独居石微晶玻璃中玻璃相含量的红外光谱定量测定

独居石微晶玻璃由偏磷酸盐玻璃和独居石两相组成。玻璃和独居石的红外吸收谱带彼此不相重叠,且1275和616cm~(-1)谱带的吸收强度随玻璃相的含量变化而变化。两谱带的对数吸光度比值与玻璃相含量(w％)的相关系数r=0.9975,其回归方程为y=48.356 25.93x。合成独居石的IR谱中952和616cm~(-1)谱带的吸光度比值也随不同的Ce_2O_3/La_2O_3比值规律变化。其r=0.9917,回归方程为y=0.2211exp(0.0221x)。高的相关性表明IR技术可在磷酸盐微晶玻璃物相定量分析中得到实际应用。     

Flash photolysis electron spin resonance and CIDEP studies of radicals derived from nitrogen heterocyclics. II. The photophysics and photochemistry of the methylpyrazines

Application of the CIDEP phenomenon is extended to the identification of the specific triplet energy levels which react to produce radicals in pyrazine and its methyl derivatives. Advantage is taken of the proximity of ³nπ and ³ππ states in which the opposite signs of the zero-field coupling constants cause triplet mechanism polarization of opposite phase in the derived radicals. The electronic natures of the radicals observed parallel those of the triplet states from which they are formed.     

Practical all-fiber source of high-power, 120-fs pulses at 1 micrometre

Amplification of femtosecond pulses at 1.03 micrometre in a standard Yb-doped single-mode fiber is reported. A pulse energy of 8 nJ and an average power of 400 mW are obtained, limited by available pump power. To our knowledge these are the highest pulse energy and average power obtained from an integrated, single-mode fiber amplifier. After dechirping, 120-fs, 6-nJ pulses are obtained. A practical fiber-based source with performance comparable with that of a bulk solid-state laser is thus demonstrated, and scaling to substantially higher powers will be possible.     

Propagation in air by field superposition of scattered light within a Fresnel fiber

Propagation of light at 1.5 microm with peak intensity in an air hole is achieved within an air-silica-structured Fresnel waveguide. Thus a simpler fiber design alternative to photonic crystal fibers is possible for high-peak-power propagation with reduced nonlinear interactions. Multiple foci are observed in the far field.     

Magnetic resonance imaging screening in women at genetic risk of breast cancer: imaging and analysis protocol for the UK multicentre study. UK MRI Breast Screening Study Advisory Group

The imaging and analysis protocol of the UK multicentre study of magnetic resonance imaging (MRI) as a method of screening for breast cancer in women at genetic risk is described. The study will compare the sensitivity and specificity of contrast-enhanced MRI with two-view x-ray mammography. Approximately 500 women below the age of 50 at high genetic risk of breast cancer will be recruited per year for three years, with annual MRI and x-ray mammography continuing for up to 5 years. A symptomatic cohort will be measured in the first year to ensure consistent reporting between centres. The MRI examination comprises a high-sensitivity three-dimensional contrast-enhanced assessment, followed by a high-specificity contrast-enhanced study in equivocal cases. Multiparametric analysis will encompass morphological assessment, the kinetics of contrast agent uptake and determination of quantitative pharmacokinetic parameters. Retrospective analysis will identify the most specific indicators of malignancy. Sensitivity and specificity, together with diagnostic performance, diagnostic impact and therapeutic impact will be assessed with reference to pathology, follow-up and changes in diagnostic certainty and therapeutic decisions. Mammography, lesion localisation, pathology and cytology will be performed in accordance with the UK NHS Breast Screening Programme quality assurance standards. Similar standards of quality assurance will be applied for MR measurements and evaluation.     

Effect of ortho‐methylation on a bis(4‐cyanatocumyl)benezene low‐dielectric resin

A synthesis procedure for and characterization of a tetra‐ortho‐methyl substituted three‐ringed cyanate ester monomer, 4,4′‐[1,3‐phenylenebis(1‐methylethylidene)]bis[2,6‐dimethyl]phenylcyanato, and the corresponding cured resin are described with comparative characterization of the resin analogue without the ortho‐methyl substitutents. The othro‐methylation lowered the dielectric from 2.64 to 2.59 at 1 GHz and from 2.64 to 2.48 GHz at 15 GHz, whereas the tan δ was not significantly affected. Other physical properties also affected included a 40 °C decrease in the glass‐transition temperature, an increase in the thermal‐expansion coefficients, and a reduction in thermogravimetric stability. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 60–67, 2003