An automated gamma-ray counting system for fast neutron activation analysis

An automated gamma-ray counting system was designed and built for use in counting long-lived nuclides (T>hr) produced during analysis by fast neutron activation. The system is mechanically simple, vet interfaced to sophisticated control and counting equipment for completely automated counting.     

Synthesis and characterisation of tungsten(VI) oxo-salicylate complexes for use in the chemical vapour deposition of self-cleaning films

Tungsten(VI) oxo-salicylate complexes were prepared in moderate yield (47 to 63%) by the reactions of WOCl4 and two equivalents of either 3-methylsalicylic acid (MesaliH2) or 3,5-di-isopropylsalicylic acid (di-i-PrsaliH2). Performing the reaction in refluxing toluene afforded the two analogous ditungsten complexes 1, [{WO(Mesali)(MesaliH)}2(mu-O)], and 2, [{WO(di-i-Prsali)(di-i-PrsaliH)}2(mu-O)], however in refluxing hexane the mononuclear tungsten complex , [WO(di-i-Prsali)(di-i-PrsaliH)Cl], was isolated. The single crystal X-ray study of revealed a pseudo-octahedral geometry around the tungsten centres. Aerosol assisted chemical vapour deposition of or afforded brown tungsten trioxide thin films. These films were converted to yellow fully oxidised WO3 on annealing in air at 550 degrees C for 30 minutes. The yellow WO3 films demonstrate preferred orientation on the substrate and show interesting functional properties-photo induced hydrophilicity and photocatalytic activity.     

Synthesis of 4'-methyl and 4'-cyano carbocyclic 2',3'-didehydro nucleoside analogues via 1,4-addition to substituted cyclopentenones

Carbocyclic 4'-methyl and 4'-cyano nucleoside analogues were synthesized using the Michael reaction to introduce the 4'-substituent and Pd-catalyzed allylic substitution to introduce the nucleoside base. Use of both the desired beta- and undesired alpha-1'-carbonate diastereomers in the Pd-catalyzed substitution was demonstrated in principle by epimerization of the alpha-diastereomer and kinetic diastereodifferentiation of a 1:1 alpha/beta mixture of 1'-carbonates.     

Self-amplification of coherent spontaneous emission in a cherenkov free-electron maser

Ultrashort pulses of microwave radiation have been produced in a dielectric-lined Cherenkov free-electron maser (FEM) amplifier. An intense initial seed pulse, due to coherent spontaneous emission (CSE), arises at the leading edge of the electron pulse. There is evidence to show that 3-4 cycle spikes are produced through the amplification of these seed pulses. A strong dependence of the start-up power on the rise time of the electron pulse has been found. The experimental results are verified by a theoretical analysis. Our study shows that amplification in a FEM amplifier is always initiated by CSE arising from the edge of the electron pulse when the rise time is comparable to the electromagnetic wave period.     

Generation of powerful subnanosecond microwave pulses by intense electron bunches moving in a periodic backward wave structure in the superradiative regime

Experimental results of the observation of coherent stimulated radiation from subnanosecond electron bunches moving through a periodic waveguide and interacting with a backward propagating wave are presented. The subnanosecond microwave pulses in Ka and W bands were generated with repetition frequencies of up to 25 Hz. The mechanism of microwave pulse generation was associated with self-bunching, and the mutual influence of different parts of the electron pulse due to slippage of the wave with respect to the electrons; this can be interpreted as superradiance. The illumination of a panel of neon bulbs resulted in a finely structured pattern corresponding to the excitation of the TM01 mode. Observation of rf breakdown of ambient air, as well as direct measurements by hot-carrier germanium detectors, leads to an estimate of the absolute peak power as high as 60 MW for the 300-ps pulses at 38 GHz. These results are compared with numerical simulations. The initial observation of 75-GHz, 10-15-MW radiation pulses with a duration of less than 150 ps is also reported.     

Theory and design of a free-electron maser with two-dimensional feedback driven by a sheet electron beam

The use of two-dimensional Bragg resonators of planar geometry, realizing two-dimensional (2D) distributed feedback, is considered as a method of producing spatially coherent radiation from a large sheet electron beam. The spectrum of eigenmodes is found for a 2D Bragg resonator when the sides of the resonator are open and also when they are closed. The higher selectivity of the open resonator in comparison with the closed one is shown. A time-domain analysis of the excitation of an open 2D Bragg resonator by a sheet electron beam demonstrates that a single-mode steady-state oscillation regime may be obtained for a sheet electron beam of width 100-1000 wavelengths. Nevertheless, for a free-electron maser (FEM) with a closed 2D Bragg resonator, a steady-state regime can also be realized if the beam width does not exceed 50-100 wavelengths. The parameters for a FEM with a 2D planar Bragg resonator driven by a sheet electron beam based on the U-2 accelerator (INP RAS, Novosibirsk) are estimated and the project is described.     

