In-focus electron microscopy of frozen-hydrated biological samples with a Boersch phase plate

We report the implementation of an electrostatic Einzel lens (Boersch) phase plate in a prototype transmission electron microscope dedicated to aberration-corrected cryo-EM. The combination of phase plate, C_s corrector and Diffraction Magnification Unit (DMU) as a new electron-optical element ensures minimal information loss due to obstruction by the phase plate and enables in-focus phase contrast imaging of large macromolecular assemblies. As no defocussing is necessary and the spherical aberration is corrected, maximal, non-oscillating phase contrast transfer can be achieved up to the information limit of the instrument. A microchip produced by a scalable micro-fabrication process has 10 phase plates, which are positioned in a conjugate, magnified diffraction plane generated by the DMU. Phase plates remained fully functional for weeks or months. The large distance between phase plate and the cryo sample permits the use of an effective anti-contaminator, resulting in ice contamination rates of <0.6 nm/h at the specimen. Maximal in-focus phase contrast was obtained by applying voltages between 80 and 700 mV to the phase plate electrode. The phase plate allows for in-focus imaging of biological objects with a signal-to-noise of 5-10 at a resolution of 2-3 nm, as demonstrated for frozen-hydrated virus particles and purple membrane at liquid-nitrogen temperature.     

静电单透镜在低能强流离子束输运中的应用

本文介绍北京大学重离子所RFQ组利用静电单透镜进行低能强流离子束束流输运的情况.在北京大学1 MeV ISR RFQ加速器的升级改造中,我们在低能输运段使用了2个静电单透镜进行调束,当引出电压为22kV时,在RPQ入口处得到了归一化均方根发射度为0.12 7× mm·mrad、峰值流强6 mA的脉冲氧离子束(相当于24mA质子束流).以及用静电单透镜聚焦强流氢离子束的初步实验实验.     

Characterization of a single electrode focusing lens for ion beam deceleration

An ion beam deceleration system was studied for the high-current ion implanter at the Laboratório de Aceleradores e Tecnologias de Radiação at the Campus Tecnológico e Nuclear, of Instituto Superior Técnico. The installed system consists of a target plate and one electrostatic focusing lens with one electrode. This article describes the results of the evaluation of the new system. With this upgrade, the ion implanter provides enhanced versatility for decelerating to 5 keV a high current ion beam at the µA level. This implantation provides a wide area and allows for a continuous magnetic beam scanning, extending the energy range to lower values, opening up a wider set of applications.     

Water relations and leaf growth rate of three Agropyron genotypes under water stress

The effects of water stress on leaf water relations and growth are reported for three perennial tussock grass genotypes under glasshouse conditions. Studies were performed in genotypes El Palmar INTA and Selección Anguil of Agropyron scabrifolium (Döell) Parodi, and El Vizcachero of A. elongatum (Host) Beauv. Agropyron scabrifolium El Palmar INTA is native to a region with warm-temperate and humid climate without a dry season, and an average annual precipitation of 900 mm. Agropyron scabrifolium Selección Anguil comes from a region with a sub-humid, dry to semiarid climate and a mean annual precipitation of 600 mm. Agropyron elongatum is a widespread forage in semiarid Argentina with well-known water stress resistance. A mild water stress treatment was imposed slowly; plants reached a minimum pre-dawn leaf water potential of about -1.83 MPa by day 21 after watering was withheld. In all genotypes, water stress led to a reduction of leaf growth. There was a tendency for a greater epicuticular wax accumulation on water-stressed plants of A. scabrifolium Selección Anguil and A. elongatum than on those of A. scabrifolium El Palmar INTA. This may have contributed to obtain greater turgor pressures and relative water contents in the first two than in the later genotype. In turn, this may have contributed to determine smaller leaf growth rate reductions in A. scabrifolium Selección Anguil and A. elongatum than in A. scabrifolium El Palmar INTA under water stress. This study demonstrated variation in water stress resistance between genotypes in A. scabrifolium, and between A. scabrifolium Selección Anguil and A. elongatum versus A. scabrifolium El Palmar INTA, which was related to their differential responses in water relations.     

钾改性Beta分子筛Brönsted酸性能对十氢萘加氢异构及加氢裂化反应的影响

为了提高十氢萘加氢转化生成桥环异构体的选择性,采用钾（K⁺）后浸渍法对Beta分子筛进行了酸性调节;采用X-射线衍射（XRD）、扫描电子显微镜（SEM）、N₂吸附-脱附（BET）、红外光谱（Py-IR）等分析手段对改性前后的分子筛进行了表征。采用孔饱和浸渍法制备了Pt/Beta-Al₂O₃催化剂,在氢气压力4.0 MPa、温度240~300 ℃、氢/油摩尔比12.62、质量空速10.56 h^-1的条件下,以十氢萘为模型化合物进行了加氢异构化和加氢裂化反应研究。结果表明：随着K₂O负载质量分数从0.105%增加到0.403%,改性前后分子筛的结构及孔道性质未发生明显变化,分子筛BrÖnsted(B)酸酸量从209 μmol/g逐渐下降到123.9 μmol/g;十氢萘加氢转化过程遵循异构化-开环-裂化连串反应机理,随着反应温度升高,异构选择性降低,开环和裂化反应选择性升高,总转化率增大;反应温度低于280 ℃时,桥环异构产物选择性可以达到90%以上。在反应温度300 ℃的条件下,随着K⁺改性分子筛的B酸酸量降低,产物异构选择性增加,开环选择性和裂化反应选择性降低。K+通过抑制裂化反应可以明显提高桥环产物的选择性。