs for GT with phenotypic variations ranged from 2.55% to 36.42%. In particular, QTL QGt.cau-5A.1 with phenotypic variations of 6.82–23.59% was detected in all the nine environments. Moreover, pleiotropic effects were detected for several QTL loci responsible for grain shape and size that could serve as target regions for fine mapping and marker assisted selection in wheat breeding programs.

QTL Mapping for Hull Thickness and Related Traits in Hybrid Rice Xieyou 9308

We conducted a quantitative trait locus (QTL) analysis of 165 rice recombinant inbred lines derived from a cross between Zhonghui 9308 (Z9308) and Xieqingzao B (XB) in Hainan and Hangzhou, China. Grain thickness (GT), brown rice thickness (BRT), hull thickness (HT) and milling quality were used for QTL mapping. HT was significantly and positively correlated with GT and BRT. Twenty-nine QTLs were detected with phenotypic effects ranging from 2.80% to 21.27%. Six QTLs, qGT3, qBRT3, qBRT4, qHT6.1, qHT8 and qHT11, were detected repeatedly across two environments. Inherited from XB, qHT6.1, qHT8 and qHT11 showed stable expression, explaining 9.92%, 21.27% and 10.83% of the phenotypic variances in Hainan and 9.61%, 6.40% and 6.71% in Hangzhou, respectively. Additionally, the QTL cluster between RM5944 and RM5626 on chromosome 3 was probably responsible for GT and milling quality. The cluster between RM6992 and RM6473 on chromosome 4 played an important role in grain filling. Three near isogenic lines (NILs), X345, X338 and X389, were selected because they contained homozygous fragments from Zhonghui 9308, corresponding to qHT6.1, qHT8 and qHT11, respectively. The hull of XB was thicker than those of X345, X338 and X389. In all the lines, qHT6.1, qHT8 and qHT11 that regulated rice HT were stably inherited with obvious genetic effects.

Simple sequence repeat markers reveal multiple loci governing grain-size variations in a japonica rice (Oryza sativa L.) mutant induced by cosmic radiation during space flight

Quantitative trait locus (QTL) for grain size traits that include grain length (GL), grain width (GW), grain thickness (GT) as well as thousand grain weight (TGW) were identified using F2 population derived from a cross between a japonica cultivar Nongken58 and its large grain-sized mutant, ‘Dali’, which was selected in SP2 generation of plants from Nongken58 seeds exposed to cosmic radiation upon space-flight, and then advanced it over eight successive generations by bagging the panicles to ensure self pollination. ‘Dali’ had similar GW and GT but 4.8 mm longer in GL, and 18.1 g heavier in TGW than those of Nongken58. Seven main-effect QTLs (M-QTLs) were identified for the grain size and weight traits. Among them, three M-QTLs, QGs3a and QGs3b for both GL and TGW, and QGs5 for GW, GT and TGW, which had strong additive effects on grain shape and grain weight, were validated in the two F2 plant-derived F3 populations. The three M-QTLs were found to be non-allelic to the cloned genes GS3, GL3.1, qSW5 and QGs5 by comparative mapping. However, there was only one pair of digenic epistasis involving QGs3b for TGW detected in this population. Interestingly, homozygous ‘Dali’ alleles at the QGs3a, QGs3b and QGs5 showed significant increase in the grain size and weight, suggesting these novel alleles of ‘Dali’ at the above three loci may be a very useful for marker-assisted improvement of grain quality for japonica cultivars.

Quantitative trait loci for grain-quality traits across a rice F2 population and backcross inbred lines

Improving grain-quality is an important goal in rice breeding programs. One vital step is to find major quantitative trait loci (QTLs) for quality related traits and then investigate the relationships among them. We crossed ‘N22’, an indica variety with good appearance but low grain weight, to a japonica variety, ‘Nanjing35’, with superior grain yield but poor appearance. This enabled us to construct an F2 population and a set of backcross inbred lines (BILs) for QTL-mapping for the traits related grain appearance. In all, 37 QTLs were identified for grain length (GL), grain width (GW), grain thickness (GT), thousand-grain weight (TGW), and the percentage of grains with chalkiness (PGWC). Of these, 17 QTLs detected from 184 plants in the F2 population explained 4.97–27.26 % of the phenotypic variance, another 20 QTLs were identified using BILs from 2009 to 2010. Quantitative trait loci for major effects were detected in different populations and across years. A new QTL hot spot (marker interval RM504–RM520) was found on Chromosome 3, which harbored QTLs for GL, GW, GT, and TGW. Among our five examined traits, grain shape was significantly correlated with TGW and PGWC. The PGWC values of two heavier grains BILs, L93, and L145 are much lower than Nanjiing35, the analysis of genotype showed that this greater weight may due to the locus for GL occurring within RM504–RM520 on Chromosome 3. Therefore, those two lines will allow us to develop a long-grain high-yield rice variety with less chalkiness.

