Nuclei and Nuclear Reactions

An a particle is identical to a helium-4 nucleus and can be represented either as 2a or 2He. 

Another type of nuclear process, known as nuclear transmutation, results from the bombardment of nuclei by neutrons, protons, or other nuclei. An example of a nuclear transmutation is the conversion of atmospheric 7N to C and jH, which results when the nitrogen isotope is bombarded by neutrons (from the sun). In some cases, heavier elements are synthesized from lighter elements. This type of transmutation occurs naturally in outer space, but it can also be achieved artificially, as we will see in Section 20.4. 

Radioactive decay and nuclear transmutation are nuclear reactions, which differ significantly from ordinary chemical reactions. Table 20.1 summarizes the differences. 

To discuss nuclear reactions in any depth, we must understand how to write and balance nuclear equations. Writing a nuclear equation differs somewhat from writing equations for chemical reactions. In addition to writing the symbols for the various chemical elements, we must also explicitly indicate the number of subatomic particles in every species involved in the reaction. 

In accordance with the notation introduced in Section 2.3, the superscript in each case denotes the mass number (the total number of neutrons and protons present) and the subscript is the atomic number (the number of protons). Thus, the atomic number of a proton is 1, because there is one proton present, and the mass number is also 1, because there is one proton but no neutrons present. On the other hand, the mass number of a neutron is 1, but its atomic number is zero, because there are no protons presenL For the electron, the mass number is zero (there are neither protons nor neutrons present), but the atomic number is — 1, because the electron possesses a unit negative charge. 

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These questions lie in the field of nuclear astrophysics, an area concerned with the connection of fundamental information on the properties of nuclei and their reactions to the perceived properties of astrological objects and processes that occur in space. The universe is composed of a large variety of massive objects distributed in an enormous volume. Most of the volume is very empty (< 1 x 10 18 kg/m3) and very cold ( 3 K). On the other hand, the massive objects, stars, and such are very dense (sun s core 2 x 105 kg/m3) and very hot (sun s core 16 x 106 K). These temperatures and densities are such that the light elements are ionized and have... 

Because neutrons are electrically neutral, their interaction with electrons is very small and primary ionization by neutrons is negligible. The interaction of neutrons with matter is practically confined to the nuclei and comprises elastic and inelastic scattering and nuclear reactions. In elastic collisions the total kinetic energy remains constant, whereas in inelastic collisions part of the kinetic energy is given off as excitation energy. 

You may recall from Chapter 4 that the nuclei of some atoms are unstable and undergo nuclear reactions. In this chapter you will study nuclear chemistry, which is concerned with the structure of atomic nuclei and the changes they undergo. An applicahon of a nuclear reaction is shown in the photo of the human neck and skull. Table 25-1 offers a comparison of chemical and nuclear reactions. 

A fluid plasma is produced in the laboratory by heating a gas to a temperature, high enough to cause ionization into electrons and positive ions. At sufficiently high temperatures atoms, stripped of all their electrons, are known as cosmic rays. Above 10 K the bare nuclei undergo nuclear reactions. Thermonuclear fusion leads to the formation of heavier nuclei with the release of excess mass as kinetic energy. 

These theoretical investigations have been very provocative to the entire field of nuclear theory and have already been helpful in producing a clarification of the equivalence of various theoretical representations of nuclei and their reactions. They are also providing a better understanding of the validity of some of the simple models employed in high energy physics. 

The energy of stars comes primarily from the fusion of hydrogen nuclei to produce helium nuclei. This nuclear reaction explains why stars shine. Eventually some stars begin to run out of hydrogen, collapse, and explode—they become supernovae. Supernovae explosions scatter heavy elements throughout space. Eventually, some of these heavy elements drawn by the force of gravity became part of the mass of planets like the Earth. 

At very high energies, the so-called Coulomb barrier may be penetrated, and nuclear reactions may take place. The NRA method produces new nuclei of specific energies. As with RBS, these energies are reduced by collisions with electrons, providing for an excellent method of depth analysis. 

Temperature range 10 to 10 K. At about 10 K, nuclei begin to form and nuclear reactions occur. Temperatures of around 10 K occur in stars and supernova where heavier elements are synthesized from H and He. The binding energy per nucleon (proton or neutron) is in the range (1.0-1.5) 10 J (6.0-9.0) xlO eV, which corresponds to (6.0-9.0) xlO kJ/mol. 

There are two kinds of chemical reactions so-called ordinary chemical reactions and nuclear reactions. In ordinary chemical reactions there are no changes to the nuclei of the atoms. The only interaction between the atoms is among the atoms electrons. In nuclear reactions the electrons do not matter. What matters are changes that take place in the atoms nuclei. This chapter is about ordinary chemical reactions nuclear reactions are discussed in Chapters 13 and 14. 

A beam of charged particles (an ion beam) with an energy from a few hundred keV to several MeV is produced in an accelerator and bombards a sample. Nuclear reactions with low-Z nuclei in the sample are induced by this ion beam. Products of these reactions (typically p, d, t, He, a particles, and y rays) are detected, producing a spectrum of particle yield versus energy. Many (p, a) reactions have energies that are too low for efficient detection. In these cases, the associated y rays are detected instead. Important examples are ... 

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