Modal behavior of photonic-crystal vertical-cavity surface-emitting diode laser analyzed with Plane Wave Admittance Method

A three-dimensional, full vectorial model has been applied for investigation the modal characteristics of a phosphide photonic-crystal vertical-cavity surface-emitting diode laser. The photonic crystal has been defined as a regular, hexagonal net of holes, etched in the upper, p-type Distributed Bragg Reflector of the laser. The electromagnetic field has been confined to the active region by a single defect of the photonic crystal providing overlapping with carriers funneled by a tunnel junction. We have determined the range of design parameters, which provide single mode operation.     

First positron experiments at NEPOMUC

The in-pile positron source NEutron induced POsitron source MUniCh (NEPOMUC) of the new Munich research reactor FRM-II is now operated at the nominal reactor power of 20 MW. Recently, intensity and positron beam profile measurements were performed at 30 eV and 1 keV, respectively. For this purpose, NaI-scintillators detect the 511 keV γ-radiation of positrons that annihilate at a removable target in the beam line. The beam profile is determined with a micro-channel plate detector and a CCD-camera. In the present arrangement of NEPOMUC's instrumentation the positron beam is connected to a coincident Doppler broadening (CDB) facility and to a positron induced Auger electron spectroscopy (PAES) analysis chamber. First experiments were carried out in order to show the performance of these new spectrometers. An overview of the positron beam facility is given and first experimental results of PAES are presented.     

Fluorimetric detector and sensor for flow analysis made of light emitting diodes

Two compact optoelectronic fluorimetric devices operating according to the paired-emitter-detector-diode concept have been developed. The fluorimetric detector, fabricated of three light emitting diodes only, has been applied for the development of fluorimetric optosensor by further integration with sensing solid phase. In these investigations as a model analyte and as a model sensing layer useful for solid phase spectrometry, riboflavin and C18-silica have been chosen, respectively. Both developed analytical devices have been applied for non-stationary fluorimetric measurements performed under conditions of flow injection analysis. The presented flow-through detector and sensor operating under given flow conditions offer riboflavin determination in mg L⁻¹ and μg L⁻¹ ranges of concentration, respectively.     

二氟甲基自由基反应及其在18F标记中的应用

随着正电子发射断层扫描(PET)技术的进步, 高效¹⁸F标记反应显现出重大的研究价值. 同时, 在有机氟化学的快速发展下, 研究人员已开发出一系列产生二氟甲基自由基(·CHF₂)的一碳合成子, 得以被引入到¹⁸F标记反应中. 本文从化学原理出发, 总结了近年来·CHF₂合成子、 碳氢键活化反应及¹⁸F标记的研究, 并展望了未来的研究趋势.     

Effect of thermal annealing on the structure and magnetism of Fe-doped ZnO nanocrystals synthesized by solid state reaction

High purity Fe₂O₃/ZnO nanocomposites were annealed in air at different temperatures between 100 and 1200 °C to get Fe-doped ZnO nanocrystals. The structure and grain size of the Fe₂O₃/ZnO nanocomposites were investigated by X-ray diffraction 2θ scans. Annealing induces an increase of the grain size from 25 to 195 nm and appearance of franklinite phase of ZnFe₂O₄. Positron annihilation measurements reveal large number of vacancy defects in the interface region of the Fe₂O₃/ZnO nanocomposites, and they are gradually recovered with increasing annealing temperature. After annealing at temperatures higher than 1000 °C, the number of vacancies decreases to the lower detection limit of positrons. Room temperature ferromagnetism can be observed in Fe-doped ZnO nanocrystals using physical properties measurement system. The ferromagnetism remains after annealing up to 1000 °C, suggesting that it is not related with the interfacial defects.     

A novel one-pot palladium-mediated synthesis of N-[(2-hydroxyphenyl)methyl]-N-(4-phenoxy-3-pyridinyl) acetamide, the precursor to [C]PBR28, a PET biomarker for the peripheral benzodiazepine receptor

Due to an urgent need to image the peripheral benzodiazepine receptor (PBR) in living human subjects using positron emission tomography imaging, we had cause to prepare N-[(2-hydroxyphenyl)methyl]-N-(4-phenoxy-3-pyridinyl) acetamide (desmethyl-PBR28 (1)), the precursor to [¹¹C]PBR28. Herein, we report a new synthesis of the precursor in which palladium-mediated reduction of the nitro pyridine to the corresponding amino pyridine, and subsequent reductive amination, can be achieved with decaborane in a convenient one-pot procedure. This procedure is operationally simpler than the current alternatives and provides high quality precursor suitable for use in clinical applications.     

Fully automated synthesis of N-labeled nitrosothiols

In the present Letter, a fast, automated, and reproducible method for the synthesis of S-[¹³N]nitrosothiols is reported. The labeling strategy is based on trapping in an anion exchange resin. The reaction with thiols in acidic media led to the formation of the desired nitrosothiols in short reaction times (60 s) with excellent radiochemical conversions (from 48.7% to 74.5%). Final radiotracers were purified by HPLC. Good radiochemical yields (from 33.8% to 60.6%, decay corrected) and radiochemical purities (>99%) were obtained in all cases. Stability of the labeled compounds was checked.     

Tetrabutylammonium fluoride-promoted α-[C]methylation of α-arylesters: a simple and robust method for the preparation of C-labeled ibuprofen

Tetrabutylammonium fluoride-promoted α-[¹¹C]methylation of α-arlylesters was developed. The method was amenable to the remote-controlled synthesis of ¹¹C-labeled ibuprofen.     

A new smoothing-regularization approach for a maximum-likelihood estimation problem

We consider the problem min _i=1 ^m (aⁱ,x–b_iloga ⁱ, z) subject tox 0 which occurs as a maximum-likelihood estimation problem in several areas, and particularly in positron emission tomography. After noticing that this problem is equivalent to mind(b, Ax) subject tox 0, whered is the Kullback-Leibler information divergence andA, b are the matrix and vector with rows and entriesa ⁱ,b _i, respectively, we suggest a regularized problem mind(b, Ax) + d(v, Sx), where is the regularization parameter,S is a smoothing matrix, andv is a fixed vector. We present a computationally attractive algorithm for the regularized problem, establish its convergence, and show that the regularized solutions, as goes to 0, converge to the solution of the original problem which minimizes a convex function related tod(v, Sx). We give convergence-rate results both for the regularized solutions and for their functional values.The research of A. N. Iusem was partially supported by CNPq Grant No. 301280/86-MA.