Radioactivity Spontaneous Nuclear Reactions

A nucleus that consists of neutrons and protons can be stable or unstable. Understanding of the factors that affect the nuclear stability is beyond the scope of this discourse, and it is not necessary to really understand for our purpose here. However, we have the facts i.e., which nuclei (isotopes) are stable and which are unstable. The unstable isotopes would not remain as such and spontaneously change into stable isotopes. In this process, the unstable nucleus emits particles and/or energy in the form of radiation (gamma (y)-ray). Particles that are emitted include electron (called i-particle), and helium nucleus (a-particle). Hence, these unstable isotopes are called radioactive, emitting a, p, and/or y radiation. There are other kinds of radiation, as well, but these three are the major ones. 

There are exactly 264 stable isotopes known on the Earth. They he on or shghtly above a line that corresponds to the composition of equal numbers of neutrons and protons, as shown in Fig. 19.4. The area that is occupied by stable isotopes is coined as stabihty peninsula (the darkened area the solid black curve represents a sort of central area where most of stable isotopes are distributed). In the hgure, each of the 

19 Chemistry s View of the Material World Basic Principles 

Atomic Number (Number of Protons) Fig. 19.4 Stable isotopes and unstable (radioactive) isotopes 

In understanding the radioactivity, we need to recognize some changes that can occur with neutron and proton. That is, they can interchange in the following manner  

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Radioactivity—Spontaneous nuclear transformations that result in the formation of new elements. These transformations are accomplished by emission of alpha or beta particles from the nucleus or by the capture of an orbital electron. Each of these reactions may or may not be accompanied by a gamma photon. 

ALPHA DECAY. The emission of alpha particles by radioactive nuclei. The name alpha particle was applied in the earlier years of radioactivity investigations, before it was fully understood what alpha particles are. It is known now that alpha particles are the same as helium nuclei. When a radioactive nucleus emits an alpha particle, its atomic number decreases by Z = 2 and its mass number by A = 4. The process is a spontaneous nuclear reaction, and the radionuclide that undergoes the emission is known as an alpha emitter. 

Calculations show that maximum annual effective population exposure dose at the boundary of Sanitary Protection Area (500 meters from radioactive release point) will be 37 mSv under the hypothetical accident related to spontaneous nuclear reaction. It will not be the necessity to evacuate the population in accordance to Radiation Safety Standards (NRB -99). Collective radiation doses for population will not exceed the annual dose of this region received from natural radiation background. 

Uranium-235 and U-238 behave differently in the presence of a controlled nuclear reaction. Uranium-235 is naturally fissile. A fissile element is one that splits when bombarded by a neutron during a controlled process of nuclear fission (like that which occurs in a nuclear reactor). Uranium-235 is the only naturally fissile isotope of uranium. Uranium-238 is fertile. A fertile element is one that is not itself fissile, but one that can produce a fissile element. When a U-238 atom is struck by a neutron, it likely will absorb the neutron to form U-239. Through spontaneous radioactive decay, the U-239 will turn into plutonium (Pu-239). This new isotope of plutonium is fissile, and if struck by a neutron, will likely split. 

Plutonium (symbol Pu atomic number 93) is not a naturally occurring element. Plutonium is formed in a nuclear reaction from a fertile U-238 atom. Since U-238 is not fissile, it has a tendency to absorb a neutron in a reactor, rather than split apart into smaller fragments. By absorbing the extra neutron, U-238 becomes U-239. Uranium-239 is not very stable, and undergoes spontaneous radioactive decay to produce Pu-239. 

The discoveries of Becquerel, Curie, and Rutherford and Rutherford s later development of the nuclear model of the atom (Section B) showed that radioactivity is produced by nuclear decay, the partial breakup of a nucleus. The change in the composition of a nucleus is called a nuclear reaction. Recall from Section B that nuclei are composed of protons and neutrons that are collectively called nucleons a specific nucleus with a given atomic number and mass number is called a nuclide. Thus, H, 2H, and lhO are three different nuclides the first two being isotopes of the same element. Nuclei that change their structure spontaneously and emit radiation are called radioactive. Often the result is a different nuclide. 

Ans. Other types of reactions require a small particle to react with a nucleus to produce a nuclear reaction radioactive decay processes are spontaneous with only the one nucleus as reactant. 

Californium is a synthetic radioactive transuranic element of the actinide series. The pure metal form is not found in nature and has not been artificially produced in particle accelerators. However, a few compounds consisting of cahfornium and nonmetals have been formed by nuclear reactions. The most important isotope of cahfornium is Cf-252, which fissions spontaneously while emitting free neutrons. This makes it of some use as a portable neutron source since there are few elements that produce neutrons all by themselves. Most transuranic elements must be placed in a nuclear reactor, must go through a series of decay processes, or must be mixed with other elements in order to give off neutrons. Cf-252 has a half-life of 2.65 years, and just one microgram (0.000001 grams) of the element produces over 170 mhhon neutrons per minute. 

Some nuclei are unstable and emit particles and electromagnetic radiation. These emissions from the nucleus are known as radioactivity the unstable isotopes are known as radioisotopes and the nuclear reactions that spontaneously alter them are known as radioactive decay. Particles commonly involved in nuclear reactions are listed in the following table ... 

