Radiation and Radioactivity on Earth and Beyond

Ivan G. Draganic, Jean-Pierre Adloff

• Publisher: CRC Press
• Year: 2020
• ISBN:
  9781000144376
• Edition: 2
• Topic: Physical Sciences
• Subtopic: Chemistry
• Index: Chemistry

This book features information regarding the Chernobyl nuclear accident, the production of elementary particles, radiation exposure, the geopolitical effects of the end of the nuclear arms race between the U.S. and the former Soviet Union, and the future of nuclear power.

This chapter concerns people and events in a story of radiation, radioactivity, and nuclear energy that covers almost a century.

The term radiation is generally used to denote the emission of any type of ray or particle from a source. If the radiation has sufficient energy to produce ions by either direct or indirect mean, it is called ionizing radiation. This book deals almost entirely with radiation of this kind. It originates in nuclear reactions and processes, appears in the form of cosmic rays, and is produced in laboratories by special devices.

Since the discovery of X-rays by Roentgen in 1895 and the identification of radiation from radioactive substances (alpha [α] and beta [β]-particles and gamma [γ]-rays), the inventory of radiation is gradually being extended. Dozens of types of nuclear and subnuclear particles are presently being produced artificially in laboratories at energies up to 10⁹ electronvolts or more, while cosmic radiation provides a variety of entities at energies up to 10²⁰ electronvolts.

Radioactivity is a property of nuclei to undergo a spontaneous disintegration accompanied by emission of radiations. The first radioelements were discovered by Pierre and Marie Curie in 1898. At present about 80 various radionuclides are known to occur in nature. Since the discovery of artificial radioactivity by Irène Curie and Frédéric Joliot in 1934, more than 2500 radioactive nuclides have been produced by man.

Energy released in a nuclear reaction involving the conversion of mass into energy is called nuclear energy. The fission of the uranium nucleus was discovered in 1939 and the first man-made nuclear reactor was put into operation in the United States in 1942, in Chicago. At the present time, there are more than 300 experimental nuclear reactors operating across the world and about 420 nuclear power plants furnish nearly 12 percent of the global output of electricity. Our era is also characterized by a prolific stockpile of nuclear weapons. Within only 4 decades following the explosion of the first atomic bomb in the desert of New Mexico (U.S.) in 1945, the world*s nuclear armament has been built up to a point sufficient to destroy not only our civilization, but also much of the Earth's capacity to support life.

1

A Story That Covers Almost A Century

THE PENETRATING INVISIBLE RADIATION

The nuclear age began in 1896 with the discovery of radioactivity by Antoine-Henry Becquerel (1852–1908) (Figure 1). It followed, by 3 months, the discovery of X-rays by Konrad Wilhelm Roentgen (1845–1923). These invisible rays traveled over a distance of several meters, and were emitted when a gas at low pressure was excited by an electric discharge. Simultaneously, a green luminescence was observed on the walls of the glass tube containing the gas.

Becquerel thought that there should be some kind of relationship between the luminescence and the emission of penetrating radiation, and that a material which becomes luminescent after being exposed to sunlight might also emit X-rays. He knew that some uranium salts show strong fluorescence when exposed to sunlight and he decided to verify whether they also emit the invisible penetrating radiation. The radiation was detected by the blackening of a photographic plate in a very simple experiment: the sensitive plate was wrapped in black paper, a crystal of the uranium salt placed on it, and the whole exposed to sunlight. In a fortuitous but crucial observation, Becquerel noticed that the sunlight and the luminescence were really not necessary for blackening of the photographic plate. He concluded that the blackening occurs from an intrinsic property of the uranium salt and he prepared a report on “the invisible radiation emitted by phosphorescent substances” for the weekly session of the Academy of Sciences in Paris.

FIGURE 1. Henri Becquerel (1852–1908).

Becquerel presented the report on March 2 (1896), a date which is usually considered as the birthday of radioactivity and nuclear science. In further experiments Becquerel found that the radiations were emitted by the chemical element uranium and that they shared several properties of the X-rays, such as the ability to discharge an electroscope.

