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| Nuclear Stability |
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| The nucleus is made of neutrons and protons. What causes them to stick together? Why do the protons not repel each other? |
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| Surprisingly, the mass of the nucleus is less than the sum of the masses of the individual protons and neutrons which makeup the nucleus. The lost mass (mass defect) has been changed into the energy necessary to bind the nucleus together. Suppose the mass defect is
Dm. Then the energy binding the nucleus together
DE is given by: |
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| where c is the speed of light. |
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If  is divided by the number of nucleons in the nucleus, then the binding energy per nucleon is obtained. The higher this value is the more stable is the nucleus. |
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| Since the positive protons in the nucleus repel each other, neutrons are required to bind the nucleus together. For lighter elements, the number of protons and neutrons are approximately equal. As the number of protons increases the force required to hold the nucleus together increases and to such an extent that more neutrons are required and the ratio of neutrons / protons is greater than one. For heavier elements, the number of neutrons in the nucleus is approximately one and a half times the number of protons. |
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Whenever a  - particle is emitted a neutron is lost and a proton gained. Therefore the neutron/proton ratio is reduced by  - radiation. |
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When an  - particle is emitted the nucleus loses two protons and two neutrons. Since there were originally more neutrons than protons it follows that the neutron/proton ratio is increased. |
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Thus, when the neutron/proton ratio is higher than the stability ratio, a  - particle is emitted to bring it nearer to the stability ratio. When the neutron/proton ratio is lower than the stability ratio, a  - particle is emitted to raise the ratio and bring it nearer to the stability ratio. |
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| Nitrogen N is stable and the neutron/ proton ratio is 1, when it is bombarded by neutrons. |
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Thus, carbon C is formed and the neutron/proton ratio is 8/6, which is greater than 1. Carbon-14 then emits a  - particle to form the stable N. |
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| Radioactive elements can undergo a number of different reactions in the process of forming a stable element. Nuclear fission is one such reaction. Fission means to divide. Elements having atomic numbers greater than 90 can undergo fission. Uranium is one such element. In nuclear fission, the nucleus of an atom breaks up. |
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In other words, heavy unstable nuclides can be
split to produce energy in a process called nuclear fission. When uranium decays naturally  - particles and  - particles are emitted. However, when uranium - 235 is bombarded by neutron it forms uranium - 236.
Uranium - 236 is unstable and breaks down, splitting into two large particles and emitting three neutrons. |
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When the exact masses of the final products are added together, the sum is found to be appreciably less than the sum of the exact masses of the uranium - 235 and the original neutron. This difference in mass  appears as energy given by |
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 [c is the velocity of light] |
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| The Uranium - 236 can fission to give
isotopes of Barium-144, Krypton - 90 and two neutrons. |
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| Nuclear Fission |
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| Uranium - 236 can also divide to produce other products. |
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| Fission in uranium atoms may result in a chain reaction. A nuclear chain reaction is a series of rapid nuclear fissions. A small sample of uranium contains billions of atoms. When one uranium - 235 nucleus is split by a neutron, it releases two neutrons. The two neutrons are used to split two more uranium nuclei. Each nuclei, causing them to split. The fission of nuclei and release of neutrons becomes a chain reaction. In a chain reaction, billions of fission reaction may occur per second. |
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| When lighter nuclides fuse together to form a heavy nucleus in a process called nuclear fusion, energy is produced and mass is lost. For example, two atoms of heavy hydrogen may fuse together to form helium and a neutron: |
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| The sum of the exact masses of the helium atom and the neutron is less than the sum of exact masses of the two heavy hydrogen atoms. This lost mass is released as energy. It is thought that the Sun's energy is produced by nuclear fusion. The two heavy hydrogen atoms must move with great speed in order to overcome the strong repulsive forces between the positively charged nuclei. The very high temperatures required for fusion reactions would melt any known material. Therefore, the atoms are held in a plasma (hot ionised gas) in an intense magnetic field so that they do not touch the walls of the container. It has not yet been possible to produce a controlled reaction, but in future this problem may well be overcome. An uncontrolled fusion reaction has been produced in the hydrogen bomb. |
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| Nuclear fusion is the opposite of nuclear fission. Elements with small masses combine to form elements with larger masses. |
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| For nuclear fusion to occur, temperatures well over one million degree Celsius must be reached. Nuclear fusion is called thermonuclear reaction. At the tremendous temperature of thermonuclear reactions, nuclear atoms no longer exist. The atoms lose their electrons (ionize) and become plasma. Recall that plasma is the
fourth state of matter consisting of nuclei and free electrons. The nuclei have enough energy to overcome forces of repulsion between them. |
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| The temperature conditions for fusion exists in the sun and stars. The sun has an internal temperature of about 20,000,000oC. In the sun, fusion occurs through a complex series of nuclear changes. During the early life of a star, four hydrogen nuclei are fused into one helium nucleus in a series of steps. |
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| The sun is constantly losing hydrogen as it forms helium. A tremendous amount of energy is released during this fusion. The energy comes from matter that is converted to energy. The helium that is formed has a mass almost one percent less than the mass of the four hydrogen atoms. This one percent mass is converted to energy. |
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| Light and radiant energy from nuclear fusion travel 150 million kilometers from the sun to the earth. However, the total energy that reaches the earth is only a tiny fraction of the total energy released by the sun. |
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