![]() Nuclear Reactions– changing the hearts of atoms
I A nuclide, A, when bombarded by energetic subatomic particles, a, changes to another nuclide is called a nuclear reaction. The energetic particles a either from radioactive decays or from particle accelerators. Often, the products consist of light particles b and another nuclide B. The reaction can be written as a + A B + b. This reaction is often written in a short form, A (a, b) B, where a and b may be an , , , neutron, proton, deuterium, a nuclide, or high-energy electron. An exothermic nuclear reaction releases energy, and an endothermic nuclear reaction requires energy. The energy required in an endothermic reaction can be supplied in the form of kinetic energy (of the incident particle a).
Ideally, the energetic particle a must approach A within 10 15 m for a nuclear reaction to take place, because the strong force will only be effective at this distance. Particles such as protons, and light nuclides with a positive charge experience a repulsion of the atomic nucleus, due to the electromagnetic force. The repulsion results in a rise of the potential energy called the Coulomb barrier. They must carry enough energy to overcome the Coulomb barrier. Once in contact (10-15 m) with any nucleon or quark of the nucleus, the strong force becomes effective, merging the incident particle with the nucleus. Such an interaction makes the potential energy uniform and low, within the nucleus forming a potential well due to the strong force. On the other hand, neutral particles (neutrons) approach the target nuclei experiencing no Coulomb repulsion. Once in contact with the nucleus, a neutron becomes part of the nucleus. However, neutrons carrying high kinetic energies will be bounced off or knock other nucleons out of the nucleus. These considerations are given in planing nuclear reaction experiments. The forgoing consideration suggests that the type of nuclear reaction depends on the target material A, the incoming particles a, and their energies. Particles from an accelerator may have the same energy before they enter the target. Interactions of incident particles with the target atoms alter the energy of particles before they react with A. Due to the range of energies of the incident particles, several modes of nuclear reactions may take place.
Discoveries of Nuclear ReactionsNuclear reactions were discovered in 1919. At that time, tracks of particles were made visible in cloud chambers. Their discovery was due to the power of mind and a keen observation.
In 1914, E. Marsden and E. Rutherford studied particles. In the vicinity of the particle source, they observed some tracks of positively charged particles that were different from those of particles.
In the cloud chamber, these particles made longer but thinner tracks than the -particle tracks. Furthermore, these particles gave more point like scintillation images on the zinc sulfide (ZnS fluorescence material) screen than the particles did. Eventually, they identified them as hydrogen nuclei or protons. At first, they thought the protons came from ionization of water molecules, but they carried out these experiments carefully under water-free conditions. The persistence of the protons around the source led them to the extraordinary conclusion that "the nitrogen atom is disintegrated under the intense force developed in a close collision with a swift particle". They considered the hydrogen atomic nuclei so liberated constituents of the nitrogen nuclei. This conclusion led to the observation of the first nuclear reaction in 1919, and they postulated the reaction to be: 14N + 4He 17O + 1H or in short form 14N (, p) 17O, which is often called an (, p) reaction. At about the same time, F. Joliot and I. Curie bombarded aluminum with alpha particles. After the bombardment, they found the aluminum metal radioactive. The induction of artificial radioactivity by particle bombardment marks another nuclear reaction,
The 30P further decay by positron emission or electron capture (EC) leading to a stable isotope of silicon, 30Si. The half-life of 30P is 2.5 min. The short notation ( , + or EC) indicates a radioactive decay process which involves no incident particles as a reactant. Another milestone in the study of nuclear reactions took place around 1929 when John D. Cockroft and Ernest T.S. Walton devised an accelerator in the Cavendish Laboratory, Cambridge, England. They applied high voltage to accelerate protons and observed the reaction:
This was actually a proton induced fission reaction because the lithium nuclei were divided into two halves. However, they called the reaction the smashing of an atom by artificially accelerated particles. Skill Developing Questions
Nuclear Reaction Experiments![]() A typical nuclear reaction experiment requires a source of energetic particles, a target containing atomic nuclei, a shield, detectors, and a data collection and analysis system as depicted here. Furthermore, the complicated data collection and analysis may be helped by the use of computers.
In an intended experiment, we usually know the particles and target nuclides used, but usually not the products. The parameters such as the types and energies of the particles and targets are set or known in an experiment, but the products are seldom as predicted. To understand a nuclear reaction, products must be detected and identified. Instruments extend our senses to see the products. Careful analysis of the data helps us to interpret the reaction. In addition to particles from radioactivity, high-energy particle accelerators provide energetic particles for the study of nuclear reactions*. Often, charged particles such as protons, alpha particles, atomic nuclei, electrons and positrons are accelerated to energies in keV, MeV, and GeV. They are used in nuclear reactions. After the bombardment, sophisticated detectors are built to detect particles emitted by the target nuclei after the reaction. Energy, charge, and type of emitted particles can be determined by specific detectors. Thus, some of the products can be identified. The unidentified products can be inferred based on the conservation of charges, particles, and masses. Research nuclear reactors usually provide neutron sources. Neutrons are captured by many nuclides and the reactions produce radioactive nuclides. Identities of the products can be determined by measuring the types of decay, the energies of the particles, and the half lives. These measurements usually lead to the identification nuclides produced by comparison with properties of known radioactive nuclides. There are many applications for nuclear reactions. For example, some information on the Basics of Boron Neutron Capture Therapy (BNCT) can be found in the URLs: http://www.mit.edu:8001/people/flavor/intro.html. and http://www.mallinckrodt.nl/nucmed/noframes/general/nucmed.htm
Neutron SourcesNeutrons are ideal bombarding particles for nuclear reactions, because they approach atomic nuclei experiencing no Coulomb barrier as do positive particles.
