Environmental influences on gamma ray spectrometry
Spatially representative sampling of both natural and anthropogenic deposits in the environment is limited by their inherent heterogenic distribution. This problem is compounded when trying to relate ground measurements which are spatially restricted to remote sensing observations which are not. This work examined these widely experienced problems in the context of the measurement of natural (K, U and Th) and anthropogenic ( 137Cs and ' 34Cs) radioactivity through the three techniques of soil sampling with laboratory based gamma ray spectrometry, in-situ gamma ray spectrometry, and airborne gamma ray spectrometry. These three methods were applied systematically to estimate the radioactivity across a tight geometry valley in Renfrewshire. Activity estimates from field based and airborne gamma spectrometry were compared with each other and with the results of high resolution gamma spectrometry of soil samples to examine the relationship between each method under variable topographic conditions. These results demonstrated that the distribution, and post depositional migration, of activity had important influences on all measurement techniques, and affected the ability to make comparisons between them. Further detailed studies were then conducted to examine these influences. The effects of variations in soil composition and characteristics on environmental gamma ray spectrometry were evaluated by calculation and experimental determination. Corrections to standard laboratory gamma spectrometric procedures were developed to improve systematic precisio:i. These investigations also reviewed soil sampling depth for direct effective comparison with in-situ gamma spectrometry. The effects of small scale sampling errors on activity estimates were demonstrated to have a quantifiable influence on the precision of activity estimation. Lateral variability of activity distribution of natural radioactivity and anthropogenic radioactivity deposited both from the atmosphere and from marine sources has been studied in detail at a number of sites. The extent of variability depends on the nature of activity, its deposition mode and local environmental characteristics. Spatial variability represents an important constraint on the interpretation of activity estimates derived from all methods examined, and on comparisons between them. Statistically representative sampling plans were developed and applied to enable spatial comparisons to be made between soil sample derived activity estimates and in-situ and remotely sensed observations. The influence of the vertical activity distribution on in-situ and airborne measurements has been recognised as an important variable affecting calibration. The use of the information from the scattered gamma ray spectrum to quantify and correct for source burial effects was examined in a series of modelling experiments. A relationship between 'Cs source burial and forward scattering was determined and subsequently applied to a salt marsh environment which showed pronounced subsurface maxima. A spectrally derived calibration correction coefficient was shown to account for variations in source burial across a single site. This provides a potential means for surmounting one of the principal limitations of in-situ gamma spectrometry. As a result of this work it has been possible to account for important environmental factors which affect gamma ray spectrometry in the laboratory, in the field and from aircraft. This has led to the development of sound methodology for comparison between sampling, field based and remote sensing techniques.