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Measurements of the Distribution And QUEENSLAND UNIVERSITY OF TECHNOLOGY SCHOOL OF PHYSICAL AND CHEMICAL SCIENCES MEASUREMENTS OF THE DISTRIBUTION AND BEHAVIOUR OF BERYLLIUM-7 IN THE NATURAL ENVIRONMENT Che Doering B. App. Sc., M. App. Sc. A thesis submitted in partial fulfilment of the requirements of the degree of Doctor of Philosophy 2007 i KEYWORDS Beryllium-7, environment, environmental radioactivity, radionuclide, cosmogenic, cosmic- rays, atmosphere, deposition, soil, surface air, atmospheric transport, erosion, depositional flux, areal activity density, Brisbane, Southeast Queensland, Australia ii ABSTRACT Beryllium-7 is a cosmogenic radionuclide produced in the atmosphere through the spallation of nitrogen and oxygen nuclei by cosmic-ray-produced neutrons and protons. It is carried in the atmosphere attached to aerosols and is deposited on land and ocean surfaces by wet and dry deposition processes. Beryllium-7 decays by electron capture to lithium-7 and has a half-life of approximately 53 days. It is a potentially useful radionuclide for studying different natural processes. This thesis presents a collection of scientific papers on the occurrence of beryllium-7 in the natural environment, particularly in the Southeast Queensland region of Australia. It shows the results of experimental measurements and discusses their implications. Overall, this thesis contributes to advancing our understanding of the distribution and behaviour of beryllium-7 in the natural environment and provides a foundation for the development of nuclear techniques for the evaluation of environmental problems. iii LIST OF PUBLICATIONS Doering, C., Akber, R., Heijnis, H., 2006. Vertical distributions of 210Pb excess, 7Be and 137Cs in selected grass covered soils in Southeast Queensland, Australia. Journal of Environmental Radioactivity 87, 135–147. Doering, C., Akber, R., submitted. Beryllium-7 in near-surface air and deposition at Brisbane, Australia. Journal of Environmental Radioactivity. Doering, C., Akber, R., submitted. Describing the annual cyclic behaviour of 7Be concentrations in surface air in Oceania. Journal of Environmental Radioactivity. iv TABLE OF CONTENTS CHAPTER ONE 1 INTRODUCTION 1.1 Environmental radioactivity and beryllium-7 1 1.2 Research objectives 2 1.3 Link between the scientific papers 2 CHAPTER TWO 4 LITERATURE REVIEW 2.1 Introduction 4 2.2 Nuclear properties 4 2.3 Activity measurement 5 2.4 Cosmogenic production 5 2.5 Production through nuclear detonations 6 2.6 Activity size distribution 7 2.7 Atmospheric residence time and concentration 8 2.8 Surface air concentrations 9 2.9 Deposition 15 2.10 Distribution in ocean waters 20 2.11 Distribution in soils and grasses 21 2.12 Environmental applications 24 2.13 Annual effective dose 28 2.14 Summary and conclusions 28 References 29 CHAPTER THREE 43 VERTICAL DISTRIBUTIONS OF 210Pb EXCESS, 7Be AND 137Cs IN SELECTED GRASS COVERED SOILS IN SOUTHEAST QUEENSLAND, AUSTRALIA v 3.1 Introduction 46 3.2 Materials and methods 47 3.2.1 Soil sampling sites 47 3.2.2 Soils 47 3.2.3 Soil sampling method 50 3.2.4 Analytical technique 50 3.2.5 Radon-222 exhalation measurements 51 3.3 Results 52 210 7 3.3.1 Pbex and Be areal activity densities 52 210 222 3.3.2 Accumulation of Pbex on the land surface and Rn activity fluxes 56 3.3.3 Fallout of 137Cs on the Southeast Queensland landscape 60 210 7 3.3.4 Natural radionuclide signatures ( Pbex and Be) for use in erosion studies 62 3.4 Conclusions 64 References 65 CHAPTER FOUR 68 BERYLLIUM-7 IN NEAR-SURFACE AIR AND DEPOSITION AT BRISBANE, AUSTRALIA 4.1 Introduction 71 4.2 Sampling and analytical techniques 72 4.3 Results and discussion 73 4.3.1 7Be concentrations in near-surface air 73 4.3.2 7Be deposition 75 4.3.3 Net cumulative 7Be areal activity density 78 4.4 Conclusion 80 References 81 vi CHAPTER FIVE 83 DESCRIBING THE ANNUAL CYCLIC BEHAVIOUR OF 7Be CONCENTRATIONS IN SURFACE AIR IN OCEANIA 5.1 Introduction 86 5.2 Data and methods of analysis 86 5.2.1 Data sources 86 5.2.2 Measurement technique 90 5.2.3 Data treatment 90 5.3 Results and discussion 92 5.3.1 Results of sinusoidal model 92 5.3.2 Atmospheric processes controlling the 7Be annual cycle 94 5.4 Conclusions 100 References 101 CHAPTER SIX 104 CONCLUDING REMARKS 6.1 Summary and conclusions 104 6.2 Future directions 106 vii LIST OF TABLES Table 2.1. Average surface air concentrations of 7Be at different locations. Data is sorted in order of descending latitude. (pp. 10–12) Table 2.2. Annual 7Be deposition and rainfall at different locations. Data is sorted in order of ascending annual rainfall. (pp. 16–18) Table 2.3. Total 7Be areal activity densities at undisturbed sites. Data is sorted in order of ascending sampling date. (pp. 22–23) Table 3.1. Site location, annual rainfall rate, and physical and chemical properties of the soils (moisture content, composition and particle size distribution are all expressed as percentage weight