Radiation ALPHA, BETA, AND GAMMA
Science Park HS -- Honors Chemistry Early Pioneers in Radioactivity Roentgen: Discoverer of X-rays 1895 Becquerel: Discoverer of Radioactivity 1896 The Curies: Discoverers of Radium and Polonium Rutherford: Discoverer Alpha and Beta rays 1897
Radioactivity Scientists quickly learned that there were three different kinds of radiation given off by radioactive materials. Alpha rays Beta rays Gamma rays The scientists called them “rays” because the radiation carried energy and moved in straight lines, like light rays.
Radioactivity We now know that radioactivity comes from the nucleus of the atom. If the nucleus has too many neutrons, or is unstable for any other reason, the atom undergoes radioactive decay. The word decay means to "break down."
Radioactivity In alpha decay, the nucleus ejects two protons and two neutrons. Beta decay occurs when a neutron in the nucleus splits into a proton and an electron. Gamma decay is not truly a decay reaction in the sense that the nucleus becomes something different.
Alpha-particle production Alpha particle – helium nucleus Examples Net effect is loss of 4 in mass number and loss of 2 in atomic number.
Beta-particle production Beta particle – electron Examples Net effect is to change a neutron to a proton.
Gamma ray release Gamma ray – high energy photon Example Net effect is no change in mass number or atomic number.
Three Common Types of Radioactive Emissions - Penetrability Alpha particles may be completely stopped by a sheet of paper, beta particles by aluminum shielding. Gamma rays, however, can only be reduced by much more substantial obstacles, such as a very thick piece of lead.
Radioactivity Radioactive decay gives off energy. The energy comes from the conversion of mass into energy. Because the speed of light (c) is such a large number, a tiny bit of mass generates a huge amount of energy. Radioactivity occurs because everything in nature tends to move toward lower energy.
Detection of Radioactivity Geiger-Muller counter – instrument which measures radioactive decay by registering the ions and electrons produced as a radioactive particle passes through a gas-filled chamber Scintillation counter instrument which measures the rate of radioactive decay by sensing flashes of light that the radiation produces in the detector
Radioactivity Radioactive decay depends on chance. It is possible to predict the average behavior of lots of atoms, but impossible to predict when any one atom will decay. One very useful prediction we can make is the half-life. The half-life is the time it takes for one half of the atoms in any sample to decay.
Half-life The half-life of carbon-14 is about 5,700 years. If you start out with 200 grams of C-14, 5,700 years later only 100 grams will still be C-14. The rest will have decayed to nitrogen-14.
Where are the Sources of Radioactivity? Naturally Occurring Sources: Radon from the decay of Uranium and Thorium Potassium -40 – found in minerals and in plants Carbon 14 – Found in Plants and Animal tissue Manmade Sources: Medical use of Radioactive Isotopes Certain Consumer products –(eg Smoke detectors) Fallout from nuclear testing Emissions from Nuclear Power plants
Nuclear Reactions and Energy A nuclear reaction is any process that changes the nucleus of an atom. Radioactive decay is one form of nuclear reaction.
Nuclear Reactions and Energy If you could take apart a nucleus and separate all of its protons and neutrons, the separated protons and neutrons would have more mass than the nucleus did. The mass of a nucleus is reduced by the energy that is released when the nucleus comes together. Nuclear reactions can convert mass into energy.
Fission reactions A fission reaction splits up a large nucleus into smaller pieces. A fission reaction typically happens when a neutron hits a nucleus with enough energy to make the nucleus unstable.
Nuclear Fission U-235 is bombarded with neutrons The nucleus absorbs neutrons It becomes unstable and splits into 2 neutrons 2-3 neutrons are emitted and bombard another U-235 atom Chain reaction
How Electricity is Produced
Fusion reactions A fusion reaction is a nuclear reaction that combines, or fuses, two smaller nuclei into a larger nucleus. It is difficult to make fusion reactions occur because positively charged nuclei repel each other.
Fusion Fuel= isotopes of hydrogen
Fusion Way of the future?? Produces no high-level waste Fuel is hydrogen (plenty of it!) Problems It takes very high temperatures (millions of degrees) to make atoms fuse Confining the plasma after it is formed Scientists have yet to be able to create energy from fusion
Rules for nuclear reactions Nuclear reactions obey conservation laws. Energy stored as mass must be included in order to apply the law of conservation of energy to a nuclear reaction. Nuclear reactions must conserve electric charge. The total baryon number before and after the reaction must be the same. The total lepton number must stay the same before and after the reaction.
Applications of radioactivity Many satellites use radioactive decay from isotopes with long half-lives for power because energy can be produced for a long time without refueling. Isotopes with a short half-life give off lots of energy in a short time and are useful in medical imaging, but can be extremely dangerous. The isotope carbon-14 is used by archeologists to determine age.
Carbon dating Living things contain a large amount of carbon. When a living organism dies it stops exchanging carbon with the environment. As the fixed amount of carbon-14 decays, the ratio of C-14 to C-12 slowly gets smaller with age.
Pros and Cons of Nuclear Energy Pros Less of an immediate environmental impact compared to fossil fuels Carbon-free source of electricity- no greenhouse gases emitted May be able to generate H-fuel Cons Generates radioactive waste Many steps require fossil fuels (mining and disposal) Expensive
Pros and Cons of Nuclear Energy
Cost of Electricity from Nuclear Energy Cost is very high 20% of US electricity is from Nuclear Energy Affordable due to government subsidies Expensive to build nuclear power plants Long cost-recovery time Fixing technical and safety issues in existing plants is expensive
Safety Issues in Nuclear Power Plants Meltdown At high temperatures the metal encasing the uranium fuel can melt, releasing radiation Probability of meltdown or other accident is low Public perception is that nuclear power is not safe Sites of major accidents: Three Mile Island Chernobyl (Ukraine)
Three-Mile Island most serious reactor accident in US 50% meltdown of reactor core Containment building kept radiation from escaping No substantial environmental damage No human casualties Elevated public apprehension of nuclear energy Led to cancellation of many new plants in US
Chernobyl worst accident in history 1 or 2 explosions destroyed the nuclear reactor Large amounts of radiation escaped into atmosphere Spread across large portions of Europe Radiation spread was unpredictable Radiation fallout was dumped unevenly Death toll is 10, ,000
Nuclear Energy and Nuclear Weapons 31 countries use nuclear energy to create electricity These countries have access to spent fuel needed to make nuclear weapons Safe storage and handling of these weapons is a concern
Radioactive Waste Low-level radioactive waste- Radioactive solids, liquids, or gasses that give off small amounts of ionizing radiation High-level radioactive waste- Radioactive solids, liquids, or gasses that give off large amounts of ionizing radiation
Radioactive Wastes Long term solution to waste Deep geologic burial –Yucca Mountain As of 2004, site must meet EPA million year standard (compared to previous 10,000 year standard) Possibilities: Above ground mausoleums Arctic ice sheets Beneath ocean floor
Radioactive Waste Temporary storage solutions In nuclear plant facility (require high security) Under water storage Above ground concrete and steel casks Need approved permanent options soon.
Case-In-Point Yucca Mountain 70,000 tons of high-level radioactive waste Tectonic issues have been identified
Decommissioning Nuclear Power Plants Licensed to operate for 40 years Several have received 20-year extensions Power plants cannot be abandoned when they are shut down Three solutions Storage Entombment Decommissioning (dismantling)