Radioactivity

Introduction

Radioactivity was discovered at the end of the 19th century by Henri Becquerel, Marie Curie (Polish native name, Maria Sklodowska-Curie), and Pierre Curie. Henri Becquerel found that uranium salts caused fogging of an unexposed photographic plate,1*1 and Marie Curie discovered that only certain elements gave off these rays of energy.12! she named this behavior “radioactivity” (natural radioactivity). A systematic search for the total radioactivity in uranium ores also guided Marie Curie to isolate a new element, polonium, and to separate a new element, radium.13'71 The two elements have chemical similarity that would otherwise have made them difficult to distinguish from each other. In 1934, Marie Curie’s daughter, Irene Joliot-Curie and her husband Frederic Jean Joliot were the first creators of artificial radioactivity. They bombarded boron with alpha particles to make the neutron-poor nitrogen isotope l3N; this isotope emitted positrons. In addition, they bombarded aluminum and magnesium with neutrons to make new radioisotopes.!81

Radioactive Decay

Radioactive decay is the process by which an unstable atomic nucleus spontaneously loses energy by emitting ionizing particles and radiation. The three main types of radiation were discovered by Ernest Rutherford, the alpha (a), beta (p), and gamma (y) rays (alpha, beta, and gamma radiation).!9'111 With Ernest Rutherford, he saw that radioactive substances are transformed from one element to another. About 10years later, he unraveled the rules for the elemental transformations that accompanied radioactive decay, first for a decay and later for P decay. Emission of an a particle changes the emitting atom to an atom of the element two places to the left in the periodic table; emission of a P' particle changes the emitting atom to an atom of the element one place to the right. These rules taken together are known as the Displacement Law; Kazimierz Fajans published it slightly earlier than did Soddy in 1913.!’21 At about the same time, Soddy came to the conclusion that several substances with different radioactive properties and different atomic weights were chemically the same element. He named such substances isotopes.113! Now, the radioactive principles are named the Soddy-Fajans periodic method.

where X and Y are symbols for nuclides, Z is the mass number, and A is the atomic number.

  • 1. Alpha (a) decay is a method of decay in large nuclei. Alpha particles (helium nuclei, He2+), consisting of two neutrons and two protons, are emitted. Because of the particles’ relatively high charge, it is heavily ionizing and will cause severe damage if ingested. However, owing to the high mass of the particle, it has little energy and a low range; typically, alpha particles can be stopped with a sheet of paper (or skin).
  • 2. Beta minus (p-) radiation consists of an energetic electron. It is less ionizing than alpha radiation, but more than gamma. The electron can be stopped with a few centimeters of metal. It occurs when a neutron decays into a proton in a nucleus, releasing the beta particle and an antineutrino. Beta-plus (p+) radiation is the emission of positrons. As these are antimatter particles, they annihilate any matter nearby, releasing gamma photons.
  • 3. 3. Gamma (y) radiation consists of photons with a frequency greater than 10l9Hz. Gamma radiation occurs to rid the decaying nucleus of excess energy after it has emitted either alpha or beta radiation.

The activity (A) of radionuclide is lost at time (t) according to the formula

where A is the radionuclide activity at time t = 0, A0 is the radionuclide activity at time t, and 2 is the decay constant of the radionuclide.

The SI unit of activity is the becquerel (Bq). One becquerel is defined as one transformation (or decay) per second. Another unit of radioactivity is the curie (Ci), which was originally defined as the amount of radium emanation (by gaseous radon-222), in equilibrium with lg of pure radium isotope 226Ra. At present, it is equal, by definition, to the activity of any radionuclide decaying with a disintegration rate of 3.7 x 10 l0Bq. The activity of a radioactive substance is characterized by its half-time—the time taken for the activity of a given amount of radioactive substance to decay to half of its value.|R15l

After the discovery of neutron in 1932, Encico Fermi and colleagues studied the results of bombarding uranium with neutron in 1934.1161 The first person that mentioned the idea of nuclear fission in 1934 was Ida Noddack.1171 After Fermi’s publication, Lise Meittner, Otto Hahn, and Fritz Strassmann began to perform a similar experiment and discovered nuclear fission of uranium 235U in 1938.|18,I91 Also, Jozef Rotblat in 1939 published the results of a study about fission of uranium 235U nuclei.120!

In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts (lighter nuclei), often producing free neutrons and photons (in the form of gamma rays), as well. Fission of heavy elements is an exothermic reaction that can release large amounts of energy both as electromagnetic radiation and as kinetic energy of the fragments (heating the bulk material where fission takes place).