of the sample). (p. 49) 210 7 137 Table 3.2. Accumulated Pbex, Be and Cs areal activity density in the surface 10 cm and portion present in grass. (p. 53) Table 4.1. Summary of near-surface air 7Be concentration measurements at Brisbane during the years 2002–2005. (p. 74) Table 4.2. Summary of 7Be deposition measurements at Brisbane during the years 2004–2006. (p. 76) Table 4.3. Comparison of measured and calculated net cumulative 7Be areal activity densities for May 2003. (p. 79) Table 5.1. Location of sampling stations, measurement periods, mean 7Be concentration in surface air, and long-term annual average rainfall. (pp. 88–89) Table 5.2. Summary of results of sinusoidal model fitting. (p. 93) Table 5.3. Values of linear correlation coefficient (R) and Spearman correlation coefficient (ρ) between mean of monthly 7Be deviations and long-term mean of monthly rainfall. (p. 99) viii LIST OF FIGURES Figure 3.1. Location of the sampling sites and map of Australia (inset). (p. 48) 210 7 Figure 3.2. Pbex (top) and Be (bottom) cumulative areal activity density profiles normalised to unity at 10 cm depth. The solid line indicates the best fitting exponential given by Equation 3.1 to the data. Error in individual data points is typically 10 %. (p. 54) Figure 3.3. 226Ra and 210Pb areal activity density plotted against 238U. The solid indicates the line of equality. Errors in 226Ra, 210Pb and 238U are typically 5, 10 and 10 %, respectively. (p. 59) Figure 3.4. 137Cs cumulative areal activity density profiles normalised to unity at 10 cm depth. The site key is the same as that shown in Figure 3.2. The solid line represents the best fitting linear function to the data. The dotted lines indicate the 95 % confidence region. Error in individual points is typically 10 %. (p. 61) Figure 4.1. Percent deviation of monthly average 7Be concentrations from the annual mean at Brisbane during the years 2002–2005. Uncertainty in 7Be concentrations is generally less than 10 %. (p. 74) Figure 4.2. Monthly 7Be deposition (columns) and rainfall (solid circles) at Brisbane during the years 2004–2006. Uncertainty in monthly 7Be deposition measurements is generally less than 10 %. The dashed line shows the trend in rainfall. (p. 76) Figure 4.3. Monthly 7Be deposition (Y) plotted against monthly rainfall (X). The solid line represents the linear least squares regression line. (p. 77) Figure 4.4. Calculated net cumulative 7Be areal activity density (columns) and measured rainfall (solid circles) at Brisbane during the years 2000–2006. ix The solid line is the decay curve for 7Be activity deposited before January 2000. The dashed line shows the trend in rainfall. (p. 79) Figure 5.1. Map of Oceania showing location of 7Be measurement stations. (p. 87) Figure 5.2. Frequency representation of normalised (top panel) and un-normalised (bottom panel) 7Be concentration data for the monitoring station at American Samoa. (p. 91) Figure 5.3. Latitude-time isopleth diagram of the zonal averaged ratio of the mean of monthly 90Sr concentrations in surface air. From Rehfeld and Heimann (1995). (p. 96) Figure 5.4. Mean of monthly 7Be deviations (filled circles) at American Samoa (top panel), Brisbane (middle panel) and Cape Grim (bottom panel), and mean of monthly 90Sr deviations (open circles) at the equivalent latitude. (p. 98) x STATEMENT OF ORIGINAL AUTHORSHIP The work contained in this thesis has not been previously submitted for a degree or diploma at any other tertiary education institution. To the best of my knowledge and belief, the thesis contains no material previously published or written by another person except where due reference is made. Signed: …………………………….. Date: …………………………….. xi CHAPTER ONE INTRODUCTION 1.1 Environmental radioactivity and beryllium-7 Radionuclides occurring in our natural environment can be classified in three general categories: (1) primordial radionuclides, i.e., nuclides which have survived since the time when the elements formed, and their progeny nuclides; (2) cosmogenic radionuclides, formed continuously by the interactions of cosmic-ray particles with matter; and (3) artificial radionuclides, introduced by human activities, e.g., by detonations of nuclear weapons. Radioecology is the study of the occurrence of these radionuclides in the natural environment. This includes the atmosphere, hydrosphere and lithosphere. Studies of environmental radionuclides can provide us with useful geophysical information, such as the timescales over which natural systems are changing or evolving and the underlying mechanistic causes. They can also be used to assess the radiological or radioecological effects of ambient radioactivity. Beryllium-7 (7Be) is a cosmogenic radionuclide formed in the atmosphere when cosmic-ray-produced neutrons and protons disintegrate the atomic nucleus of nitrogen and oxygen into lighter fragments. It has a relatively short half-life of approximately 53 days.
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