Determining the efficiency of a broad-energy HPGe detector using Monte Carlo simulations

Broad-energy HPGe detectors have a useful range of 3 keV to 3 MeV, making them ideal for the assay of environmental samples. Such measurements however, are hindered by variations in the sample matrix, summing effects, and the Compton continuum. Detectors may be characterised by proprietary software in such a situation, however Monte-Carlo modelling is a useful, inexpensive alternative that also provides greater flexibility when determining the detector response and efficiency during a measurement. In the current work, a full GEANT4 model of a broad-energy HPGe detector is presented, and simulations of various samples are compared to experimental data. These are found to be accurate within 3 % at a confidence level of 95 % for energies from 30 to 3,000 keV, with greater variations below 100 keV due to an increased sensitivity to geometrical inaccuracies.     

An External Matrix-Assisted Laser Desorption Ionization Source for Flexible FT-ICR Mass Spectrometry Imaging with Internal Calibration on Adjacent Samples

We describe the construction and application of a new MALDI source for FT-ICR mass spectrometry imaging. The source includes a translational X-Y positioning stage with a 10×10 cm range of motion for analysis of large sample areas, a quadrupole for mass selection, and an external octopole ion trap with electrodes for the application of an axial potential gradient for controlled ion ejection. An off-line LC MALDI MS/MS run demonstrates the utility of the new source for data- and position-dependent experiments. A FT-ICR MS imaging experiment of a coronal rat brain section yields ～200 unique peaks from m/z 400-1100 with corresponding mass-selected images. Mass spectra from every pixel are internally calibrated with respect to polymer calibrants collected from an adjacent slide.     

Self-assembly and conductivity of hydrogen-bonded oligothiophene nanofiber networks

Symmetric oligothiophene derivatives containing hydrogen bond forming segments create self-supporting organogels consisting of self-assembled 1D nanostructures at low concentrations. Hydrogen bond formation and π-π stacking were both found to be crucial for the formation of conductive supramolecular networks of 1D nanostructures.     

Controlling the melting of kinetically frozen poly(butyl acrylate-b-acrylic acid) micelles via addition of surfactant

We have studied the melting of polymeric amphiphilic micelles induced by small-molecule surfactant and explained the results by experimental determination of the interfacial tension between the core of the micelles and the surfactant solutions. Poly(n-butyl acrylate-b-acrylic acid) (PBA-b-PAA) amphiphilic diblock copolymers form kinetically frozen micelles in aqueous solutions. Strong interactions with surfactants, either neutral or anionic [C12E6, C6E4, sodium dodecyl sulfate (SDS)], were revealed by critical micelle concentration (cmc) shifts in specific electrode and surface tension measurements. Since both polymer and surfactant are either neutral or bear negative charges, the attractive interactions are not due to electrostatic interactions. Light scattering, neutron scattering, and capillary electrophoresis experiments showed important structural changes in mixed PBA-b-PAA/surfactant systems. Kinetically frozen micelles of PBA-b-PAA, that are hardly perturbed by concentration, ionization, ionic strength, and temperature stresses, can be disintegrated by addition of small-molecule surfactants. The interfacial energy of the PBA in surfactant solutions was measured by drop shape analysis with h-PBA homopolymer drops immersed in small-molecule surfactant solutions. The PBA/water interfacial energy gammaPBA/H2O of 20 mN/m induces a high energy cost for the extraction of unimers from micelles so that PBA-b-PAA micelles are kinetically frozen. Small-molecule surfactants can reduce the interfacial energy gammaPBA/solution to 5 mN/m. This induces a shift of the micelle-unimer equilibrium toward unimers and leads, in some cases, to the apparent disintegration of PBA-b-PAA micelles. Before total disintegration, polymer/surfactant mixtures are dispersions of polydisperse mixed micelles. Based on core interfacial energy arguments, the disintegration of kinetically frozen polymeric micelles was interpreted by gradual fractionation of objects (polydisperse dispersion mechanism), whereas the disintegration of polymeric micelles in a thermodynamically stable state was interpreted by an exchange between a population of large polymer-rich micelles and a population of small surfactant-rich micelles (bidisperse dispersion mechanism). Finally, in our system and other systems from the literature, interfacial energy arguments could explain why the disintegration of polymer micelles is either partial or total as a function of the surfactant type and concentration and the hydrophobic block molar mass of the polymer.     

Hybrid fixed point theorems for multivalued mappings in Banach algebras under a weak topology setting

In this paper, we introduce a new class of multivalued mappings of the form \({\frac{I-C}{A}}\), where \({A}\) and \({C}\) are multivalued mappings acting on Banach algebras. Using this concept, we present some fixed point theorems in Banach algebras satisfying a certain sequential condition (\({P}\)) in the weak topology setting. Our main tool is the axiomatic measure of weak noncompactness. Our results generalize, extend and improve wellknown results on the subject. An application to illustrate our theory is included.     

A method for detection of spectral spin diffusion from minor peaks, and its application to F MAS NMR of antimony(III)-doped fluorapatite

A new technique for detecting spectral spin diffusion in solids under MAS NMR conditions that is particularly well suited for accurately measuring cross-relaxation from minor spectral components is presented. The pulse sequence, SINK (Saturation Inter-Nuclear Kinetics), selectively saturates the magnetization of a minor spectral component with a series of rotor-synchronized DANTE pulse trains and monitors spin diffusion to other peaks with a non-selective 90° pulse. We have used SINK with ¹⁹F MAS NMR on samples of calcium fluorapatite doped with Sb³⁺ to measure spin diffusion between a weak peak at 68.6 ppm due to fluoride ions associated with Sb³⁺ and other peaks in the spectrum. The SINK experiment clearly demonstrates that spin diffusion from the former peak to the main resonance of fluorapatite at 64.0 ppm is faster than spin diffusion to a second antimony-related peak at 65.6 ppm. These results strengthen our previous conclusion that antimony(III) occupies a phosphate site in the apatite lattice, with an SbO₃³⁻ group replacing a PO₄³⁻ group. The SINK experiment also enables the detection of a “hidden” peak at approximately 62.9 ppm that is otherwise obscured by the intense main peak at 64.0 ppm.