Radioactivity in the Environment

Physicochemical aspects and applications

Vlado Valkovic

• Publisher: Elsevier Science
• Year: 2000
• Print ISBN:
  9780444829542
• eBook ISBN:
  9780080540245
• Topic: Physical Sciences
• Subtopic: Analytic Chemistry

Numerous sources of ionizing radiation can lead to human exposure: natural sources, nuclear explosions, nuclear power generation, use of radiation in medical, industrial and research purposes, and radiation emitting consumer products. Before assessing the radiation dose to a population one requires a precise knowledge of the activity of a number of radionuclides. The basis for the assessment of the dose to a population from a release of radioactivity to the environment, the estimation of the potential clinical heath effects due to the dose received and, ultimately, the implementation of countermeasures to protect the population, is the measurement of radioactive contamination in the environment after the release.It is the purpose of this book to present the facts about the presence of radionuclides in the environment, natural and man made. There is no aspect of radioactivity, which has marked the passing century, not mentioned or discussed in this book.

CHAPTER 1

Introduction

Radioactivity is a part of nature—in the process of element formation by nuclear reactions taking place in stars, both stable and radioactive isotopes of elements are formed. The isotopic composition of elements is characterized by properties of nuclear reactions that led to the formation of the elements. Elemental composition of the planet Earth, thought to be about 4.5×10⁹ years old, although not yet in chemical equilibrium, reflects the composition of the material from which it was formed. Therefore, a number of radionuclides occur in nature, having long half-lives (longer than the age of Earth). In addition there are natural processes which continuously produce new radioisotopes. Recently, human activities have also contributed to the increased concentration of some of the radionuclides.

Ever since its discovery, the influence of radioactivity on man and his wellbeing has been in question. The development of technology, especially the use of nuclear processes like fusion and fission, in both energy production and weapon manufacturing and testing, has led to occasional contamination of the environment by radioactivity.

Numerous sources of ionizing radiation can lead to human exposure: natural sources, nuclear explosions, nuclear power generation, use of radiation in medical, industrial and research purposes and radiation-emitting consumer products. Before assessing the radiation dose to the population, one requires a precise knowledge of the activity of a number of radionuclides. The basis for the assessment of the dose to the population from a release of radioactivity to the environment, the estimation of the potential clinical health effects due to the dose received and, ultimately, the implementation of countermeasures to protect the population is the measurement of radioactive contamination in the environment after the release. The types of radiation one should consider include:

1. Alpha radiation which consists of heavy positively charged particles emitted by atoms of elements such as uranium and radium. Alpha radiation can be stopped completely by a sheet of paper or by the thin surface layer of our skin (epidermis). However, if alpha-emitting materials are taken into the body by breathing, eating, or drinking, they can expose internal tissues directly and may, therefore, cause more biological damage.

2. Beta radiation which consists of electrons. They are more penetrating than alpha particles and can pass through 1–2 centimetres of water. In general, a sheet of aluminum a few millimetres thick will stop beta radiation.

3. Gamma rays which are electromagnetic radiation similar to X-rays, light, and radio waves. In general, gamma rays, depending on their energy, can pass right through the human body, but can be stopped by thick walls of concrete or lead.

4. Neutrons which are uncharged particles. Therefore, they do not produce ionization directly. But, their interaction with the atoms of matter can give rise to alpha, beta, gamma, or X-rays which then produce ionization. Neutrons are penetrating and can only be stopped by thick masses of concrete, water, or paraffin.

The two basic quantities in the assessment of radiation levels and effects are the activity of a radioactive material and the radiation dose. The activity of a radioactive material is the number of nuclear disintegrations per unit time, unit becquerel (Bq). One becquerel is one disintegration per second. The term radiation dose can mean several things (e.g. absorbed dose, dose equivalent or effective dose equivalent). The absorbed dose of radiation is the energy imparted per unit mass of the irradiated material. The unit of absorbed dose is joule/kg, for which the special name gray (Gy) is used:

It is the purpose of this book to present the facts about the presence of radionuclides in nature. The use of technology can significantly modify the exposure to natural radiation. Among the human activities which should be considered in this context are: (i) the electricity generation by coal-fired power plants, (ii) the use of phosphate fertilizers, and (iii) many consumer products. Man-made radioactivity has found many useful applications in everyday life. The best known are medical applications. The use of radionuclides and radioactivity in diagnosis and treatment of diseases is well established practice.

