Simulation of electromagnetic shower formation in a Germanium crystal

Abstract

When high-energy electrons penetrate crystalline matter, the successive processes of photon emission and pair production form an electromagnetic shower. If the incident electrons are directed along the crystal axis, the cross section for photon emission is drastically enhanced because electrons in ‘channeling’ states feel a strong electric field continuously. Experiments designed to detect this effect were performed at CERN. The results showed an anomalous peak in the energy loss spectrum of the emerging electrons. In this paper, we report results of a Monte-Carlo simulation of shower formation in a Germanium crystal. Our results agree with the experimental data more quantitatively than previous simulations. We simulated a shower formation by incident photons as well.     

A C4N4 Diaminopyrimidine Fluorophore

A new scaffold for producing efficient organic fluorescent materials was identified: 2,5-diamino-4,6-diarylpyrimidine featuring a C4N4 elemental composition. Single-step installation of two aryl groups at the 4,6-positions of the pyrimidine core delivered fluorescent organic materials in a modular fashion. A range of fluorescent compounds with distinct absorption/emission properties was readily accessed by changing the aromatic attachments. A generally high absorption coefficient and quantum yield were observed, including C4N4 derivatives that could fluoresce even in the solid state. The two amino groups at the 2,5-positions of the pyrimidine were essential for intense fluorescence with a large Stokes shift, which was corroborated by structural relaxation to a p-iminoquinone-like structure in the excited state. Besides live-cell imaging capabilities, fluorescent labeling of a protein involved in autophagy elucidated a new protein–protein interaction, supporting potential utility in bioimaging applications.     

Development of the 8-aza-3-bromo-7-hydroxycoumarin-4-ylmethyl group as a new entry of photolabile protecting groups

A significant substitution effect of the position of the bromo group on the photosensitivity of the 8-azacoumarin chromophore leads to the development of a highly photosensitive 8-aza-3-bromo-7-hydroxycoumarin-4-ylmethyl (aza-3-Bhc) group that shows excellent photolytic efficiency and hydrophilicity with long-wavelength absorption maxima. The newly identified aza-3-Bhc group can be applied to caged glutamates for ester-type and carbamate-type protections of carboxyl and amino functionalities.     

Alternative two‐step matrix application method for imaging mass spectrometry to avoid tissue shrinkage and improve ionization efficiency

Mass spectrometry (MS) was used to measure the concentrations of drug and biological compounds in plasma and tissues. Matrix‐assisted laser desorption/ionization (MALDI) imaging MS (IMS) has recently been applied to the analysis of localized drugs on biological tissue surfaces. In MALDI‐IMS, matrix application process is crucial for successful results. However, it is difficult to obtain homogeneous matrix crystals on the tissue surface due to endogenous salts and tissue surface heterogeneity. Consequently, the non‐uniform crystals degrade the quality of the spectrum and likely cause surface imaging artifacts. Furthermore, the direct application of matrix solution can cause tissue shrinkage due to the organic solvents. Here, we report an alternative two‐step matrix application protocol which combines the vacuum deposition of matrix crystals and the spraying of matrix solution to produce a homogeneous matrix layer on the tissue surface. Our proposed technique can also prevent cracking or shrinking of the tissue samples and improve the ionization efficiency of the distributed exogenous material. Copyright © 2013 John Wiley & Sons, Ltd.     

A Payne�CRayner Type Inequality for the Robin Problem on Arbitrary Minimal Surfaces in {\mathbb{R}^N}

We prove a Payne?CRayner type inequality for the first eigenfunction of the Laplacian with Robin boundary condition on any compact minimal surface with boundary in ${\mathbb{R}^N}$ . We emphasize that no topological condition is necessary on the boundary.     

Fast single-photon avalanche diode arrays for laser Raman spectroscopy

We incorporate newly developed solid-state detector technology into time-resolved laser Raman spectroscopy, demonstrating the ability to distinguish spectra from Raman and fluorescence processes. As a proof of concept, we show fluorescence rejection on highly fluorescent mineral samples willemite and spodumene using a 128×128 single-photon avalanche diode (SPAD) array with a measured photon detection efficiency of 5%. The sensitivity achieved in this new instrument architecture is comparable to the sensitivity of a technically more complicated system using a traditional photocathode-based imager. By increasing the SPAD active area and improving coupling efficiency, we expect further improvements in sensitivity by over an order of magnitude. We discuss the relevance of these results to in situ planetary instruments, where size, weight, power, and radiation hardness are of prime concern. The potential large-scale manufacturability of silicon SPAD arrays makes them prime candidates for future portable and in situ Raman instruments spanning numerous applications where fluorescence interference is problematic.     

Efficient Light‐Harvesting Antennae Resulting from the Dense Organization of Dyes into DNA Junctions through d‐Threoninol

Herein we report the construction of efficient light‐harvesting antennae by hybridization of DNA oligonucleotides containing high densities of fluorophores into DNA junctions through d ‐threoninol. Six pyrene donors could be incorporated into each arm without self‐quenching. A perylene acceptor was located at the center of the junction. Antenna effects of a duplex and three‐ to eight‐way junctions were systematically compared. Six‐ and eight‐way junctions had the highest antenna effects, and their effective absorption coefficients were 8.5 times higher than that of perylene. Interestingly, even‐numbered junctions had higher efficiencies than odd‐numbered junctions. Nondenaturing gel analyses and fluorescence lifetime measurements demonstrated that the strong odd–even effects were derived from differences in the stability of junctions. The results presented will guide the design of efficient artificial photosynthetic systems.