RADIOACTIVITY AND THE CURIES

Two years after Becquerel’s discovery of radiation emitted by uranium, Gerhart Carl Schmidt (1865–1949) reported that thorium, another heavy chemical element, could also discharge an electroscope and blacken a photographic plate. The same result was obtained independently by a young Polish woman, Marya Sklodowska (1867–1934), whose civil status changed in 1895 to Marie Curie. She became the most prominent figure in the history of “radioactivity”, the name she gave to the new phenomenon of spontaneous emission of invisible radiation.

In 1882, her husband Pierre Curie (1859–1906) had discovered piezoelectricity, a phenomenon related to the appearance of electric charges when a quartz foil is subjected to pressure. He conceived a simple but very effective device to measure the electric charge. Even small charges, such as those resulting from radiation emitted by uranium, could be measured accurately. The device known as the piezoelectric quartz electrometer was used by generations of radiochemists, up to the advent of the Geiger-Müller counter and simple electronic tubes (Figure 2).

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FIGURE 2. Pierre Curie (1859–1906) and Marie Curie (1867–1934).

Marie Curie chose the study of rays emitted by uranium and thorium as a subject for her doctoral thesis. With the aid of the quartz electrometer, she also undertook precise measurements of every mineral and chemical compound she could find. In her first publication (April 1898), she noted that potassium salts are weakly radioactive, and further reported that two uranium minerals are more radioactive than uranium itself. The latter finding implied that the minerals pitchblende and chalcolite must contain some unknown constituent which was more “active” than uranium.

Together with Gustave Bémont (1867–1932), the Curies undertook the chemical processing of pitchblende. The procedure was in no way innovative, but the quartz electrometer enabled a simple and reliable control of the radioactivity after each separation step. In July 1898, they announced the discovery of a new element whose properties were similar to those of bismuth. It was named polonium in honor of Marie’s native country.

The Curies soon discovered a second radioactive element, which resembled barium and for which they proposed the name radium. The optical spectrum of the new element showed characteristic lines, but final proof for its existence required an accurate determination of atomic weight.

Too many “discoveries” of new chemical elements had already been reported toward the end of the 19th century and the scientific community had become skeptical and cautious. Any claim had to be supported by “visible proofs”. In addition to the establishment of characteristic spectral lines, such proofs comprised exhibition in a test tube and an unequivocal determination of the atomic weight.

The latter required a weighable quantity of radium, which the Curies extracted tediously from several tons of pitchblende residues remaining from the separation of uranium used for the manufacture of colored ceramic and glass. Finally, 100 milligrams of radium chloride was recovered after 4 years of exhausting physical labor in a miserable and unheated open shed. But this tiny amount was sufficient to provide visible evidence for the unquestionable existence of a new radioactive element. In 1903, the Curies shared with Becquerel the Nobel Prize in physics. Elemental radium in the metallic state was obtained by Marie Curie in 1910 and the following year this achievement brought her a second Nobel Prize, this time in chemistry. Marie Curie was the first person to obtain this prestigious award twice.

THE NATURE OF RADIOACTIVITY, RUTHERFORD AND SODDY

A common property of X-rays and uranium rays is their ability to ionize a gas. In 1899, Ernest Rutherford (1871–1937) began a systematic study of radiation emitted by uranium and examined the ionization under various conditions. He soon established that this radiation consists of two different types of particles. One of these, which he named α-particles (α-rays), produced a large number of ions, but was completely stopped by a thin sheet of glass, aluminum, or paper; these particles were identified as positive ions of the element helium. The other type of radiation was more penetrating but produced fewer ions. Rutherford called this form β-radiation (β-rays), which was found to consist of electrons, i.e., negatively charged particles. Since α- and β-rays bear electric charges, they are affected by electric and magnetic fields. This is not the case, however, for the third type of radiation, the γ-rays, which are electromagnetic waves like X-rays. γ-Radiation produces little ionization and travels over greater distances (Figure 3).

FIGURE 3. Ernest Rutherford (1871–1937).

The years following the discovery of radium brought many, frequently confusing, observations. Several new “activities’’ were described but could not be ascribed to a specific element. One of the most important observations was that the radioactivity of a substance does not necessarily continue indefinitely; for a given substance, the activity decreases within a specific time scale.