The determination of radionuclides in environmental samples is an important task in relation to the protection of human health. This is especially the case when there is an accidental release of radioactivity into the environment, as was the case with the Chernobyl accident. Assessment of the situation requires knowledge of the type and form of released radionuclides and reliable, practical techniques for the analysis of various radionuclides.

All the instruments for the detection and measurement of radioactivity are based on the physical processes of radiation interaction with matter. The understanding of these processes has led to the development of many types of radiation detectors. The methods used for the measurement of radionuclide concentrations in various matrices are numerous. Some are better then others, but the best is always a combination of several techniques.

With the development of understanding of radiation interaction with living matter, better safety standards have been developed. Dosimetry is an important factor in the beneficial use of radiation.

The most frightening experience in the history of humankind was the use of the nuclear bomb. The development of this device has resulted in many disturbed sites on our globe. It has also left us with the fear that this powerful weapon could be developed and used again. Mankind is trying to prevent this through the international treaties being signed by a majority of states.

Let us hope we shall continue to live in this radioactive environment by using radioactivity to improve the quality of our lives.

CHAPTER 2

Radioactive Nuclides in Nature

Except for the simplest nucleus, that of hydrogen, all other nuclei consist of neutrons and protons. The ratio of neutrons to protons is unity for lighter isotopes on the so-called “stability line” and increases gradually as one approaches the behaviour elements at the end of the periodic table or moves off the “stability line”. As this ratio increases, a stage is reached where the nuclide is no longer stable. The heaviest stable nuclide is

. Nuclides heavier than this are unstable because they have excess energy to dissipate. Unstable nuclides are called radionuclides and they dissipate their surplus energy by the emission of radiation. This process is called radioactivity or radioactive decay.

The decay of a radionuclide is a statistical process in the sense that it is not possible to predict exactly when a particular nucleus will disintegrate. One may, however, ascribe a probability that a nucleus will decay in unit time. This probability is known as the radioactive decay constant (transformation constant), λ, of the radionuclide. The number of atoms of a radioactive substance disintegrating per unit time, dN/dt, which is referred to as the activity of the substance, is proportional to the total number, N, of radioactive atoms present at time t; the constant of proportionality being λ.

Thus,

(2.1)

Integrating this equation, one obtains

(2.2)

where N₀ is the initial number of radioactive atoms present, and N, as already stated, the number of radioactive atoms at time t. Rewriting eq. (2.1), it follows that

(2.3)

Equation (2.3) indicates that the number of radioactive atoms present as well as the disintegration rate (activity) decrease exponentially with time. The time taken for half the radioactive atoms originally present to decay is called the half-life of the radionuclide.

Substituting N = N₀/2 and t = t_1/2 in eq. (2.2), one obtains

(2.4)

or

(2.5)

or

(2.6)

The number of radioactive atoms present and hence the rate of disintegration decreases to one-half in one half-life, to one-quarter in two half-lives, to one-eighth in three half-lives, and so on. The half-life is characteristic of any particular radioisotope. Another useful quantity is the mean life or the average life of a radionuclide which is the reciprocal of the decay constant, t_m = 1/λ.

A radionuclide, upon undergoing disintegration of a particular type, yields a specified nuclide. The original radionuclide is called the parent and the decay product is called the daughter. The daughter may also be a radionuclide. A succession of nuclides, each of which transforms by radioactive disintegration into the next until a stable nuclide results, is called a radioactive series. Examples of such series are the uranium series and the thorium series.

Radioactive equilibrium refe...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Preface
• Chapter 1: Introduction
• Chapter 2: Radioactive Nuclides in Nature
• Chapter 3: Technologically Modified Exposure to Natural Radiation
• Chapter 4: Man-made Radioactivity
• Chapter 5: Measurements of Radioactivity
• Chapter 6: Radiation Safety
• Chapter 7: The Nuclear Fuel Cycle
• Chapter 8: The Bomb
• Chapter 9: Monitoring Accidentally Released Radionuclides in the Environment
• Chapter 10: Bomb Test Sites
• Chapter 11: International Safeguards
• Chapter 12: Environmental Monitoring for Safeguards
• Chapter 13: Comprehensive Test Ban Treaty (CTBT)
• INDEX