Unit 9, Chapter 30

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

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

3. 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.

4. 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

5. Key Question: How do we model radioactivity? 30.1 Radioactivity *Students read Section 30.1 AFTER Investigation 30.1

6. 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.

7. 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.

8. 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 decaymeans to "break down."

9. 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.

10. 30.1 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.

11. 30.1 Radioactivity • If you started with one kilogram of C-14 it would decay into 0.999988 kg of N-14. • The difference of 0.012 grams is converted directly into energy via Einstein’s formula E = mc2.

12. 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.

13. 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.

14. 30.1 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.

15. 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.

16. 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).

17. 30.1 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.

18. 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.

20. 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?

21. Key Question: What are some types and sources of radiation? 30.2 Radiation *Students read Section 30.2 AFTER Investigation 30.2

22. 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.

23. 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.

24. 30.1 Intensity Power (watt) I = P A Intensity (W/m2) Area (m2) Intensity = 7.96 W/m2 Intensity = 1.99 W/m2

25. 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.

26. 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.

27. 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.

28. 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.

29. 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. 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.

31. 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.

32. 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.

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

34. 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.

35. 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.

36. 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.

38. 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.

39. 30.3 Fusion reactions • A fusion reaction is a nuclear reaction that combines, or fuses, two smaller nuclei into a larger nucleus.

40. 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.

41. 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.

42. 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.

43. 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.

44. 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.

45. 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 muonsand pions. • Thousands of particles called neutrinosfrom the sun pass through you every second and you cannot feel them.

46. 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.

47. 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.

48. 30.3 Neutrinos • Despite the difficulty of detection, several carefully constructed neutrino experiments have detected neutrinos coming from nuclear reactions in the sun.

49. Application: Nuclear Power