Radioactivity: Ordinary hydrogen has one proton and no neutrons, so it has mass number 1. Heavy hydrogen, or deuterium, has mass number 2, because it has one proton and one neutron. A radioactive form of hydrogen, tritium, has mass number 3. It has one proton and two neutrons. Ordinary hydrogen, deuterium, and tritium are isotopes of hydrogen. All isotopes of an element have the same chemical properties. The uranium nucleus has 92 protons. The most plentiful isotope of uranium has 146 neutrons. Its mass number is therefore 238 (the sum of 92 and 146). Scientists call this isotope uranium 238 or U-238. The uranium isotope that almost all nuclear reactors use as fuel as 143 neutrons, and so its mass number is 235. This isotope is called uranium 235 or U-235. A nuclear reaction involves changes in the structure of a nucleus. As a result of such changes, the nucleus gains or loses one or more neutrons or protons. It thus changes into the nucleus of a different isotope or element. If the nucleus changes into the nucleus of a different element, the change is called transmutation. Radioactivity is the process by which atoms emit radiation, or atomic particles and rays of high energy, from their nuclei (cores). Of more than 2,300 different kinds of known atoms, more than 2,000 are radioactive. Only about 50 radioactive types exist in nature. Scientists make the rest artificially. Antoine Henri Becquerel of France discovered natural radioactivity in 1896. He found that uranium compounds emitted radiation which affected a photographic plate even when they are wrapped in black paper; they also ionised a gas. Soon afterwards, Marie Curie discovered an even more strongly radioactive substance, namely radium. Every element with an atomic number greater than that of lead (82) is radioactive. The nuclei of some of these elements can decay by splitting in two: this is Spontaneous fission. Natural radioactivity occurs in nine of the lighter elements also. Of these the most important are146C (carbon) and 4019K (Potassium). The isotope was probably formed when the earth was created. Its present existence is due to its long half-life of 1.25 x 109 years; though it only constitutes 0.01% of natural potassium, its presence makes living tissue appreciably radioactive. It may decay either by b-emission or electron capture. It is produced continuously from the action of the neutrons in cosmic rays on atmospheric nitrogen, by a nuclear reaction. Of the seventh row elements, only five are round in nature; radium, actinium, thorium, protactinium and uranium. Emission of Radiation: Different forms of radiation originate in the nuclei of radioactive atoms. There are three kinds of radioactive radiation: alpha particles, which were first identified by Becquerel; beta rays; identified by Ernest Rutherford of New Zealand; and gamma rays, identified by Marie and Pierre Curie of France. Emission of alpha or beta rays causes transmutation, but gamma radiation does not result in transformation. Alpha particles have a positive electrical charge. They consist of two protons and two neutrons, and are identical with the nuclei of helium atoms. Alpha particles are emitted with high energies, but lose energy rapidly when passing through matter. These are stopped by a thick sheet of paper; in air they have a range of a few centimetres, being eventually brought to rest by collisions with air molecules. They cause intense ionisation in a gas (by attracting electrons out of their molecules) and are deflected by electric and very strong magnetic fields. All alpha-particles emitted by a particular radioactive substance have the same speed, about one-twentieth of the speed of light. Americium emits alpha-particles only. Alpha radiation occurs in 238U, an isotope of uranium. After losing an alpha particle, the nucleus has 90 protons and 144 neutrons. The atom with atomic number 90 is no longer uranium, but thorium. The isotope formed is 23490Th. Beta rays are electrons. Some radioactive nuclei emit ordinary electrons, which have negative electrical charges. But others emit positrons, or positively charged electrons. For example, an isotope of carbon, 146C , gives off negative electrons. Carbon 14 has eight neutrons and six protons. When its nucleus transforms, a neutron changes into a proton, an electron, and an antineutrino. After emission of the electron and antineutrino, the nucleus contains seven protons and seven neutrons. Its mass number remains the same, but its atomic number 7 is nitrogen. Thus, 146C changes to 147N after emission of a negative beta particle. A carbon isotope, 116C , emits positrons. Carbon 11 has six protons and five neutrons. When it emits a positron, one proton changes into a neutron, a positron, and neutrino. After emission of the positron and the neutrino, the nucleus contains five protons and six neutrons. The mass number remains the same, but the atomic number drops by one. The element of atomic number 5 is boron. Thus,116C changes into 115B after emission of a positron and a neutrino. Strontium emits beta particles only. Beta particles travel with almost the speed of light. Some can penetrate 13 millimetres of wood. Gamma radiation may occur in several ways. In one process, the alpha or beta particle emitted by a nucleus does not carry off all the energy available. After emission, the nucleus has more energy than in its most stable state. It rids itself of the excess by emitting gamma rays. Gamma rays have no electrical charge. They are similar to X-rays, but they usually have a shorter wavelength. Whereas X-rays are due to energy changes outside atomic nuclei, as are all forms of electromagnetic radiation, gamma-rays, like alpha- and beta-particles, come from inside atomic nuclei. These rays are photons (particles of electromagnetic radiation) and travel with the speed of light. They are much more penetrating than alpha and beta particles. Radium emits alpha-, beta- and gamma-rays. Cobalt is a pure gamma source. Radioactive Decay and Half-Life: Radioactive decay is the process by which a nucleus spontaneously (naturally) changes into the nucleus of another isotope or element. The process releases energy chiefly in the form of nuclear radiation. The decay process happens of its own accord and cannot be controlled; it is unaffected by temperature changes, and occurs whether the material is pure or combined chemically with other elements. Uranium, thorium and several other natural elements decay spontaneously and so add to the natural, or background, radiation that is always present on the earth. Nuclear reactors produce radioactive decay artificially. Nuclear radiation accounts for about 10 per cent of the energy produced in a nuclear reactor. Scientists measure radioactive decay in units of time called half-lives. A half-life equals the time required for half the atoms of a particular radioactive element or isotope to decay into another element or isotope. The number of particles emitted in a given length of time by a sample of a radio-isotope (radioactive isotope) equals a definite percentage of the number of atoms in the sample. For example, in any sample of 11C, 3.5 per cent of the atoms break down each minute. At the end of a minute, only 96.5 per cent of the sample will remain. At the end of a second minute, only 96.5 per cent of the previous 96.5 per cent, or 93.1 per cent of the original amount, will remain. At the end of 20 minutes, only half of the original quantity will remain. This shows that the half- life of 11C is 20 minutes. This dying away of a substance is called radioactive decay or nuclear transformation. Different radio-isotope have different half- lives. They may range from fractions of a second to billions of years. With a few exceptions, the only radio-isotope found in nature in detectable quantities are those with half-lives of many millions or even billions of years. Scientists believe that when the elements that made up the earth were formed, all possible isotopes were present. Generally, those with short half-lives have decayed to undetectably small amounts. But some naturally occurring short-lived radio-isotope have been formed by the decay of long-lived radio-isotope. For example, thorium-234, which has a short-life, is produced from uranium, which has a long half-life. Hundreds of short-lived radio-isotopes are produced artificially by bombarding nuclei with neutrons and other fast nuclear particles in nuclear reactors. When a neutron or other particle strikes an atom’s nucleus, the nucleus is likely to capture it. In some cases, a nucleus captures a particle and immediately gives off some of its own particles.