P. Sci. Unit 12 Nuclear Radiation Chapter 10

Nuclear Radiation Strong Nuclear force – the force that holds protons and neutrons together. Remember that like charges repel each other. The strong nuclear force is what holds the positive protons together.

Very large atoms are unstable because of their size. (uranium and the transuranium elements) the nuclear strong forces have a hard time holding the nucleus together.

Over time the nucleus will decay (break up) When this happens, particles (protons or neutrons) or energy is emitted by the nucleus.

Radioactivity Process of nuclear decay in which an unstable nucleus emits particles or energy in the form of electromagnetic radiation.

Radiation vs Nuclear Radiation Radiation – Transfer of energy by electromagnetic waves. Also a type of heat transfer (conduction, convection, radiation). Nuclear Radiation – radiation associated with nuclear changes.

Types of Nuclear Radiation 1)Alpha particles 2)Beta particles 3)Gamma rays 4)Neutron emission

Alpha Particles Made of 2 protons and 2 neutrons (the same as a helium nucleus) Positively charged Largest particle Can be stopped by a sheet of paper.

Beta Particles Made of fast moving electrons Negatively charged Small – fast moving particles Stopped by thin metal or wood

Beta cont. Occurs when a neutron (no charge) decays to form a proton and an electron. The electron, having very little mass, is then ejected from the nucleus at a high speed.

Gamma Rays Not made of matter – this is a form of electromagnetic energy No charge High energy Can penetrate 7cm of lead Pose a danger to health

Neutron Emission Made of a neutron The atom becomes a different isotope (same # of protons – different # of neutrons) A hazard of neutron radiation is neutron activation +

Neutron Activation the ability of neutron radiation to induce radioactivity in most substances it encounters, including the body tissues of people. accounts for much of the radioactive material released by the detonation of a nuclear weapon.

Neutron activation cont. It is also a problem in nuclear fission and nuclear fusion installations, because it gradually makes the equipment radioactive; eventually the equipment must be replaced and disposed of as low- level radioactive waste. This is also how many artificial elements are made. Neutrons are shot in the nucleus of an atom and the atom becomes unstable giving off more radioactive particles.

Half-Life During radioactive decay – the time it takes for half the atoms to decay is the “half-life” of that substance. The half-life of each radioactive substance is different. A half-life can be a millionth of a second to billions of years.

Half-Life progression First SecondThird FourthFifth Etc.

Half-Life progression of Iodine 131 First SecondThird FourthFifth Etc hr. (8.1 days) hr. (16.2 days) hr. (32.4 days) hr. (40.5 days) hr. (24.3 days) 8.1 days later

If you know how much of a particular radioactive isotope was present in an object at the beginning, you can predict how old the object is. Carbon 14 is used to date organic (plant or animal) remains.

Fission vs. Fusion Fission is the process by which a nucleus splits into two or more smaller fragments, releasing neutrons and energy. Fusion is the process in which light nuclei combine to form heavier nuclei and releasing energy.

Energy Theory of relativity (Einstein) – matter can be converted into energy and energy into matter. E = m x c 2. The mass-equivalent energy of 1 kg of matter is more than the chemical energy of 8 million tons of TNT.

Fission What occurs in nuclear reactors. Example: Uranium-235 when hit with neutrons produce barium-137 and krypton-84 as well as 15 neutrons and energy.

Chain Reaction A chain reaction refers to a process in which neutrons released in fission produce an additional fission in at least one further nucleus. This nucleus in turn produces neutrons, and the process repeats. The process may be controlled (nuclear power) or uncontrolled (nuclear weapons).

Critical Mass The minimum amount of a substance that can sustain a chain reaction. It takes very little Uranium-235 to reach critical mass.

Control Rods In a nuclear reactor the chain reaction is controlled by rods of materials (like cadmium or boron) that absorb some of the neutrons thus slowing and controlling the chain reaction.

Fusion What occurs in stars (including our sun) Four hydrogen atoms fuse together in the sun to produce a helium atom. A large amount of energy is needed to start the reaction.

Fusion Reactors Fusion reactions produce much more energy per gram of fuel and produce less radioactive waste than fission. So why do we not have fusion reactors?

Answer Fusion can occur only in the plasma state of matter (super-heated gas). Once it gets going it generates enough heat to keep going as long as there is fuel. It requires a heat of about 10 million degrees Celsius. Scientists have to find a way of producing and containing that much heat.

Extra Credit Explain what “cold fusion” is and what has been done in this area.

Dangers and Benefits Dangers – –nuclear waste –Nuclear radiation Benefits – –Medical Cancer Treatment Radioactive tracers –Nuclear Power Provides more energy than current forms of energy production