Three heavy radionuclides, natural 235U and artificial 239Pu and 233U, are capable of reactions (nuclear fission) in which an atom’s nucleus splits into smaller parts, releasing a large amount of energy in the process. During the fission of 235U, three neutrons are released in addition to the two daughter atoms (see reaction below) (Figure l).1211

Fission yield as a function of mass number for the slow neutron fission of U

FIGURE 1 Fission yield as a function of mass number for the slow neutron fission of 2!5U.

Source: Environmental radiochemistry and radiological protection.1211

Natural and Artificial Radionuclides

Radionuclides present in the natural environment are classified as either of natural or anthropogenic origin. Naturally occurring radionuclides occur in different ecosystems with cosmogenic and primordial providence.122,23!

  • 1. Cosmogenic radionuclides: Cosmic ray-produced radionuclides are generated in the upper- atmosphere gases, e.g., 02, N2> and Ar. They are transported to the lower atmosphere and next to the oceans and to the continents. Most of the cosmic radionuclides are produced in very small amounts and only four of them, 3H, "Be, l4C, and 22Na, constitute significant contributions to the radiation dose to humans. Cosmogenic radionuclides have been measured in humans, topsoil, polar ice, surface rocks, sediments, the biosphere, the ocean floor, and the atmosphere.1241
  • 2. Primordial radionuclides: Among non-series radionuclides of terrestrial origin, only 40K and 87Rb are significant sources of radiation to humans. They are characterized by a long half-time (more than 109years) and small concentrations in crustal rocks (below 1 mBq/kg)J25,261

The serially occurring radionuclides are contained in four natural decay series—uranium, thorium, actinium, and neptunium—and, except for the actinium series, are named after their parent nuclides (Figures 2-5).

Uranium-238 decay series

FIGURE 2 Uranium-238 decay series.

Source: Wikipedia, uranium series decay chain, http://11pload.wikimedia.0rg/wikipedia/commons/a/al/Decay_ chain%284n%2B2%2C_Uranium_series%29.PNG.1401

Thorium-232 decay series

FIGURE 3 Thorium-232 decay series.

Source: Wikipedia, Thorium series decay chain, http://upload.wikimedia.Org/wikipedia/commons/l/lc/Decay_ chain%284n%2CThorium_series%29.PNG.1411

Anthropogenic Radionuclides

Anthropogenic-derived radionuclides have been mainly released from several sources since the 1940s. Major sources in the environment are nuclear weapons, nuclear power production, accidents (e.g., the Chernobyl accident in 1986), radioactive waste disposal, solid radioactive waste disposal, and manmade radionuclides as tracers of environmental processes. Fallout from nuclear weapons explosions represents the largest contribution of anthropogenic-derived radionuclides to the natural environment. Anthropogenic radionuclides are divided into three groups:!22-34!

1. Neutron activation products: By neutron irradiation of objects, it is possible to induce radioactivity. This activation of stable isotopes enables to create radioisotopes. A lot of artificial radionuclides in the natural environment are produced as a result of the activation process

Actino-uranium 235U decay series

FIGURE 4 Actino-uranium 235U decay series.

Source: Wikipedia, Actinium series decay chain, http://upload.wikimedia.Org/wikipedia/commons/l/le/Decay_ chain%284n%2B3%2C_Actinium_series%29.PNG.|42l

during nuclear weapons tests, the work of reprocessing plants and nuclear reactors used in power plants, as well as in nuclear studies. Owing to the use of new radioanalytical techniques, activation products such as 22Na, 51Cr, 54Mn, 65Zn, 110mAg, and 124Sb could be detected in the natural environment.1261

2. Fission radionuclides: During the fission of 235U, three neutrons are released in addition to two daughter atoms. In the detonation of a nuclear bomb, radioactive fission products are generated from the primary fission of 235U or 239Pu. The most important radionuclides from two families are 90Sr, 95Zr, 1311,132I, !32Te, l37Cs,I40Ba, and 144Ce. These radionuclides are deposited from the atmosphere to the surface of earth, with the fallout comprising components from the stratosphere (78%), local radioactive pollution (12%), and the troposphere (10%).1271

Neptunium-237 decay series

FIGURE 5 Neptunium-237 decay series.