Rutherford discovered that thorium releases a radioactive gas, which he called “emanation”. This finding was useful in explaining the instability of radioactive species. Together with Frederick Soddy (1877–1956), Rutherford formulated a revolutionary idea on the nature of radioactivity at the beginning of 1903. It was recognized that radioactive decay is a process within the atom and involves its spontaneous transformation, i.e., the atom changes its chemical identity, and is “transmuted” into the atom of another element.

FITTING RADIOELEMENTS AND ISOTOPES INTO MENDELEEV’S PERIODIC TABLE

The flourishing field of radioactivity left many scientists rather indifferent, possibly because of the complicated or confusing cookbook type of procedures described by many chemists in their publications.

However, the chemists had their own troubles. The problem was how to fit the multitude of new radioactive species into the periodic table of chemical elements, as set up in 1869 by Dmitri Ivanovich Mendeleev (1834–1907) (Figure 4). Mendeleev’s idea was that the properties of the elements depend in a periodic manner on their atomic weight. In such an arrangement, elements which have similar properties recur at regular intervals and fall into related groups. Mendeleev was able to predict the existence and properties of hitherto undiscovered elements by means of his original table, and subsequently the periodic law was extremely useful in explaining and correlating the properties of elements. In 1895, the table listed 66 elements, together with at least 10 others whose position was still uncertain. A disturbing fact was the incompatibility of several known atomic weights with the attractive and simple hypothesis of William Prout (1785–1850) that the atoms of chemical elements are composed of hydrogen atoms. The advance in experimental procedures used in the determination of atomic weights has shown that more and more data differ substantially from the values expected for a multiple of the atomic weight of hydrogen.

FIGURE 4. Dmitri Ivanovich Mendeleev (1834–1907).

By the end of 1911, three dozen new radioactive species had been definitely identified and characterized by their radiation and the kinetics of their decay, although room remained for only 14 further elements in the periodic table. The puzzle was progressively solved when it was recognized that one radioactive entity could be transformed into another and that a certain genetic relationship existed within a radioactive family. After a great deal of confusion and controversy it was found that all known radioactive elements could be assigned to one of three radioactive families. Two of these had uranium as the parent element and one was headed by thorium; the final product in all cases was lead.

The next task was to force the members of radioactive families into the vacancies of the periodic chart. Radium was easy to accommodate on the basis of its chemical analogy with barium and its known atomic weight. The three forms of emanation, which behaved like the inert gas argon, also had definitive positions.

For the five remaining spaces between bismuth and uranium there were still too many candidates. The idea emerged that some of the radioactive substances might represent varieties of the same element. The way to this hypothesis was opened in 1913 when Frederick Soddy and Kasimir Fajans (1887-1975) suggested that the lead which appears as a stable end product of the three radioactive families, and the practically inseparable ordinary lead of different atomic weight, occupy the same place in the periodic table. Some of the radioactive species also had similar, if not identical, chemical properties and thus belonged together. Their characteristic radioactive behavior indicated, however, that they are distinguishable as different substances.

A great step forward was made when Fajans announced the “displacement laws” in 1912. According to this scheme, emission of an α-particle is accompanied by a shift of the radioactive element in the periodic table by two places from right to left in the horizontal row, whereas β-emission results in a shift of one position from left to right in the horizontal row. Fajans was able to fit all known radioactive elements into the periodic table and assign their atomic weights. The valid prediction of chemical properties was maintained.

It was now firmly est...

Table of contents

• Cover
• Title Page
• Copyright Page
• Preface to the Second Edition
• Preface
• Authors
• Table of Contents
• Chapter 1 A Story that Covers Almost a Century
• Chapter 2 Radioactivity and Matter
• Chapter 3 Ionizing Radiation
• Chapter 4 Chemical Aspects of Nuclear Processes
• Chapter 5 Nucleosynthesis, Cosmic Radiation, and the Universe
• Chapter 6 The Interstellar and Interplanetary Medium
• Chapter 7 The Early Earth
• Chapter 8 The Contemporary Earth
• Chapter 9 Nuclear Energy
• Chapter 10 Perspectives
• Glossary
• Index