Environmental Radionuclides (eBook)

Tracers and Timers of Terrestrial Processes

Klaus Froehlich (Herausgeber)

eBook Download: EPUB

2009 | 1. Auflage
432 Seiten
Elsevier Science (Verlag)
978-0-08-091329-2 (ISBN)

Infos

The book presents a state of the art summary of knowledge on the use of radionuclides to study processes and systems in the continental part of the Earth's environment. It is conceived as a companion to the two volumes of this series which deal with isotopes as tracers in the marine environment (Livingston Marine Radioactivity) and with the radioecology of natural and man-made terrestrial systems (Shaw, Radioactivity in Terrestrial Ecosystems). Although the book focuses on natural and anthopogenic radionuclides (radioactive isotopes), it also refers to stable environmental isotopes, which in a variety of application, especially in hydrology and climatology, have to be consulted to evaluate radionuclide measurements in terms of the ages of groundwater and climate archives, respectively.
The basic principles underlying the various applications of natural and anthropogenic radionuclides in environmental studies are described in the first part of the book. The book covers the two major groups of applications: the use of radionuclides as tracers for studying transport and mixing processes: and as time markers to address problems of the dynamics of such systems, manifested often as the so-called residence time in these systems. The applications range from atmospheric pollution studies, via water resource assessments to contributions to global climate change investigation. The third part of the book addresses new challenges in terms of development of new methodological approaches including analytical methods and fields of applications.

* a state of the art summary of knowledge on the use of radionuclides
* is conceived as a companion to the two volumes of this series which deal with isotopes as tracers

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Environmental Radionuclides presents a state-of-the-art summary of knowledge on the use of radionuclides to study processes and systems in the continental part of the Earth's environment. It is conceived as a companion to the two volumes of this series, which deal with isotopes as tracers in the marine environment (Livingston, Marine Radioactivity) and with the radioecology of natural and man-made terrestrial systems (Shaw, Radioactivity in Terrestrial Ecosystems). Although the book focuses on natural and anthropogenic radionuclides (radioactive isotopes), it also refers to stable environmental isotopes, which in a variety of applications, especially in hydrology and climatology, have to be consulted to evaluate radionuclide measurements in terms of the ages of groundwater and climate archives, respectively. The basic principles underlying the various applications of natural and anthropogenic radionuclides in environmental studies are described in the first part of the book. The book covers the two major groups of applications: the use of radionuclides as tracers for studying transport and mixing processes: and as time markers to address problems of the dynamics of such systems, manifested commonly as the so-called residence time in these systems. The applications range from atmospheric pollution studies, via water resource assessments to contributions to global climate change investigation. The third part of the book addresses new challenges in the development of new methodological approaches, including analytical methods and fields of applications. - A state-of-the-art summary of knowledge on the use of radionuclides- Conceived as a companion to the two volumes of this series, which deal with isotopes as tracers

Inhalt

Front cover 1
Environmental Radionuclides: Tracers and Timers of Terrestrial Processes 4
Copyright page 5
Contents 6
Contributors 10
Foreword 12
Chapter 1. Origin and Distribution of Radionuclides in the Continental Environment 16
1. Introduction 16
2. Primordial and Natural Decay-Series Radionuclides 17
3. Cosmogenic Radionuclides 22
4. Anthropogenic Radionuclides 31
References 38
Chapter 2. Radionuclides as Tracers and Timers of Processes in the Continental Environment - Basic Concepts and Methodologies 42
1. Introduction 42
2. Radioactive Decay - Fundamentals of Radiometric Dating 43
3. The Concept of Residence Time in Environmental Systems 56
4. On the Definition of Environmental Tracers 63
References 64
Chapter 3. Radionuclides as Tracers of Atmospheric Processes 66
1. Introduction 67
2. Atmospheric Radionuclides 67
3. Behaviour of Radionuclides in the Atmosphere 78
4. Application of Radionuclides in Atmospheric Studies 90
References 99
Chapter 4. Radiocarbon as a Tracer in the Global Carbon Cycle 104
1. Introduction 105
2. Global Distribution of Carbon 107
3. The Conventional Radiocarbon Dating Technique 112
4. Methods of Isotope Measurement 122
5. Definition, Calculation and Reporting of C Isotope Enrichment Values 128
6. 14C as a Tracer 135
7. Radiocarbon Literature 143
8. A Look Forward 144
References 144
Chapter 5. Radionuclides as Tracers and Timers in Surface and Groundwater 154
5.1. Surface Water, Unsaturated Zone, and Glacial Systems 155
References 189
5.2. Radionuclides and Transient Gas Tracers in Studies of Lakes and Inland Seas 197
References 215
5.3. Dynamics and Pollution of Groundwater 219
References 240
Chapter 6. Examining Processes and Rates of Landscape Change with Cosmogenic Radionuclides 246
1. Introduction 247
2. Principles of Cosmogenic Nuclide Research 248
3. Mineral-Nuclide Pairs 264
4. Determining Rates of Landscape Evolution with Cosmogenic Nuclides 269
5. New Directions and Outlook 286
Acknowledgments 290
References 290
Chapter 7. Soil Erosion and Sedimentation Studies Using Environmental Radionuclides 310
1. Introduction 310
2. Radionuclides as Tracers of Soil Movement 313
3. Standardisation of the 137Cs Technique to Measure Soil ErosionsolDeposition 319
4. Recent Developments of Fallout Radionuclide Techniques 324
5. Applications of Fallout Radionuclides in Soil ErosionsolSedimentation Studies 326
6. Future Research 326
7. Concluding Remarks 329
References 330
Chapter 8. Isotopic Tracers in Climatology 338
1. Introduction 338
2. Role of Environmental Isotopes in Understanding Climate Changes 340
3. Environmental Stable Isotopes and Radionuclides in Palaeo-Climatic Archives 349
4. Conclusions 370
Acknowledgements 370
References 370
Chapter 9. Analysis of Radionuclides 378
1. Introduction 378
2. Radiometry 380
3. Isotopic Enrichment 403
4. Mass Spectrometry 405
5. Future Perspectives 413
References 418
Author Index 422
Subject Index 442

