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| Radioactivity |
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| It is a spontaneous emission or disintegration of an unstable nucleus resulting in certain radiations. The elements exhibiting this phenomena are called radioactive elements. e.g., radium, thorium, actinium, polonium etc. |
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| Radioactivity does not occur due to any external provocation and is a property of heavy nuclei. |
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| Nuclei are subjected to two competing forces, the attractive nuclear force and the repulsive coulomb force between the protons. The stability of a nucleus is high when it contains more neutrons than protons. |
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| Radiations emitted by radioactive elements are of three kinds depending upon their ability to penetrate matter. These are alpha rays, beta rays and gamma rays. |
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| The following picture shows what happens when U238 decays. Notice that the decay chain continues for many years and ends up as lead. Lead is the most stable element. In many of the intermediate steps, the nucleus gets rid of some energy by emitting gamma radiation along with alpha or beta rays. |
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| (i) a-rays consists of a-particles carrying double positive charge having a mass roughly four times that of hydrogen atom. |
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| (ii) Due to large mass, their penetrating power is very small. |
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| (iii) Due to large mass and velocity they have a large ionizing power. |
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| (iv) a- rays produce fluorescence in certain substances. |
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| (v) a- particles affect photographic plate. |
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| (vi) a-rays are deflected by electric and magnetic fields. |
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| (i) b-rays consist of negatively charged particles and have the same mass as that of an electron. |
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| (ii) Since their velocities are 1/10th of the speed of light they are considered as fast moving electrons. |
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| (iii) The penetrating power of b-particles is larger than that of a -particles. |
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| (iv) The b-particles can ionise the gas through which they pass, but their ionising power is only 1/100th of a-particles. |
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| (v) The range of b-particles in air is several meters and is greater than a-particles. |
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| (vi) b-particles can also produce fluorescence. |
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| (vii) They affect a photographic plate. |
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| (viii) They are also deflected by electric and magnetic field showing that they carry a negative charge. |
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| (i) g-rays are not deflected by electric and magnetic fields and this shows that they do not have any charge. |
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| (ii) The rest mass of g-ray photon is zero. |
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| (iii) The g-rays travel with the speed of light. |
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| (iv) g-rays have a very large penetrating power and small ionizing power. |
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| (v) g-rays also produces fluorescence. |
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| (vi) g-rays affect photographic plate more than a or b-particles. |
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| (vii) g-rays can produce nuclear reactions. |
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