Beam chopper development at LAMPF

In order to reduce pileup limitations on μSR data rates, a fast chopper for surface muon beams was built and tested at LAMPF. The system allowed one muon at a time to be stopped in a μSR sample in the following way: A surface beam from the LAMPF Stopped Muon Channel was focused through a crossed-field beam separator and onto a chopper slit. With the separator E and B fields adjusted properly, the beam could pass through the slit. The beam to the μSR sample was turned on or off (chopped) rapidly by switching the high voltage applied to the separator plates on or off within approximately 500 ns; with the E field off, the B field deflected the beam, dumping it near the slit. We demonstrated that, with improved electronics, we will be able to stop a single muon in a μSR sample as frequently as once every 20 μs and that data rates for the system can be a factor of five higher than is attainable with unchopped beams. The observed positron contamination of the beam was less than five percent, and the ratio of the muon rate with beam on to the rate with beam off was 1540.     

The Krein Spectral Shift Function in Semifinite von Neumann Algebras

We show the existence of a spectral shift function in the sense of Krein for bounded trace class perturbations of a self-adjoint operator affiliated with a semifinite von Neumann algebra     

Stretching liquid bridges with bubbles: the effect of air bubbles on liquid transfer

Liquid bridges containing bubbles are relevant to industrial printing and are also a topic of fundamental scientific interest. We use flow visualization to study the stretching of liquid bridges, both with and without bubbles, at low capillary numbers. We find that whereas the breakup of wetting fluids between two identical surfaces is symmetric about the bridge midpoint, contact line pinning breaks this symmetry at slow stretching speeds for nonwetting fluids. We exploit this observation to force air bubbles selectively toward the least hydrophilic plate confining the liquid bridge.     

Estimation of the lateral to basal surface ratio of talc by inverse gas chromatography

Inverse Gas Chromatography at Infinite Dilution, i.e. IGC‐ID, allows following the evolution of surface properties of talc impregnated with increasing concentrations of polyethyleneglycol of molecular weight 20,000. Coupling Inverse chromatography in infinite dilution conditions measurements with controlled surface modification, we have developed a new method to estimate the lateral to basal surface ratio of talc crystals.     

Azaarenes: 13 Rings in a Row by Cyclopentannulation

Cyclopentannulation was explored as a strategy to access large, stable azaarenes. Buchwald–Hartwig coupling of previously reported di- and tetrabrominated cyclopentannulated N,N′-dihydrotetraazapentacenes furnished stable azaarenes with up to 13 six-membered rings in a row and a length of 3.1 nm. Their optoelectronic and semi-conducting properties as well as their aromaticity were investigated.