# Foundations of Physics

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Foundations of Physics
CPO Science Foundations of Physics Unit 9, Chapter 30

Chapter 30 Nuclear Reactions and Radiation
Unit 9: The Atom Chapter 30 Nuclear Reactions and Radiation 30.1 Radioactivity 30.2 Radiation 30.3 Nuclear Reactions and Energy

Chapter 30 Objectives Describe the cause and types of radioactivity.
Explain why radioactivity occurs in terms of energy. Use the concept of half-life to predict the decay of a radioactive isotope. Write the equation for a simple nuclear reaction. Describe the processes of fission and fusion. Describe the difference between ionizing and nonionizing radiation. Use the graph of energy versus atomic number to determine whether a nuclear reaction uses or releases energy.

Chapter 30 Vocabulary Terms
radioactive alpha decay beta decay gamma decay radiation isotope radioactive decay energy barrier intensity inverse square law shielding fission reaction CAT scan ionizing nonionizing ultraviolet fusion reaction Geiger counter rem nuclear waste neutron antimatter x-ray neutrino background radiation dose fallout detector half-life

*Students read Section AFTER Investigation 30.1

30.1 Radioactivity The word radioactivity was first used by Marie Curie in 1898. She used the word radioactivity to describe the property of certain substances to give off invisible “radiations” that could be detected by films.

30.1 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.

30.1 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."

30.1 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.

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.

30.1 Radioactivity If you started with one kilogram of C-14 it would decay into kg of N-14. The difference of grams is converted directly into energy via Einstein’s formula E = mc2.

30.1 Radioactivity Systems move from higher energy to lower energy over time. A ball rolls downhill to the lowest point or a hot cup of coffee cools down. A radioactive nucleus decays because the neutrons and protons have lower overall energy in the final nucleus than they had in the original nucleus.

30.1 Radioactivity The radioactive decay of C-14 does not happen immediately because it takes a small input of energy to start the transformation from C-14 to N-14. The energy needed to start the reaction is called an energy barrier. The lower the energy barrier, the more likely the atom is to decay quickly.

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.

30.1 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.

30.1 Half-life Most radioactive materials decay in a series of reactions. Radon gas comes from the decay of uranium in the soil. Uranium (U-238) decays to radon-222 (Ra-222).

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.

30.1 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.

30.1 Calculating with isotopes
A sample of 1,000 grams of the isotope C-14 is created. The half-life of C-14 is 5,700 years. How much C-14 remains after 28,500 years? 1) You are asked for the amount of C-14 left after 28,500 years. 2) You are given the half-life for carbon as 5,700 years. 3) One half the C-14 decay s every half-life. 4) 28,500 years is 5 times the half-life. The amount of C14 is reduced by half every 5,700 years. Start: 1,000 grams 5,700 years: 500 grams 11,400 years: 250 grams 17,100 years: 125 grams 22,800 years, 62.5 grams 28,500 years, 31.2 grams Answer = 31.2 grams

What are some types and sources of radiation? *Students read Section AFTER Investigation 30.2

30.2 Radiation The word radiation means the flow of energy through space. There are many forms of radiation. Light, radio waves, microwaves, and x-rays are forms of electromagnetic radiation. Many people mistakenly think of radiation as only associated with nuclear reactions.

30.2 Radiation The intensity of radiation measures how much power flows per unit of area. When radiation comes from a single point, the intensity decreases inversely as the square of the distance. This is called the inverse square law and it applies to all forms of radiation.

30.1 Intensity I = P A Power (watt) Intensity (W/m2) Area (m2)

30.2 Harmful radiation Radiation becomes harmful when it has enough energy to remove electrons from atoms. The process of removing an electron from an atom is called ionization. Visible light is an example of nonionizing radiation. UV light is an example of ionizing radiation.

30.2 Harmful radiation Ionizing radiation absorbed by people is measured in a unit called the rem. The total amount of radiation received by a person is called a dose, just like a dose of medicine. It is wise to limit your exposure to ionizing radiation whenever possible. Use shielding materials, such as lead, and do your work efficiently and quickly. Distance also reduces exposure.

30.2 Sources of radiation Ionizing radiation is a natural part of our environment. There are two chief sources of radiation you will probably be exposed to: background radiation. radiation from medical procedures such as x-rays. Background radiation results in an average dose of 0.3 rem per year for someone living in the United States.

30.2 Background radiation Background radiation levels can vary widely from place to place. Cosmic rays are high energy particles that come from outside our solar system. Radioactive material from nuclear weapons is called fallout. Radioactive radon gas is present in basements and the atmosphere.

