Nicholas C. Darnton

Electrodeless Dielectrophoresis of Single- and Double-Stranded DNA

Dielectrophoretic trapping of molecules is typically carried out using metal electrodes to provide high field gradients. In this paper we demonstrate dielectrophoretic trapping using insulating constrictions at far lower frequencies than are feasible with metallic trapping structures because of water electrolysis. We demonstrate that electrodeless dielectrophoresis (EDEP) can be used for concentration and patterning of both single-strand and double-strand DNA. A possible mechanism for DNA polarization in ionic solution is discussed based on the frequency, viscosity, and field dependence of the observed trapping force.

Influence of topology on bacterial social interaction

The environmental topology of complex structures is used by Escherichia coli to create traveling waves of high cell density, a prelude to quorum sensing. When cells are grown to a moderate density within a confining microenvironment, these traveling waves of cell density allow the cells to find and collapse into confining topologies, which are unstable to population fluctuations above a critical threshold. This was first observed in mazes designed to mimic complex environments, then more clearly in a simpler geometry consisting of a large open area surrounding a square (250 × 250 μm) with a narrow opening of 10–30 μm. Our results thus show that under nutrient-deprived conditions bacteria search out each other in a collective manner and that the bacteria can dynamically confine themselves to highly enclosed spaces.

A large-scale low-background liquid scintillation detector: the counting test facility at Gran Sasso

A 4.8 m3 unsegmented liquid scintillation detector at the underground Laboratori Nazionali del Gran Sasso has shown the feasibility of multi-ton low-background detectors operating to energies as low as 250 keV. Detector construction and the handling of large volumes of liquid scintillator to minimize the background are described. The scintillator, 1.5 g PPO/L-pseudocumene, is held in a flexible nylon vessel shielded by 1000 t of purified water. The active detector volume is viewed by 100 photomultipliers, which measure time and charge for each event, from which energy, position and pulse shape are deduced. On-line purification of the scintillator by water extraction, vacuum distillation and nitrogen stripping removed radioactive impurities. Upper limits were established of < 10−7 Bq/kg-scintillator for events with energies 250 keV < E < 800 keV, and < 10−9 Bq/kg-scintillator due to the decay products of uranium and thorium. The isotopic abundance of 14C12C in the scintillator was shown to be approximately 10−18 by extending the energy window of the detector to 25–250 keV. The 14C abundance and uranium and thorium levels in the CTF are compatible with the Borexino Solar Neutrino Experiment.

Lifetimes of intermediates in the β-sheet to α-helix transition of β-lactoglobulin by using a diffusional IR mixer

The extremely slow α-helix/β-sheet transition of proteins is a crucial step in amylogenic diseases and represents an internal rearrangement of local contacts in an already folded protein. These internal structural rearrangements within an already folded protein are a critical aspect of biological action and are a product of conformational flow along unknown metastable local minima of the energy landscape of the compact protein. We use a diffusional IR mixer with time-resolved Fourier transform IR spectroscopy capable of 400-μs time resolution to show that the trifluoroethanol driven β-sheet to α-helix transition of β-lactoglobulin proceeds via a compact β-sheet intermediate with a lifetime of 7 ms, small compared with the overall folding time of β-lactoglobulin.

Measurement of the 14C abundance in a low-background liquid scintillator

Abstract The 14 C/ 12 C ratio in 4.8 m 3 of high-purity liquid scintillator was measured at (1.94±0.09)×10 −18 , the lowest 14 C abundance ever measured. At this level the spectroscopy of low-energy solar neutrinos, in particular a measurement of the 7 Be neutrino flux, will not be obstructed by the 14 C β decay intrinsic to a liquid scintillator detector. A comprehensive study of the deviation of the shape of the 14 C β spectrum from the allowed statistical shape reveals consistent results with recent observations and calculations. Possible origins of the 14 C in the liquid scintillator are discussed.

Light propagation in a large volume liquid scintillator

The fluorescence light propagation in a large volume detector based on organic liquid scintillators is discussed. In particular, the effects of the fluor radiative transport and solvent Rayleigh scattering are emphasized. These processes have been modelled by a ray-tracing Monte Carlo method and have been experimentally investigated in the Borexino prototype which was a 4.3 ton, 4π sensitive detector. The comparison between the model prediction and the experimental data shows a satisfactory agreement indicating that the main aspects of these processes are well understood. Some features of the experimental time response of the detector are still under study.

Hydrodynamics in 2½ dimensions: making jets in a plane

We show that a careful analysis of the Navier-Stokes equation in the low Reynolds number limit has two distinct solutions, one valid for a deep, thin curtain of flow and the other for a thin wide flow. We derive a solution to the latter situation and use the results to develop a new way to control fluid flows in thin, wide sheet flow.

REMOVAL OF COSMOGENIC 7BE FROM SCINTILLATORS

Abstract Cosmogenic Be can provide a substantial background to low threshold and low counting rate scintillation detectors such as the Borexino solar neutrino detector. The efficacy of various 7 Be removal techniques is examined. Distillation appears to be the best with a reduction factor of at least 1000 per stage.