Mixtures used as Neutron Sources Neutron Source Reaction energy / MeV Ra and Be 9Be (, n) 12C up to 13 Po and Be up to 11 Pu and B 11B (, n) 14N up to 6 In 1932 James Chadwick* bombarded beryllium with alpha particles, and discovered a neutral particle, the neutron. The reaction is now used as a neutron source, and the reaction is 9Be (, n) 12C. Further study showed that bombardment of boron by alpha particles also produced neutrons in the reaction, 9B (, n) 12N. Since particles do not travel more than a few centimeters, emitting radioactive nuclides Ra, Po, and Pu are mixed with beryllium or boron to produce neutrons. Only small fractions (in the order of 0.005% to 0.05% depending on the mixture) of the alpha particles emitted result in the production of neutrons. These mixtures are called neutron sources. The energies of the neutrons so produced are in the order of MeV.
Ra, Be 9Be (, n) 8Be 0.6 Ra, D2O 2D (, n) 1H 0.1
Neutrons can also be produced using accelerated particles. A d-d reaction, 2D (d, n) 3He, gives different yields depending on the energy (100 KeV to 2 MeV) of the accelerated deuterium, d (2D). Better yields of neutrons are obtained with the d-t reaction, 3T (d, n) 4He. These fusion reactions are well studied, and they will be discussed in Chapter 9 on nuclear fusion. Another type of neutron source is provided by spontaneous fission. For example, the nuclide 252Cf decays by 97 % alpha decay and by 3% spontaneous fission. Every fission reaction releases an average of 3.8 neutrons. Nuclear fission reactions are discussed in Chapter 8. Major sources with very high numbers (intensities or densities) of neutrons (1015 n cm-2 s-1 or higher) are close to the core area of nuclear reactors. More information will be provided for these sources in conjunction with nuclear fission and nuclear reactor technology in Chapter 8. Skill Building Questions
Neutron Induced RadioactivityNeutrons, discovered in 1932, are ideal projectiles for inducing nuclear reactions. Neutrons are captured by most stable nuclides. The increase of neutrons in these reactions produces radioactive materials, mostly beta emitters.
Emission of light particles , , and in neutron-induced reactions are often delayed. Half-lives of nuclei produced and their decay energies are determined by experiments, and these data provide identification for the products. Once the products are identified, the reactions are deduced. Almost every element absorbs neutrons, but some more than others. S 19F (n, ) 16N 27Al (n, ) 24Na ( , ) 24Mg. After that, he told his student Segré to buy all possible pure elements found in Mendeleyev's periodic table, and then they bombard what they have bought with neutrons. Using a pure element as target material reduced complication due to other elements. They produced radioactive nuclides with various half-lives for the elements iron, silicon, phosphorous, vanadium, copper, arsenic, silver, tellurium, chromium, barium, samarium, gold, neodymium, etc. They identified (n, ), (n, p) and (n, ) reactions. The neutron bombardments gave them many new radioactive nuclides, and Fermi was awarded with the Nobel Prize for Chemistry in 1938 for his identification of new radioactive elements produced by neutron bombardment and his discovery, made in connection with this work, of nuclear reaction affected by slow neutron. After receiving this prize on Dec. 12, he went to the United States directly from Stockholm, fulfilling his wish since the day Italy joined Hitler. Skill Building Questions
Nuclear Reactions Induced by Cosmic RaysT ![]() he primary cosmic rays arriving at the top of the earth's atmosphere consist mostly of positively charged particles, mainly protons (83 %). Most cosmic protons have energy in the range between 1 and 2 GeV (2 giga eV or 109 eV), and a few reach high energies of ~1018 eV. Other components of the cosmic rays include nuclei of He (0.6 %), C, N, O and most elements of the periodic Table.
Cosmic rays interact with atomic nuclei in the atmosphere as well as those of liquids and solids. The impact of primary cosmic rays near the top of the atmosphere produces violent nuclear reactions in which many neutrons, protons, alpha particles and other fragments are produced. Some light nuclides such as 3H, 4He, 7Be, 10B are also produced. Lithium, beryllium and boron are practically absent in stellar objects, but are abundant in cosmic rays. They are probably produced in interstellar space through collisions of protons and alpha particles with interstellar gases. One interesting nuclear reaction due to cosmic rays is the formation of 14C, 14N (n, p) 14C The half-life of the -emitting 14C is 5730 y. Carbon atoms circulate around the planet Earth forming a carbon cycle. Thus, carbon in systems actively exchange carbon in this cycle contains a certain amount of the radioactive 14C. This type of carbon has a specific radioactivity (radioactivity per unit weight of say gram) of 14.9 disintegration per minute per gram. This radioactivity is readily measurable. When a carbon-containing sample is isolated from the carbon cycle, no isotope exchange takes place. Its 14C isotope decay according to a half-life of 5730 y. Thus, the specific radioactivity decreases. Thus, by measuring the specific radioactivity of a sample enables us to determine the age (of isolation) for the sample. This method is called 14C-dating or carbon dating. Meteorites are exposed to a high level of cosmic rays. Nuclear reactions generate many radioactive nuclides, and as a result, the radioactivity of meteorites is usually high. Analysis of isotope distribution reveals interesting results of cosmic rays and history of meteorites, but this subject is a spin-off from a general discussion of nuclear reactions.
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