30.2 X-ray machines X-rays are photons, like visible light photons only with much more energy. Diagnostic x-rays are used to produce images of bones and teeth on x-ray film. Xray film turns black when exposed to x-rays.

30.2 X-ray machines Therapeutic x-rays are used to destroy diseased tissue, such as cancer cells. Low levels of x-rays do not destroy cells, but high levels do. The beams are made to overlap at the place where the doctor wants to destroy diseased cells.

30.2 CAT scan The advent of powerful computers has made it possible to produce three-dimensional images of bones and other structures within the body. To produce a CAT scan, computerized axial tomography, a computer controls an x-ray machine as it takes pictures of the body from different angles.

30.2 CAT scan People who work with radiation use radiation detectors to tell when radiation is present and to measure its intensity. The Geiger counter is a type of radiation detector invented to measure x-rays and other ionizing radiation, since they are invisible to the naked eye. A Geiger counter detects radiation by electrically collecting ions of gas. The cylinder is positive and the wire in the center is negative. Ionizing radiation knocks electrons from molecules of gas in the cylinder. The ions and electrons make an electric current that is proportional to the intensity of the radiation.

30.3 Nuclear Reactions and Energy
Key Question: How do we describe nuclear reactions? *Students read Section AFTER Investigation 30.3

30.3 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.

30.3 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.

30.3 Nuclear Reactions and Energy
When separate protons and neutrons come together in a nucleus, energy is released. The more energy that is released, the lower the energy of the final nucleus. The energy of the nucleus depends on the mass and atomic number.

The graph above compares the energy of the nucleus in one kilogram of matter for
elements 2 (helium) through 92 (uranium). Note that the units of energy are hundreds of trillions (1012) of joules per kilogram of material! Nuclear reactions often involve huge amounts of energy as protons and neutrons are rearranged to form different nuclei. A nuclear reaction releases energy when it produces a nucleus that is lower down on the graph. A nuclear reaction uses energy when it creates a nucleus that is higher up on the graph.

30.3 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.

30.3 Fusion reactions A fusion reaction is a nuclear reaction that combines, or fuses, two smaller nuclei into a larger nucleus.

30.3 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.

30.3 Fission reactions The average energy of the nucleus for a combination of molybdenum-99 (Mo-99) and tin-135 (Sn-135) is 25 TJ/kg. The fission of a kilogram of uranium into Mo-99 and Sn-135 releases the difference in energies, or 98 trillion joules.

30.3 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.

30.3 Conservation Laws There are conservation laws that apply to the type of particles before and after a nuclear reaction. Protons and neutrons belong to a family of particles called baryons. Electrons come from a family of particles called leptons.

30.3 Calculating nuclear reactions
The nuclear reaction above is proposed for combining two atoms of silver to make an atom of gold. This reaction cannot actually happen because it breaks the rules for nuclear reactions. List two rules that are broken by the reaction. 1) You are asked for the rules that would be broken if this reaction were ever to happen. 2) You are given the reaction and the atomic mass of each isotope. 3) Check the rules: Two silver nuclei have a total charge of 94 ( ). One gold nucleus has a charge of 79. The reaction violates the rule of conservation of charge. Two silver nuclei have a total of 214 protons and neutrons ( ). One gold nucleus has 197 protons and neutrons. The reaction also breaks the rule about the total number of baryons (protons and neutrons).

30.3 Antimatter, neutrinos and others particles
The matter you meet in the world ordinarily contains protons, neutrons, and electrons. Cosmic rays contain particles called muons and pions. Thousands of particles called neutrinos from the sun pass through you every second and you cannot feel them.

30.3 Antimatter, neutrinos and others particles
Every particle of matter has an antimatter twin. Antimatter is the same as regular matter except properties like electric charge are reversed. An antiproton is just like a normal proton except it has a negative charge. An antielectron (also called a positron) is like an ordinary electron except that it has positive charge.

30.3 Neutrinos When beta decay was first discovered, physicists were greatly disturbed to find that the energy of the resulting proton and electron was less than the energy of the disintegrating neutron. The famous Austrian physicist Wolfgang Pauli proposed that there must be a very light, previously undetected neutral particle that was carrying away the missing energy. We now know the missing particle is a type of neutrino.

30.3 Neutrinos Despite the difficulty of detection, several carefully constructed neutrino experiments have detected neutrinos coming from nuclear reactions in the sun.

Application: Nuclear Power