David Benton

Developments in particle tracking using the Birmingham Positron Camera

Abstract The RAL/Birmingham Positron Camera consists of a pair of MWPCs for detecting the pairs of back-to-back 511 keV photons arising from positron-electron annihilation. It was constructed in 1984 for the purpose of applying PET to engineering situations, and has been widely used for the non-invasive imaging of flow, including extensive studies on geological samples. The technique of Positron Emission Particle Tracking (PEPT), whereby a single positron-emitting tracer particle can be tracked at high speed, was developed at Birmingham and has proved a very powerful tool for studying the behaviour of granular materials in systems such as mixers and fluidised beds. In order to extend its effective field of view, the camera has recently been mounted on a motorised translation stage under computer control so that the motion of a tracer particle can be followed over a length of up to 1.5 m. A preliminary investigation into the feasibility of enhancing the PEPT technique using the singles count rates in the two detectors has also been undertaken.

Investigation of paste flow using positron emission particle tracking

Abstract The method of positron emission particle tracking (PEPT) has been adapted for use during extrusion. A particle 2 mm in diameter was tracked through a 45° conical die geometry using ram speeds of 5 and 10 mm min −1 . The particle was placed at different points across the diameter of the barrel in order to determine the dependence of the speed and strain rate on the initial position. Analysis of the speed of the particle during extrusion showed that the clay based paste used in the experiment exhibited plug flow in the barrel and the die land regions. The maximum extensional strain rate was shown to be independent of the ram speed, whereas the shear strain rate at the wall was highly dependent on the ram speed.

The use of positron emission tomography to monitor unsaturated water flow within porous construction materials

In principle, positron emission tomography (PET) offers a non-destructive method to monitoring water flows in all types of porous construction materials.

Microwave Properties of an Inhomogeneous Optically Illuminated Plasma in a Microstrip Gap

The optical illumination of a microstrip gap on a thick semiconductor substrate creates an inhomogeneous electron-hole plasma in the gap region. This allows the study of the propagation mechanism through the plasma region. This paper uses a multilayer plasma model to explain the origin of high losses in such structures. Measured results are shown up to 50 GHz and show good agreement with the simulated multilayer model. The model also allows the estimation of certain key parameters of the plasma, such as carrier density and diffusion length, which are difficult to measure by direct means. The detailed model validation performed here will enable the design of more complex microwave structures based on this architecture. While this paper focuses on monocrystalline silicon as the substrate, the model is easily adaptable to other semiconductor materials such as GaAs.

Behavioural interactions between imidazoleacetic and γ-hydroxybutyric acid in rats and mice

Abstract Mice and rats were injected with hypnotic doses of imidazoleacetic acid and γ-hydroxybutyric acid either singly or in combination. γ-Hydroxybutyric acid, a normal metabolite of GABA in the brain, was the more potent drug in the rat and imidazoleacetic acid, a normal metabolite of histamine, the more potent drug in the mouse. Imidazoleacetic acid given 30 min prior to the γ-hydroxybutyric acid produced a potentiation of the hypnotic effects in the rat. When the drugs were given either simultaneously or in the reverse order, no potentiation was observed. Since the potentiation was specific to the rat it was proposed that the reflect differences in the activities of histamine-metabolizing enzymes between the two species.

Hyperfine structure and isotope shifts in natural vanadium

High-resolution crossed beam laser spectroscopy has been performed on an atomic beam of natural vanadium. The hyperfine structure was measured in highly resolved spectra for 25 transitions from the first metastable states, 3d 4 .a 5 D/4s a 6 DJ , to two high-lying multiplets, 3d 3 4s.a 5 F/4p z 6 D o and 3d 3 4s.a 5 F/4p z 6 F o . This yielded the hyperfine A.J/ and B.J/ coefficients of 16 51 V fine structure states, including the six previously unstudied levels in the z 6 F o multiplet. Excellent agreement was found between these values and those obtained from atomic beam magnetic resonance experiments for the metastable states. In addition, the hyperfine structure of 50V was measured in eight transitions to the upper state multiplet z 6DJ . This resulted in the evaluation of hyperfine coefficients for all 10 levels of the upper and lower states, previously unknown, as well as the first optical isotope shift measurements for vanadium.

Developments towards practical free-space quantum cryptography.

We describe a free space Quantum cryptography system which is designed to allow continuous unattended key exchanges for periods of several days, and over ranges of a few kilometres. The system uses a four laser faint pulse transmission system running at a pulse rate of 10MHz to generate the required four alternative polarization states. The receiver module similarly automatically selects a measurement basis and performs polarization measurements with four avalanche photodiodes. The controlling software can implement the full key exchange including sifting, error correction, and privacy amplification required to generate a secure key.

Optical isotope shifts in the iron atom

Isotope shifts between and have been measured for five transitions between 300 and 306 nm in the optical spectrum of the iron atom. The data were obtained by means of high-resolution laser spectroscopy utilizing frequency doubling into the ultraviolet. The data have been examined by means of a King plot. Field shifts factors have been obtained allowing electron densities at the nucleus to be determined. Hyperfine structure in the odd isotope has also been observed in both natural and isotopically enriched samples and hyperfine structure coefficients determined for the upper states. An average mutual screening factor for the ground state electrons is presented.

High resolution laser spectroscopy measurements of the hyperfine structure of 138La

Laser spectroscopy of a thermal atomic beam of natural lanthanum has been performed. An active filtering technique has been used to remove spurious cavity modes from the laser output, leading to unambiguous identification of hyperfine structure associated with 138La. Hyperfine structure coefficients and isotope shifts for several transitions of the isotopes 138La and 139La are presented.

Technique for passive scene imaging of gas and vapor plumes using transmission-waveband modulation

A new approach to locating gas and vapor plumes is proposed that is entirely passive. By modulating the transmission waveband of a narrow-band filter, an intensity modulation is established that allows regions of an image to be identified as containing a specific gas with absorption characteristics aligned with the filter. A system built from readily available components was constructed to identify regions of NO. Initial results show that this technique was able to distinguish an absorption cell containing NO gas in a test scene.