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Past weapons production activities have resulted in mass quantities of trans-uranic waste being buried in drums at several sites in the United States. In an effort to relocate these waste drums to more permanent storage sites, Fluor Hanford has begun characterizing their contents to ensure compliance with various shipping and storage requirements. Non-destructive analysis techniques are regularly employed, among them passive radiation detection using a Canberra Gamma-Energy-Analyzer germanium detector vault. Necessary strict legal tolerances require strong quality assurance. The detectors are frequently calibrated in the traditional method with check sources, but it would be advantageous to have an estimate of system minimum detectable activity (MDA). However, any estimate is complicated by the fact that sources are distributed stochastically in the waste drums. In this study, a method was developed to predict system detector efficiency for a variety of detector configurations and drum fill materials and calculate MDA based on these efficiencies. The various system designs were modeled in Monte Carlo N-Particle Code, version 4b, to determine photopeak detection efficiency. An external code written in C programming language was used to randomly assign between one and 20 sources to volumetric regions of the waste drum. Twenty simulations were performed for each design and drum fill material combination, each time redefining the stochastically distributed source. This provided a normally distributed spectrum of 20 efficiencies for each situation. From this, mean and lower 95% confidence limit efficiencies were used to calculate MDA. The patterns among the results were then compared with values predicted by the MDA formula. Finally, an examination was made of the impact on the MDA of the system's true design in the case of single or multiple detector failure. The results indicate that this method of estimating minimum detectable activity, although costly in computing time, provides results consistent with intuitive and calculated expectations. Future work would allow easy calibration of the model to measured efficiency results. Used in coordination with physical experiments, this method may eventually prove useful in benchmarking system performance and accurately ensuring reliable waste drum characterizations. / Graduation date: 2003

This thesis is concerned with the estimation of the strength of a radioactive source when decay is rapid so that the usual assumption of a Poisson distribution of counts is not applicable. We distinguish two main cases according as the background radiation is small or large. In the former case the common procedure of simply subtracting off a constant background radiation is adequate. If the background radiation is neglected altogether the method of maximum likelihood may be used to estimate both source strength and decay-constant for counts taken at any arbitrary times. The large-sample variances and covariances of these estimates are obtained and the procedure is illustrated on an actual set of experimental results. When the background radiation is large their randomness should be taken into account. The exact distribution of counts is the sum of two independent variates, one a binomial and one a Poisson. However, a normal approximation with the same mean and variance will often suffice. We draw attention to a procedure due to Tandberg who considered the problem of obtaining a single optimum count. The method of maximum likelihood applied in conjunction with the assumption of normality of total counts is compared with his method in a numerical example. The possibility of taking two counts is also considered and the source strength is estimated by a least squares approach. / M.S.

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In this thesis, I discuss coherent manipulation of a trapped ytterbium ion using long wavelength radiation and the results of experiments towards the performance of a two-ion entanglement gate using a static magnetic field gradient of 23.3(6) T/m to create coupling between an ion's internal state and its motion. After using these experiments to explain the requirements for high-fidelity entanglement operations, I continue by examining existing methods for creating this gradient, the current limiting factor in producing the highest fidelity operations. This includes a full characterisation of the gradients produced by symmetric scheme permanent magnets and buried current carrying wires including development of scaling laws in order to create optimum gradients for a given trap geometry. I continue by proposing a new method by which extremely high gradients over 100 T/m can be created for planar chip traps with minimal modification to an existing experiment. These gradients are tailored for axial as well as radial entanglement schemes and aim to show that the technology exists in order to produce a two-qubit gate over the fault tolerant threshold. Subsequently, I discuss the implementation of this new scheme in an experiment before constructing the apparatus to accurately align a chip with these magnets and documenting their installation into two new experimental setups. This includes a preliminary measurement of the gradient produced by an imperfect setup outside of vacuum which verifies those simulated at ~ 110 T/m. Lastly, I discuss the prospects of on-chip magnetic materials and propose a new method which when sufficiently developed should allow for high magnetic field gradients to be produced on-chip at higher ion heights than when solely using current carrying wires. Additionally this scheme should allow for switchable gradients with maximised stability in geometries previously not possible to create.

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Proefschrift--Utrecht. / Summaries in English, French, German and Dutch. Bibliography: p. [81]-82.