Radioactivity and Nuclear Reactions

Transcription

1 Radioactivity and Nuclear Reactions Radioactivity Protons and Neutrons are attracted to each other in the nucleus of an atom by the strong force The total force between these particles depends on how far apart they are Smaller nuclei have a stronger attraction to each other and are more stable Larger nuclei are held together less tightly and the strong force may not be enough to keep the atom stable. When the strong force is not enough to hold a nucleus together, the nucleus begins to decay and give ff matter and energy. This is called radioactivity 2 Radioactive decay is the process by which the unstable nuclei lose mass and/or energy by emitting radiation. Eventually unstable nuclei achieve a more stable state when they are transformed into atoms of a different element. All nuclei that contain more than 83 protons are radioactive Almost all elements with more than protons don t exist naturally on Earth and are only produced in laboratories These are called synthetic elements and decay soon after they are created due to being unstable 3 4 Radioactivity is all around you, even in your body It cannot be detected by smell, sight, sound, taste, or touch Radioactivity can be captured on photographic plates (x-rays) Radioactivity Radioactivity was st discovered by Antoine Becquerel, when a photographic plate never exposed to Sunlight in his lab had become exposed. The only possible culprit was a nearby uranium salt sitting on the bench top. 5 6

2 Nuclear Decay There are three types of nuclear radiation Alpha Beta Gamma radiation Alpha and Beta are particles Gamma radiation behaves like a wave of light at a very high frequency Alpha Particles Alpha Particles are the least penetrating form of radiation. They can be stopped by a piece of paper Ex. Smoke detectors give off alpha particles that ionize the surrounding air. Smoke particles absorb the ions and electrons and break a circuit which causes the alarm to go off 7 8 Beta Particles Beta particles are faster and more penetrating than alpha particles. They can pass through paper and are stopped by a sheet of aluminum foil Gamma Rays The most penetrating form of radiation are gamma rays Gamma rays carry energy Thick blocks of lead and concrete are required to stop gamma rays 9 0 Penetrating Power of Radiation Radioactive Half-Life Half-Life (t /2 ) is the time required for half of the atoms of a radioisotope to emit radiation and to decay to products. 2 2

3 Half-Life Example It takes 4.5 X 0 9 years for one half of a sample of uranium-238 to decay to lead-206. Therefore, it would take another 4.5 X 0 9 years for one half of the remaining uranium to decay, et cetera, et cetera, et cetera. 00g 50g U U half-life 2nd half-life g 25g 25g 3rd half-life g U 4th half life 2.5g g U 3 How many atoms of a 2.97g. sample of molybdenum-9 would remain after 62 min. if the half-life of molybdenum-9 is 5.49 min.? How many ½ lives is this? # Half-Lives Time Spent (min) Amount Remaining (g) Answer = 0.9 g 4 half-lives 4 Types of Nuclear Reactions There are two types of nuclear reactions Fission splitting the nucleus Fusion fusing or combining of nuclei Nuclear Fission Fission is the breaking apart of a very heavy nucleus into parts. 4 U 36Kr + 56Ba n 5 6 Fusion is the combining of 2 small nuclei into larger one. Fusion of hydrogen into helium occurs in the sun. Fusions reactions should produce much more energy than fission and use much more accessible fuels. However, currently many problems exist in fusion reactions such as the extremely high temperature needed for the reaction. Film badges are used to monitor the amount of radiation exposure people have received. Radiation Detection 7 8 3

4 Geiger Counter Scintillation Counter Instrument that detects radiation by measuring current produced by gas particles ionized by radioactivity 9 Instrument that converts light to an electric signal for detecting radiation. 20 Uses for Nuclear Radiation Since the physical and chemical properties of radioisotopes of an element are the same as stable ones, many uses for radioactive nuclides are possible. In medicine radioactive nuclides are used to destroy cancer cells and as tracers to tract substances through the body or identify cancer and other diseases. 2 Cobalt - 60 Radioactive Tracer 22 In agriculture, radioactive nuclides are used as tracers in fertilizer to determine the effectiveness or to prolong shelf life of food by irradiating to destroy microorganisms. 23 In dating radioactive nuclides are used to determine the age of objects. Example: Carbon -4 is used to date organic materials. 24 4

5 In energy production, currently nuclear fission is used to create energy. Example: Comanche Peak nuclear power plant in Glen Rose produces energy that is used by TXU. Nuclear Waste Nuclear fission produces radioactive wastes that must be contained and stored on-site (temporary) or disposed of (permanent) What part of the atom is involved? How is the reaction started? What is the outcome of the reaction? How much energy is absorbed or released? What are some examples? Chemical Reaction Outermost electrons (valence electrons) Atorms are brought close together with high temperature or pressure, or catalysts, or by increasing concentrations of reactants Atoms form ionic or covalent bonds A small amount Burning fossil fuels, digesting food, housecleaning, making medicines and commercial products Nuclear Reactions Protons and neutrons in the nucleus High temperature is required or atoms are bombarded with highspeed particles The number of protons and neutrons in an atom usually changes A huge amount Nuclear energy, taking x- rays, treating cancer, irradiating food to sterilize it, the sun generating heat and light 27 5