The Kinds of Nuclear Reactions. Many different kinds of nuclear reactions have now been studied. Spontaneous radioactivity is a nuclear reaction in which the reactant is a single nucleus. Other known nuclear reactions involve a proton, a deuteron, an alpha particle, a neutron, or a photon (usually a gamma ray) interacting with the nucleus of an atom. The products of a nuclear reaction may be a heavy nucleus and a proton, an electron, a deuteron, an alpha particle, a neutron, two or more neutrons, or a gamma ray. In addition, there occurs the very important type of nuclear reaction in which a very heavy nucleus, made unstable by the addition of a neutron, breaks up into two pans of comparable size, plus several neutrons. This process of fission has been mentioned in Chapter 25 and, is described in a later section of the present chapter. 

Radioactive xenon (radioxenon) is produced by the fissioning of nuclear material, either via neutron-induced or spontaneous fission, and also via neutron activation or other nuclear reactions involving xenon gas. The most abundant radioactive xenon isotopes in... 

All the nuclear reactions that have been described thus far are examples of radioactive decay, where one element is converted into another element by the spontaneous emission of radiation. This conversion of an atom of one element to an atom of another element is called transmutation. Except for gamma emission, which does not alter an atom s atomic number, all nuclear reactions are transmutation reactions. Some unstable nuclei, such as the uranium salts used by Henri Becquerel, undergo transmutation naturally. However, transmutation may also be forced, or induced, by bombarding a stable nucleus with high-energy alpha, beta, or gamma radiation. 

Medieval alchemists spent years trying to convert other metals into gold without success. Years of failure and the acceptance of Dalton s atomic theory early in the nineteenth century convinced scientists that one element could not be converted into another. Then, in 1896 Henri Becquerel discovered radioactive rays (natural radioactivity) coming from a uranium compound. Ernest Rutherford s study of these rays showed that atoms of one element may indeed be converted into atoms of other elements by spontaneous nuclear disintegrations. Many years later it was shown that nuclear reactions initiated by bombardment of nuclei with accelerated subatomic particles or other nuclei can also transform one element into another—accompanied by the release of radiation (induced radioactivity). 

Becquerel presented the problem to Marie and Pierre Curie for further study. Their conclusion was that a nuclear reaction was taking place within the uranium atoms. Marie Curie named this spontaneous emission of radiation by an unstable atomic nucleus radioactivity. 

Unlike chemical reaction rates, which are sensitive to factors such as temperature, pressure, and concentration, the rate of spontaneous nuclear decay cannot be changed. Because the decay of an individual nucleus is a random event, it is impossible to predict when a specific nucleus in a sample of a radioactive material will undergo decay. However, the overall rate of decay is constant, which allows you to predict when a given fraction of the sample will have decayed. 

Natural radioactivity is a spontaneous process artificial radioactivity is nonspontaneous and results from a nuclear reaction that produces an unstable nucleus. 

Reactions between an atomic nucleus and another particle are called nuclear reactions. In some such reactions, new nuclei are formed nuclear transmutations) in others the original nucleus is excited to a higher energy state (inelastic scattering) in a third class, the nucleus is unchanged (elasticscattering). Spontaneous nuclear transformations, which are involved in the radioactive decay of unstable nuclei, have be discussed in Chapter 4. In this chapter the enqrhasis is on the mass and energy relationships when a projectile interacts with a nucleus. 

The nuclide gCf emits neutrons through spontaneous fission in 3% of all decays, the rest being a-decays. All the other neutron sources listed involve a radioactive nuclide whose decay causes a nuclear reaction in a secondary substance which produces neutrons. For example, ffSb produces neutrons in beryllium powder or metal as a result of the initial emission of 7-rays, in which case there is no coulomb barrier to penetrate. Radium, polonium, plutonium, and americium produce neutrons by nuclear reactions induced in beryllium by the a-particles from their radioactive decay. For the neutrons produced either by spontaneous fission in californium or by the a-particle reaction with beryllium, the... 

Nuclear reactions In the late 1890s, scientists noticed that some substances spontaneously emitted radiation in a process they named radioactivity. The rays and particles emitted by the radioactive material were called radiation. Scientists discovered that radioactive atoms undergo changes that can alter their identities. A reaction that involves a change in an atoms nucleus is called a nudear reaction. The discovery of these nuclear reactions was a major breakthrough, as no chemical reaction had ever resulted in the formation of new kinds of atoms. 

Most nuclei in nature are stable and remain intact indefinitely. Radionuclides, however, are unstable and spontaneously emit particles and electromagnetic radiation. Emission of radiation is one of the ways in which an unstable nucleus is transformed into a more stable one that has less energy. The emitted radiation is the carrier of the excess energy. Uranium-238, for example, is radioactive, undergoing a nuclear reaction emitting helium-4 nuclei. The helium-4 particles are known as alpha ( ) particles, and a stream of them is called alpha radiation. When a nucleus loses an alpha particle, the remaining fragment has an atomic number of 90 and a mass number of 234. The element with atomic number 90 is Th, thorium. Therefore, the products of uranium-238 decomposition are an alpha particle and a thorium-234 nucleus. We represent this reaction by the nuclear equation... 

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