Units of modulus 1

If k is an algebraic number field which is normal over the field of rational numbers then it is shown that k has nontrivial units of modulus 1 if and only if the maximal real subfield of k is also a normal extension of the rationals. A characterization of the units is given for fields which satisfy the above conditions. A new proof of Kummer's Theorem on the units of cyclotomic fields is also obtained.     

Shift endomorphisms and compact Lie extensions

We consider skew-products with an arbitrary compact Lie group, when the base map is a one-sided shift of finite type endowed with an equilibrium state of a Hölder continuous function. First we show that the weak-mixing property of the skew-product implies exactness and exponential mixing. Then we address the problem of classification under measure-theoretic isomorphisms. We show that for a generic set of equilibrium states the isomorphism class of the skew-products corresponds essentially to the cohomology classes of the defining skewing function and the isomorphism is essentially a homeomorphism.Dedicated to Ricardo MañéPartially supported by CNPq.     

The valence band of graphite monofluoride

Good agreement is shown to exist between the theoretically computed (modified extended Hückel) and the experimentally determined (X-ray photoelectron spectroscopy) density-of-states for graphite monofluoride.     

A novel method for the determination of migration of contaminants from food contact materials

A neutron activation method has been developed for the analysis of high density polyethylene, low density polyethylene, polypropylene, polyethylene terephthalate and polystyrene food contact plastics. The method provides determination of over 50 elements at concentrations below 1 mg kg^–1. This technique has now been extended to study migration from food contact materials into standard food simulants (olive oil, acetic acid, ethanol and water). Samples of plastic are irradiated in a thermal neutron flux to produce radionuclides of the elements present in the plastic. Over a period of time the radionuclides of these elements may travel from the plastic into the food simulants, and hence the migration can be determined. Gamma ray spectrometry is performed on the simulants at the end of the test to quantify the migration. Any activity present must be due only to the migration of radionuclides of elements in the plastic and nothing else. This eliminates the need for a blank determination, which is required with existing migration methods. Preliminary studies have shown that detection limits of around 0.002 mg kg^–1 can be achieved for Sb in a retail polyethylene terephthalate (PET) bottle. This can be compared to levels of 0.005     

Platinum metal concentrations in urban road dust and soil in the United Kingdom

Concentrations of Pt, Pd, Rh and Au in soils and road dusts taken from areas of high and low traffic flows in the London Borough of Richmond and from a section of the Kingston bypass (A3) at New Malden, Surrey, have been measured. High concentrations of platinum are associated with high traffic densities. Samples taken from streets of lower traffic flows were found to contain the lower concentrations of the ranges. These values correlated well with the levels of lead which were also high at roundabouts. If the preliminary results obtained in this study apply more generally throughout the UK, then the potential for exposure to enhanced levels of Pt would appear to be higher for road users and for those living in urban environments or along major highways.     

The validation of a method for determining the migration of trace elements from food packaging materials into food

A new radiotracer method has been developed to measure the migration of trace elements from food contact packaging into four standard food simulants; acetic acid, ethanol, olive oil, deionised water. A sample of material is irradiated in a thermal neutron flux of 10¹⁶n·m^–2·s^–1 to activate the trace elements and produce a range of radionuclides. The sample is then placed in the food simulant and the migration of the radionuclides is monitored by performing -ray spectrometry on a sample of the simulant. Any radionuclides measured must be due entirely to the migration of the elements present in the plastic, since the simulant itself is not radioactive. Preliminary studies have shown that detection limits of around 0.2g·dm^–2 (0.002 mg/kg) can be achieved for antimony in a sample of polyethylene terephthalate. This method can now been extended to measure migration into real foods. This will highlight any differences between the standard simulants currently used and real foods. Since the method only involves irradiation of the packaging material, any food matrix can be studied.