Acquisition and reconstruction of data in three-dimensional positron emission tomography (3D PET) was introduced in 1990 almost 20 years after the first PET scanners were developed. 3D PET offers a significant sensitivity improvement over conventional, sliceoriented 2D PET, but at the cost of a three-fold increase in acceptance of scattered events. In addition, processing time is increased and new methods for applying corrections such as for photon attenuation, calibration, and detector/geometry normalisation are required. 3D PET raised concerns that the high quantitative accuracy that was possible with 2D PET (with its moderate sensitivity) would not be matched in 3D, primarily because of the greatly increased scattered photon component in the measured data. The aim of this thesis was to develop methods that enable quantitatively accurate measurements with 3D PET. A technique to correct for scattered photons prior to reconstruction has been developed, implemented and assessed. A device for normalising the data for detector efficiency and the geometry of the cylindrical detector system has been developed, and the factors affecting reconstruction investigated. A new approach to calibration of the reconstructed data to produce images of activity concentration which is independent of scatter has been implemented. Finally, the techniques have been applied to data from brain scans of human subjects. Evaluation of images reconstructed from 3D PET demonstrates that the methodology developed in this work produces data accurate to within 10% of the true activity concentration in an object with reasonably homogeneous density. 3D PET is shown to be as accurate as 2D PET, but with a sensitivity advantage that improves signal-to-noise by approximately a factor of three in the human brain and slightly less in other regions of the body.

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In the studies related to trace analysis in meteorology, hydrology, medical and clinical research, the measurement and monitoring of tritium released from nuclear facilities and its health physics aspects on human and the environment are of growing importance. The techniques employed currently for the enrichment of tritium are carried out using large samples and require long periods of operation (7 days or more) to obtain the enriched tritium sample. In the present investigation, a laboratory scale chromatography based system was designed, constructed and commissioned successfully to collect enriched tritiated water samples which were then counted to determine the levels of activity. The total time taken to conduct the complete enrichment procedure and counting of the collected samples was less than one working day. This system also showed the capability of enriching quantities of water samples as small as 20 ml. This was about ten times less than required in the conventional enrichment techniques. Extensive efforts were made to attain optimum operational, reproducible and efficient measurements of tritium enrichment for quantitative analysis. The developed experimental technique involved injecting a known volume of water into a preheated furnace to react with magnesium turnings, in the presence of a carrier gas, to produce the hydrogen isotopes. These isotopes were separated using the principles of chromatography. Liquid scintillation counting method was employed to determine the activity of the collected samples. Two gases, nitrogen and helium, were utilized as the carrier gases during this investigation. Best values of enrichment were obtained with the use of nitrogen as a carrier gas, but the samples collected were difficult to count due to the formation of ammonia with consequent chemical complications.