Solid-state 19F NMR spectroscopy reveals that Trp41 participates in the gating mechanism of the M2 proton channel of influenza A virus

The integral membrane protein M2 of influenza A virus assembles as a tetrameric bundle to form a proton-conducting channel that is activated by low pH. The side chain of His37 in the transmembrane alpha-helix is known to play an important role in the pH activation of the proton channel. It has also been suggested that Trp41, which is located in an adjacent turn of the helix, forms part of the gating mechanism. Here, a synthetic 25-residue peptide containing the M2 transmembrane domain was labeled with 6F-Trp41 and studied in lipid membranes by solid-state 19F NMR. We monitored the pH-dependent differences in the 19F dipolar couplings and motionally narrowed chemical shift anisotropies of this 6F-Trp41 residue, and we discuss the pH activation mechanism of the H+ channel. At pH 8.0, the structural parameters implicate an inactivated state, while at pH 5.3 the tryptophan conformation represents the activated state. With the aid of COSMOS force field simulations, we have obtained new side-chain torsion angles for Trp41 in the inactivated state (chi1 = -100 degrees +/- 10 degrees , chi2 = +110 degrees +/- 10 degrees ), and we predict a most probable activated state with chi1 = -50 degrees +/- 10 degrees and chi2 = +115 degrees +/- 10 degrees . We have also validated the torsion angles of His37 in the inactivated state as chi1 = -175 degrees +/- 10 degrees and chi2 = -170 degrees +/- 10 degrees .     

Azo-containing phosphine complexes of palladium(II) and platinum(II) and their effectiveness in the Heck reaction: crystal and molecular structure of [PdCl2{PPh2{1-(4-MeC6H4N2)-2-OC(O)Me---C10H5}]·2EtOH

Reaction of Na[MCl₄] (M=Pd or Pd) with the azo-containing phosphines Ph₂P{1-(4-RC₆H₄N₂)-2-OR′-C₁₀H₅} {R=Me (I), NMe₂ (II); R′=C(O)Me} affords the complexes [MCl₂L₂] (1–4) in good yield. Complexes 1–4 have all been fully characterised by elemental analysis, ¹H-, ¹³C{¹H}-, and ³¹P{¹H}-NMR spectroscopy and UV–visible spectroscopy. The use of 1 in the Heck reaction has been investigated and shown to effect up to 1000 turnovers.     

C Selective Polarization and Spin Diffusion in a Lipid Bilayer-Bound Polypeptide by Solid-State NMR

While ¹⁵N solid-state NMR has proven to be very advantageous for the development of structural biological methods, ¹³C spectroscopy has increased sensitivity and spectral dispersion. However, large natural abundance signals and homonuclear dipolar interactions pose significant problems. Here we have used a pair of ¹³C-labeled sites in a lipid-solubilized polypeptide to show the selective polarization can be used in combination with spin diffusion to achieve simplified spectra. Both unoriented and oriented samples have been used, with the latter providing a well-resolved homonuclear dipolar splitting.     

A new distortion energy-based equivalent stress for multiaxial fatigue life prediction

A new equivalent stress amplitude expression has been developed for the assessment of fatigue life in components under multiaxial loading. The expression was generated by incorporating non-linear/plastic stress–strain relation into a mechanical energy calculation, and then applying the calculation to the distortion energy theory for a cyclic loading case. Therefore, the new uniaxial equivalent stress expression determines an appropriate stress amplitude value for multiaxial cyclic loading. The purpose of the equivalent stress value is to determine multiaxial fatigue failure using an energy-based fatigue life prediction criterion. The governing understanding behind the criterion states that the physical damage quantity for failure is equal to the accumulated strain energy in a monotonic fracture, which is also equal to the accumulated strain energy during fatigue failure. Using the new equivalent stress amplitude expression and the energy-based life prediction method, a comparison is made between prediction results and multiaxial empirical data. The multiaxial data was acquired by a vibration-based biaxial bending fatigue test and a torsion fatigue test with an assumed axial misalignment. The results of the comparison provide encouragement regarding the capability of the newly developed equivalent stress amplitude expression for fatigue life prediction.