Source: Wikipedia, Neptunium series decay chain, http://upload.wikimedia.0rg/wikipedia/commons/8/8c/Decay_ ehain%284n%2Bl%2CNeptunium_series%29.PNG.1'131

Transuranic elements: In chemistry, transuranic elements are chemical elements with atomic numbers greater than 92 (the atomic number of uranium). All transuranic elements are radioactive; 20 transuranic elements have been discovered to date: neptunium (Np), plutonium (Pu), americium (Am), curium (Cm), berkelium (Bk), californium (CO, einsteinium (Es), fermium (Fm), mendelevium (Md), nobelium (No), lawrencium (Lr), rutherfordium (Rf), dubnium (Db), seaborgium (Sg), bohrium (Bh), hassium (Hs), meitnerium (Mt), darmstadium (Ds), roentgenium (Rg), and copernicium (Cn). Small quantities of neptunium and plutonium are found in nature (in uranium rocks), but most of them are synthesized in nuclear reactors. The most important sources of transuranic elements (generally, neptunium, plutonium, americium, and curium) in the natural environment are nuclear weapons explosions and nuclear power production as a result of the activation process of uranium, 23SU.121,27,281 yiie environmental chemistry of some transuranic elements, such as plutonium, is complicated by the fact that solutions of this element can undergo disproportionation, and as a result many different oxidation states can coexist at once.

The most important (long half-time, type of decay, and strong radio toxicity) natural and anthropogenic radionuclides present in the environment are as follows:1211 Naturally occurring radionuclides

a. Radionuclides of terrestrial origin—primordial nonseries radionuclide (e.g., 40K and 87Rb)

b. Cosmogenic radionuclides (e.g., 3H and l4C).

c. Primary radionuclides—primordial series radionuclide: long-lived; have been ubiquitous on Earth since their formation (i.e., ca. 4.5 x 109years ago). The radionuclides 238U, 232Th, and 235U are the parent members of the uranium, thorium, and actinouranium radioactive decay series, respectively

Anthropogenic Radionuclides

a. Neutron activation products (e.g., 54Mn, 55Fe, 60Co,

b. 235U and 239Pu fission radionuclides (e.g., 90Sr, 95Zr, l311, 1321, 132Te, l37Cs, and 144Ce)

c. Transuranic elements (e.g., 237Np, 238Pu, 239Pu, 240Pu, 241Pu, 24lAm, and 243Am)

Natural and artificial radionuclides in different environmental samples (natural water, sediments, soils, biological organisms) are determined by many radiometric methods, in particular neutron activation analysis (NAA), and alpha, beta, and gamma spectrometry.129,301

Sources of Radionuclides in the Environment and Pollution Problem

of the phosphate fertilizers plant, their concentration in soil, flora, and water samples is much higher than in non-contaminated areas.136,371 Radionuclides are strongly accumulated by some species and the bioaccumulation factor values for some radioactive elements (polonium, plutonium, americium) in sea algae, benthic animals, and fish are more than 1.000.138,391 Some of these organisms are often used as bioindicators of radioactive pollution of the natural environment.1211 Also, transuranic elements (especially plutonium) belong to the group of radioactivity caused by humans. These radionuclides are important from the radiological point of view due to their high radiotoxicity, long physical lifetime, high chemical reactivity, and long residence in biological systems in the natural environment.1391

Solution to Radioactive Pollution

A possible solution to radioactive pollution is by the reduction of radioactive emission to the natural environment, change of nuclear technology, and recognition of determination and accumulation processes in living organisms. Plants and animals are capable of accumulating natural and artificial radionuclides from the environment. That is why it is very important to recognize the impact of radionuclides on living organisms and their possible transfer to the human body by way of feeding.1211 Due to the importance of water, air, and food (also cigarette smoking) to human life, their quality must be strictly controlled and monitored. For this reason, studies of food for human consumption must be performed to guarantee that food materials have a low level of radioactivity, both natural and artificial. Especially, long-lived alpha emitters are the most dangerous nuclides in case of ingestion, because the long-term effects of their intake on the human body are the most important from the radiochemical and radiological points of view. A large contamination to the radiation dose received by humans comes from naturally occurring radionuclides accumulated in the body. At the moment, knowledge about accumulation of natural alpha radionuclides by organisms and their ingestion by humans is still very poor.

Conclusion

Radioactivity and radionuclides have been widely applied for more than a hundred years.

Radioactive substances are used to study living organisms, to diagnose and treat diseases (nuclear medicine), to sterilize medical instruments and food, to produce energy for heat and electric power, and to monitor various steps in all types of industrial processes, as well as in research studies (nuclear physics, chemistry, radiology, geochronology and geology, and cosmic research). Many natural and artificial radionuclides are strong radiotoxicants (generally, alpha emitters with a long half-time) to biological organisms. Studies on the bioaccumulation and distribution of radionuclides in different components of the natural environment are very important for radioprotection. Radionuclides used to produce energy in nuclear power plants generate the present and future problems of adequate and responsible utilization of nuclear wastes. For these reasons, the determination and distribution of some important natural and artificial radionuclides in the natural environment should be controlled and monitored.

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