Atmospheric testing of nuclear weapons and nuclear reactor accidents has resulted in widespread distribution of radioactive fission products into the environment. Determination of these radionuclides in biological and environmental samples presents serious difficulties because of interferences (90Sr with a continuous beta spectrum needs separation from other radionuclides, such as 90Y) and the poor limits of detection established by currently available counting equipment. In this project an ion exchange chromatography procedure for the separation of Cs, Sr and Y was developed. Since these radionuclides are hazardous, the method was developed using stable isotopes and inductively coupled plasma mass spectrometry (ICP-MS) as an elemental detector. In the first instance, the optimum operating conditions of the instrument for the analysis of Cs, Sr and Y were determined. These values were found to be 1.1 kW incident power and 1 1 min-1 nebuliser flow rate. The accuracy of the ICP-MS measurments was then assessed through the use of international reference materials NIST:SRM 1572 Citrus Leaves and NIST:SRM 1573 Tomato Leaves. In general a good agreement was observed for most of the elements between certified values and the results obtained in this study. The developed separation technique consisted of three stages. In the first stage ammonium molybdophosphate (AMP) and BioRad AG 50W-X8 resins were used. Caesium was separated from the other two isotopes using 2 M NH4OH. In the second stage Sr and Y were eluted from the above mentioned column using 4 M HNO3. The solution was then loaded onto a Sr-SpecRTM column where Sr is retained. In the third stage Sr was eluted from the column using deionised water. This setup provided excellent recoveries for the three stages namely: 85 % for Cs, 93 % for Y and 95 % for Sr. The analysis of Cs, Sr, and Y in foodstuffs, namely beans and milk was carried out after using the aforementioned separation procedure. Different growth media (commercial white silica sand, Lightwater garden soil, commercial garden soil, Rose Batch compost, and commercial compost) were used to assess the effect of organic content on the uptake of Cs and Sr by bean plants. These experiments involved the addition of different doses of added elemental concentration (100 to 1000 ?g ml-1) to the substrate and also various solution pH's (4 to 9). It was observed that the higher uptake of Cs and Sr by bean plants was at solution pH 7 and also for growth media with high organic content, i.e. Rose Batch compost and commercial compost. The distribution of Cs and Sr into the bean plant tissue resulting through absorption from commercial compost, showed that Cs is absorbed mostly in the stem (95 %) and with minimal levels in root and leaves (5 %). On the other hand, Sr was readily absorbed to the aboveground parts of the bean plants (36 % stem, 63 % in leaves) in contrast to other parts; 0.3 % root and 0.05 % beans. A study was undertaken to assess the uptake of Cs and Sr by bean plants in the presence of K and Ca at various solution pH's. It was found that in general, for all solution pH's, potassium inhibits the uptake of Cs. On the other hand, there appears to be a competition between Sr and Ca at high pH (7 to 9) which resulted in a rapid reduction of the Sr uptake. The addition of Ca to the substrate decreased the uptake of Sr by stems/leaves and increased the uptake of Sr by beans. As an application, solutions with different pH and radionuclide 137Cs and 90Sr) levels were added to mature bean plants. The developed ion exchange chromatography procedure was then adopted to separate 137Cs and 90Sr from these plants. In this study a Ge(Li) gamma ray spectrometer (ND 66) was used as a detector for 137Cs whilst a liquid scintillation counter was used for measuring 90Sr. Prior to the 137Cs measurements, the detection limits, resolution, absolute full energy peak and calibration of the instrument were evaluated. In general the values obtained in this work were in good agreement with the certified values.

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The biogeochemical behaviour of plutonium and americium was investigated in contaminated soils from the UK to help determine possible remediation and management options. Stimulating anoxic sediments from Aldermaston, through the addition of a carbon substrate (glucose), induced reducing conditions resulting in a negligible change in Pu mobility. This was despite a substantial shift in the bacterial profile from a diverse community to one dominated by fermentative Beta proteobacteria and Clostridia. The latter group also includes organisms associated with metal reduction, such as close relatives to Clostridium species, reported previously to facilitate the reduction of Pu(IV) to Pu(III). A sequential extraction was performed on soils from Aldermaston and the Esk Estuary to identify which selected fractions the Pu and Am are most strongly associated with. The majority of Pu was associated with the 'residual fraction': 63.8 – 85.5 % and 91.9 – 94.5 % in the Aldermaston and Esk Estuary soils respectively. Metals associated with this fraction are highly recalcitrant and are unlikely to be released into solution over a significant time span under most geological conditions. The Am was more evenly distributed with the 'organic fraction' being the most dominant. Degradation of organic matter under oxidising conditions may result in mobilization of metals associated with this fraction. The Aldermaston soil was also subjected to bioleaching using a sulfuric acid producing microbial community, which resulted in a maximum 0.18 % of Pu released into solution. However, up to 12.5% of Am was found in solution suggesting Am is more susceptible to mobilization than Pu. The potential for Pu mobilization through abiotic oxidative leaching was investigated using permanganate. Even when carbonate was added to act as a potential complexant for the Pu, less than 1% of the Pu was leached. Greater success was observed when leaching was attempted using citric acid; an estimated 25 – 30% of Pu was released into solution offering a potential route for remediation of Pu-contaminated soils. These data would suggest that the Pu is highly recalcitrant, and may exist in a small particulate form in the Aldermaston soils, possibly in the oxide form, and is unlikely to mobilize under natural biogeochemical conditions.