Identification of QTLs for Grain Traits in Rice Using Extreme Materials in Grain Size

The grain length (GL),grain width (GW),grain thickness (GT),ratio of grain length to width (RLW),and 1000-grain weight (TGW) were evaluated in Beijing.Using a BC2F2 population including 216 lines derived from a backcross combination between GSL156 with large grain (71.9 g) and Chuanqi with small grain (12.1 g).The quantitative trait loci (QTLs) for above five grain traits were identified by composite interval mapping using SSR markers.The results showed that the five grain traits exhibited a normal continuous distribution in BC2F2 population,indicating that they were quantitative traits controlled by multiple genes.A total of 28 QTLs conferring the five grain traits were detected on chromosomes 1,2,3,4,5,6,and 12,respectively.Fourteen QTLs,namely qGL3-2,qGL3-3,qGT12-1,qGT2-1,qGT5-1,qGW1-1,qGW12-1,qGW2-1,qGW5-1,qRLW3-1,qTGW12-1,qTGW2-1,qTGW3-3,and qTGW5-1,were main-effect QTLs and explained 13.70%,52.51%,21.13%,18.79%,20.92%,14.59%,18.33%,30.03%,20.05%,24.53%,13.47%,11.43%,21.30%,and 15.68% of the observed phenotypic variance,respectively.Among them,most QTLs were mapped on chromosome 3.Six QTLs had the alleles contributing to positive effect which were derived from small grain parent Chuanqi while the other 22 QTLs alleles from large grain parent GSL156.The modes of gene action were mainly additive or partial dominance.The marker interval RM7580-RM8208 on chromosome 3 was common to the three QTLs for GW,RLW,and TGW,respectively.The marker interval,RM7636-RM5812 on chromosome 2,RM3351-RM26 on chromosome 5,and RM1103-RM17 on chromosome 12,were common to the three major QTLs,which were associated with GW,GT and TGW,respectively.The eight SSR markers used in this study would be useful in molecular breeding in rice.The alleles from parent with larger grain were showed significant effects on GL,GW,GT,and TGW.

Development of Classification Drying of High Moisture Wheat Grain According to the Moisture Content

A new method of drying early harvested wheat grains having high moisture content has been developed: classification drying based on moisture content using a thickness grader. Grain size is known to be roughly proportional to moisture content. We therefore clarified the relation between grain thickness and moisture content before the drying process, and demonstrated the possibility of new classification drying. Based on the results, we found that a 3.2-mm sieve was optimal for a roughly uniform separation of grains having moisture content of around 35%. For the drying process, the thicker grain group had higher moisture content and its flour was vulnerable to discoloration and degraded starch quality. Moreover, we found that it is important leave grains of high moisture content— the thicker grain group in classification drying—exposed to airflow in order to prevent degraded quality. Practical experiments conducted with a circulating batch dryer showed that the thinner grain group having lower moisture needs less time for drying due to the lower initial moisture content.

Analysis of Surface Integrity in Dry and Cryogenic Machining of <i>AZ31B Mg</i> Alloys

Surface integrity of machined products can have a critical impact on their performance, such as corrosion, wear and/or fatigue resistance. It has been reported that reducing the grain size of AZ31B Mg alloys could significantly enhance its corrosion resistance, which is often the limiting factor for its wide application. Severe plastic deformation (SPD) has proved to be an effective way to induce grain refinement. In this study, the potential of cryogenic machining as a novel SPD method to induce grain refinement on the surface of AZ31B Mg alloys was investigated. The microstructures of the workpiece surface/sub-surface and the machined chips after both dry and cryogenic machining were studied. A surface layer where nanocrystallized grains exist was found in the machined surface under cryogenic conditions. Increasing the edge radius of the cutting tool resulted in a thicker grain refinement layer. In addition to the experimental study, an FE model based on the Johnson-Cook constitutive equation was developed and validated using experimental data in terms of chip morphology and forces. The capability of this model to predict critical deformation parameters for dynamic recrystallization (DRX), such as strain, strain-rate and temperature, was demonstrated. With further development, the model can be used to predict the onset of DRX and the grain size on the machined surface.