Textauszug

Chapter 2 Radionuclides as Tracers and Timers of Processes in the Continental Environment – Basic Concepts and Methodologies

Klaus Froehlich1 and Jozef Masarik2,∗

1Viktor-Wittner-Gasse 36/7, 1220 Vienna, Austria

2Department of Nuclear Physics and Biophysics, Comenius University, Mlynská dolina, 842 48 Bratislava, Slovakia

∗Corresponding author. Tel.: [phone]; Fax: [phone]

E-mail address: [e-mail]

Publisher Summary

This chapter discusses the basic principles underlying the various applications of environmental radionuclides in tracer studies of terrestrial systems. There are two major groups of applications: (1) providing timescales of past and present processes and (2) tracing substances involved in terrestrial transport, exchange, and mixing processes. The applications of both groups are based on the phenomenon of radioactivity. Environmental radionuclides have the distinct advantage over injected (artificial) tracers in that they facilitate the study of various processes on a much larger temporal and spatial scale through their natural distribution in environmental systems. Thus, environmental radionuclides are unique tools in regional studies for investigating the time- and space-integrated characteristics of environmental systems. The use of injected artificial tracers is generally effective for site-specific local applications.

1. Introduction

Applications of radionuclides in environmental studies are based on the general concept of ‘tracing’, in which either intentionally introduced radionuclides or naturally occurring (environmental) radionuclides are employed. Environmental radionuclides have the distinct advantage over injected (artificial) tracers in that they facilitate the study of various processes on a much larger temporal and spatial scale through their natural distribution in environmental systems. Thus, environmental radionuclides are unique tools in regional studies to investigate the time- and space-integrated characteristics of environmental systems. The use of injected artificial tracers is generally effective for site-specific local applications.

This chapter deals with the basic principles underlying the various applications of environmental radionuclides in tracer studies of terrestrial systems. There are two major groups of applications: (1) providing timescales of past and present processes; (2) tracing substances involved in terrestrial transport, exchange and mixing processes. The applications of both groups are based on the phenomenon of radioactivity.

2. Radioactive Decay – Fundamentals of Radiometric Dating

2.1 Radioactivity

Radioactivity is a phenomenon related to the spontaneous transmutation (decay) of an unstable atomic nucleus accompanied by emission of radiation. In this definition, a number of terms is used that need to be explained. In the following, a brief summary of the definitions and fundamentals is given. For more details, the reader may refer to a suitable textbook (e.g. Turner, 2007; Magill and Galy, 2005).

The nucleus of an atom consists of protons and neutrons and is surrounded by a cloud of orbiting electrons forming the shell of the atom (Figure 1). The mass of a proton is about 2,000 times heavier than that of an electron. The proton carries a positive and the electron a negative elementary charge. The neutron is electrically neutral and only slightly heavier than the proton (Figure 1). In a neutral atom, the number of electrons is equal to the number of protons, which is called the atomic number Z. While the mass of an atom is nearly equal to the mass of its nucleus (about 2×10–27 to 400×10–27 kg), the size (diameter) of the atom is controlled by the atomic shell (about a fraction of a nanometre), that is about 10,000 times larger than the nucleus.

Figure 1 Structure of the atom.

