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Thesis (Ph. D.)--University of Hong Kong, 2002. / Includes bibliographical references (leaves 174-186).

Thesis (Ph. D.)--University of Wisconsin--Madison, 1962. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Measurements of the radioactive content of environmental samples are potentially very costly, especially when these are made ex situ in a laboratory. A less expensive alternative is to acquire in situ measurements in the field. Both measurement types are subject to uncertainties, some of which arise from different sources depending on the measurement method used. Surveys on radioactively contaminated land found that in situ measurements produced results that were as useful in satisfying the typical objectives of such surveys as ex situ measurements. The random component of analytical uncertainty estimated from duplicated in situ measurements was 2-4 times higher than would have been expected from Poisson statistics, however the sampling uncertainty (0-10 %) was found to be much lower than that for ex situ measurements (44-73 %). This resulted from the combined effects of high heterogeneity of the target radionuclide (137Cs) in the ground, and the comparatively large primary sample mass associated with in situ measurements of gamma-emitting radionuclides. A large sampling mass also means that in situ measurements have an advantage in finding small hotspots of activity, although they may not provide sufficient resolution for spatially mapping lateral distributions of contaminants for remediation purposes. The degree of resolution can be readily changed in the field, however, by the simple expedient of changing the detector height. Experiments with an in situ detector close to the ground surface enabled the position of a small hotspot to be determined to within a few centimetres. To evaluate activity concentrations in the soil, assumptions need to be made about the dimensions of the measured sample, and the distributions of activity within it. This requires some information that might be best obtained from ex situ measurements of excavated samples. However, well planned in situ surveys have the potential to significantly reduce the requirement for these expensive laboratory measurements. A new method of optimising the design of in situ surveys has been developed, based on a generic model for predicting the detector response to small particles of activity at different positions relative to the detector. The new mathematical model used by this method compares well with field measurements, and also with predictions made using a commercially available calibration program.

In order to determine the physical location of contaminants in soil, two solidified soil "thin" sections, which preserve the undisturbed structural characteristics of the original soil, containing weapons-grade plutonium from the Rocky Flats Environmental Test Site were prepared. Two autoradiographic methods were used in radionuclide mapping: contact autoradiography using CR-39�� plastic alpha track detectors and neutron-induced autoradiography that produced fission fragment tracks in Lexan�� plastic detectors. The combination of the two autoradiographic methods distinguished alpha-emitting particles of natural U, from ������������������Pu and non-fissile alpha-emitters. The locations of 990 alpha "stars" caused by plutonium "hot particles" in two soil sections were recorded, particles were sized, their size-frequency and depth distributions were analyzed. Several large colloidal conglomerates of "hot particles" were found in soil profiles. One such conglomerate with a geometric size of about 500 ��m contained over 94% of the total recorded contaminant alpha activity in the sample. It was found that the upper 6.5 cm of soil contained 20% of all recorded particles (mean equivalent size 0.35 ��m). The deeper portion of the surface 6.5 cm soil layer contained 80% of the particles (with mean equivalent size 0.25 ��m). The average specific activity (SA) for 989 hot particles (with the conglomerate of the particles excluded) with equivalent diameters over 80 nm was found to be greater than 23.9 Bq g����� (about 90% of the overall average SA). For dissolved and defragmented (below 80 nm of equivalent diameter) actinide particles, SA was found smaller than 2.9 Bq g�����. Over 99% of the total actinide contaminant activity in the analyzed soil sample (with the conglomerate of the particles included) was found in particles with equivalent diameters over 0.08 ��m. it suggests that larger particles of plutonium oxide moved down more slowly than smaller ones and no significant breakup of plutonium oxide particles occurred since the original plutonium soil deposition. Both profiles of the depth contaminant activity and number of actinide particles have distinctive peaks at the same depth, about 10 cm. Independent from nuclear track analysis, this pattern of the actinides depth distribution was observed in the measured gamma activity depth profile of �������Am (daughter product of �������Pu beta decay) in solidified soil blocks. / Graduation date: 2001

Soil that is contaminated with radioactive elements poses an exposure hazard to those whom may take up temporary or permanent residence on such a site. Of particular interest is the internal exposure from ingestion of this radioactive soil. Although most ingestion of soil is inadvertent, usually being attached to foodstuffs that are not properly cleaned, it is possible that a person might consume a larger quantity. Childhood soil ingestion from simple hand-to-mouth activities is one explanation for this larger intake, as well as geophagia (eating dirt) or pica (craving and eating non-food items). The assumption that any person might consume a "mouthful" of dirt is a rare but possible occurrence that, when analyzed, will help with decisions about safe contamination levels of soil. Samples of soils contaminated with radium-226 were sent from an engineering and environmental firm to Oregon State University's Department of Nuclear Engineering and Radiation Health Physics for assessment. The analysis of the samples was aimed at the determination of bioavailability and bioaccessibility of the radioactive species found in the soils. Subsequent site remediation actions for the New Jersey-based project would be partially dictated by the results of Oregon State University's testing. Initially, the soils were tested for the presence of carbonates, for leachability of radioactivity in water and in acid, and for particle size distribution, i.e., soil type. Each of the eight samples was then subjected to a stomach/intestinal analogue to determine how much of the radioactivity would be transferred to solution upon human ingestion, (bioaccessibility). Mass balance and gamma spectrometry outputs for the soil samples before and after the digestion was one way the loss to solution was assessed. Another method to determine the loss of radioactivity to solution was to count aliquots of the digestive fluids in a high purity germanium detector, using a library of only radium isotopes and their progeny to locate peaks. The combination of results from mass balance and gamma spectrometry outputs allowed for OSU's researchers to determine the bioaccessibility of each soil's radioactive components. Using the determined bioaccessibility and previous animal models, the determination of bioavailability varied between the samples, from zero to 28% of the total initial radioactivity in the samples. A hot particle estimation of the dose from the non-bioavailable portion of the samples yielded a high dose to a small number of cells. Assuming ingestion of the most radioactive sample, (Sum-03a), the amount of damaged (killed) tissue in each section of the gastrointestinal tract was estimated to be less than 0.0407 cm³. This small volume of tissue is not likely to result in evident damage as the healthy human gastrointestinal tract regenerates all surface cells approximately every six days and most items are resident in the digestive system for less than 48 hours. / Graduation date: 2006

No description available.

Graduation date: 1979

No description available.

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