Surface Integrity in Dry and Cryogenic Machining of AZ31B Mg Alloy with Varying Cutting Edge Radius Tools

Surface integrity of machined products has a critical impact on their functional performance. Magnesium alloys are lightweight materials for transportation industry and are also emerging as a potential material for temporary biomedical implants. However, their unsatisfactory corrosion resistance limits their application to a great extent. Surface integrity factors, such as grain size, crystallographic orientation and residual stresses, were reported to have significant influence on corrosion resistance of AZ31 Mg alloys. In this study, AZ31B Mg discs were orthogonally turned using cutting tools with two edge radii under both dry and cryogenic conditions. The influence of cutting edge radius and cooling method on surface integrity was investigated. Cryogenic machining using a large edge radius tool led to a thicker grain refinement layer, larger compressive residual stresses and stronger intensity of basal texture, which may remarkably enhance the corrosion performance of magnesium alloys.

QTL analysis for yield related traits using populations derived from an indica-japonica hybrid in rice (Oryza sativaL.)

Introgression has been achieved from a wild species Oryza minuta (2n = 48, BBCC, Acc No.101141) into O. sativa subsp. indica IR71033-121-15. This introgression line was developed at International Rice Research Institute (IRRI) through embryo rescue as well as three backcrosses using IR31917-45-3-2 as a recurrent parent. These two IR lines resemble each other but differ in several important agronomic traits, which can be attributable to O. minuta introgressions. Out of 530 STS markers tested for introgression analysis, at least 14 introgressed chromosomal segments from O. minuta were detected throughout 12 chromosomes. Most of the introgressed segments were quite small in size, as they were detected by single markers and flanking markers were negative for introgressions. A population from the cross between japonica cultivar Junambyeo and introgressed indica line IR71033-121-15, consisting of 146 plants, was evaluated for ten agronomic traits. Genotyping of F2 lines and phenotyping from both F<sub>2</sub> and F<sub>3</sub> were considered for QTL analysis. A total of 11 single-locus QTLs (S-QTLs) were identified for ten traits, i.e. days to heading (DTH), panicle number (PN), spikelet fertility (SF), panicle length (PL), spikelet per panicle (SPP), grain length (GL), grain width (GW), grain length to width ratio (GLW) and grain thickness (GT) in both populations. The O. minuta derived alleles of QTLs, spp6, gl3, glw3, glw5 and t12 were detected in both populations indicating robust QTLs for these traits. QTLs from O. minuta introgression could be new sources of natural genetic variation for genetic improvement of rice.

Residual Stress Relief and Redistribution of Welded Metals by Vibratory Stress Relaxation

In this study, research work on the effects of vibrational energy on the microstructure of welds and Charpy toughness is performed. The results show that vibration during welding exhibits positive effects on the microstructure constituent formation by reducing the amount and thickness of grain boundary ferrite and suppressing the formation of the Widmanstatten structure. And also due to the finer microstructure developed by the vibration, toughness value of the weld metal increases.

Quantification of 39Ar recoil ejection from GA1550 biotite during neutron irradiation as a function of grain dimensions

This study presents the first measurement of 39Ar recoil ejection loss from individual, dimensionally characterized mineral grains due to neutron irradiation, and reveals the extent to which this recoil loss is problematic for 40Ar/ 39Ar dating. Using the well-characterized biotite standard GA1550, known to have between grain reproducibility of 40Ar*/ 39Ar K of order 0.1%, we measured the thicknesses (3–210 μm) and surface areas (0.07–0.90 mm 2) of 159 grains selected to span the dimensional range represented in the aliquot. Thinner grains with high surface area/volume (SA/V) reveal elevated 40Ar/ 39Ar, as much as 26% higher than thicker grains expected to suffer proportionately negligible depletion. Although the thinner grains yield intrinsically less precise measurements due to small 39Ar ion beams, a regular decrease in net recoil loss with increasing biotite grain thickness is clear for grains thinner than ca. 50 μm. Grains thicker than 50 μm reveal essentially no 39Ar loss within analytical uncertainties. The measured 39Ar loss spectrum is significantly higher than predicted by previous modeling approaches. These results suggest a practical threshold of ca. 50 μm grain thickness for biotites, and probably other phyllosilicates, irradiated with 235U fission spectrum neutrons in order to avoid recoil artifacts. Poor agreement between our data and simulation results indicates that recoil displacement models should be revisited in order to resolve the discrepancy. Further empirical work to determine the recoil loss of 39Ar in other minerals is important not only for routine age measurements, but also to shed more light on the role of recoil in multi-diffusion domain theory and other thermochronologic applications exploiting variable diffusion radii and/or grain size effects.