The term nuclide defines an atomic species with a given number of protons and neutrons in the nucleus. The number of protons and neutrons is called the atomic mass number A=N+Z, where N is the number of neutrons and Z the number of protons (atomic number). The common name of proton and neutron is nucleon, that is A is the number of nucleons. A nuclide is characterised by the symbol , where X stands for the chemical element to which the nuclide belongs. For example the nuclide of carbon with 12 nucleons is described by .

Nuclides belonging to the same chemical element, that is having the same atomic number but different atomic mass numbers, are called isotopes. For example the following nuclides are isotopes of the element carbon: (most abundant carbon isotope), (heavy stable carbon isotope) and (heavy unstable carbon isotope). Since the chemical element is defined by the atomic number, the latter can be omitted in the nuclide symbol. Hence, the above carbon isotopes can be written as 12C, 13C and 14C.

Nucleons are bound in the nucleus by nuclear forces, also called strong forces. These are short-range attractive forces that exist between protons and neutrons and are strong enough to overcome the electrostatic repulsion which exists between the charged protons. Like atomic electrons of the shell, nucleons ‘occupy’ discrete energy levels of the nucleus. The situation in the nucleus is, however, complicated by the fact that two types of nucleons, protons and neutrons, are to be accommodated. The energy levels involved are of the order of megaelectron volt, much higher than the energies of atomic electron levels. Because the charged protons experience an electrostatic repulsion between them, which the uncharged neutrons do not, the proton levels appear at slightly higher energies than the neutron levels. As in the case of atomic electrons, each nuclear level can only hold a certain number of protons or neutrons. Because of the higher energy of the proton levels, stable nuclei tend to have slightly more neutrons than protons. There are possible transitions between neutron and proton energy levels. These transitions lead to the emission of particles and transformation of one kind of nucleons to another. If the nucleus has, for example a single proton in a high energy level with a vacancy for a neutron in a lower level, that proton can transmute into a neutron and emit a particle with positive charge, called a positron, and a gamma quantum that takes the excess energy. This process is called beta plus decay. The opposite process, called beta minus decay, occurs if a nucleus has too many neutrons and thus transmutes a neutron into a proton.

The gamma quantum is a photon that has no rest mass and no charge. Therefore, the gamma transition changes neither Z nor A of the emitting nucleus. Like X-rays, the gamma quantum is equivalent to electromagnetic radiation. Thus, the alternative term is gamma ray (radiation). The energy of the gamma quantum Eγ is given by the energy difference between the corresponding energy states E2 (higher state) and E1 (lower state):

where h is Planck’s constant=6.626068×10–34 m2 kg–1 s–1 and f the frequency of the equivalent electromagnetic radiation. In most cases, the nucleus remains in the higher energy state for less than 10–14 s.

The basic condition for the existence of stable nuclides is the following: the total energy of a stable nucleus must be lower than the total energy of any other combination of the same number of neutrons and protons. If this condition is not fulfilled, the nucleus is unstable and will spontaneously decay into another configuration. The latter process is called radioactive decay and the decaying nuclide is called a radionuclide. There are radionuclides that can decay in two or three different ways, but the majority of radionuclides decay in a single way. In either case, radioactive decay changes Z and N of the nucleus. The product nuclide, also called the daughter nuclide, may also be radioactive and transmute spontaneously, as happens in the natural decay series (Chapter 1). In the following sections, a short overview of the various radioactive decay types will be given.

2.1.1 Beta minus decay

A nuclide with too many neutrons can get closer to stability by converting a neutron into a proton. The simultaneous emission of an electron e– (in this process also called beta minus particle β–) and an antineutrino (a particle that carries energy but no charge and nearly no mass) ensures the conservation of charge and energy, respectively. This process

increases Z by 1 and decreases N by 1 so that A remains unchanged. The decay product (daughter nucleus) can initially be in an excited state and emit the excess energy in the form of one or more gamma quantum (gamma rays). A typical example is the decay of the radionuclide 131I into its daughter nuclide 131Xe:

In this process, the atomic number increased by 1 and therefore the iodine nucleus changed to a xenon nucleus under emission of a beta minus particle β–, an antineutrino and a gamma ray.

Details of a radioactive decay process are given by the decay scheme. In Figure 2a, it is shown that 131I decays with a half-life of 8 days under emission of a beta minus...

Produkt-Details

• Erscheint lt. Verlag: 23.9.2009
• Sprache: englisch
• Themenwelt: Sachbuch/Ratgeber
• Themenwelt: Naturwissenschaften ► Biologie ► Ökologie / Naturschutz
• Themenwelt: Naturwissenschaften ► Geowissenschaften ► Geologie
• Themenwelt: Naturwissenschaften ► Physik / Astronomie ► Atom- / Kern- / Molekularphysik
• Themenwelt: Technik ► Umwelttechnik / Biotechnologie
• ISBN-10: 0-08-091329-6 / 0080913296
• ISBN-13: 978-0-08-091329-2 / 9780080913292