Conceptual Physics Fundamentals Chapter 16: THE ATOMIC NUCLEUS AND RADIOACTIVITY

This lecture will help you understand: Radioactivity Alpha, Beta, and Gamma Rays The Atomic Nucleus and the Strong Force Radioactive Half-Life Transmutation of the Elements Radiometric Dating Nuclear Fission Mass-Energy Equivalence—E = mc2

The Atomic Nucleus and Radioactivity “The release of atomic energy has not created a new problem. It has merely made more urgent the necessity of solving an existing one.” —Albert Einstein

Radioactivity Radioactivity Radioactivity is the process of nuclear decay (radioactive decay). Nothing new in the environment; it’s been going on since time zero. It warms Earth’s interior, is in the air we breathe, and is present in all rocks (some in trace amounts). It is natural.

The radioactive decay of nature’s elements occurs in the Radioactivity CHECK YOUR NEIGHBOR The radioactive decay of nature’s elements occurs in the A. soil we walk on. air we breathe. interior of Earth. all of the above D. all of these.

The radioactive decay of nature’s elements occurs in the Radioactivity CHECK YOUR ANSWER The radioactive decay of nature’s elements occurs in the A. soil we walk on. air we breathe. interior of Earth. all of the above. D. all of these.

Alpha, Beta, and Gamma Rays Radioactive elements emit three distinct types of radiation:  —alpha: positively charged (helium nuclei)  — beta: negatively charged (electrons)  —gamma (electromagnetic radiation)

Alpha, Beta, and Gamma Rays Relative penetrations

The origins of radioactivity go back to Alpha, Beta, and Gamma Rays CHECK YOUR NEIGHBOR The origins of radioactivity go back to A. military activities in the mid-20th century. the industrial revolution two centuries ago. the beginning of human error. before humans emerged on Earth. D. before humans emerged on Earth.

The origins of radioactivity go back to Alpha, Beta, and Gamma Rays CHECK YOUR ANSWER The origins of radioactivity go back to A. military activists in the mid-20th century. the industrial revolution two centuries ago. the beginning of human error. before humans emerged on Earth. D. before humans emerged on Earth.

Any atom that emits an alpha particle or beta particle Alpha, Beta, and Gamma Rays CHECK YOUR NEIGHBOR Any atom that emits an alpha particle or beta particle A. becomes an atom of a different element, always. may become an atom of a different element. becomes a different isotope of the same element. increases its mass. A. becomes an atom of a different element, always.

Any atom that emits an alpha particle or beta particle Alpha, Beta, and Gamma Rays CHECK YOUR ANSWER Any atom that emits an alpha particle or beta particle A. becomes an atom of a different element, always. may become an atom of a different element. becomes a different isotope of the same element. increases its mass. Explanation: Contrary to the failures of alchemists of old to change elements from one to another, this was going on all around them—unnoticed. A. becomes an atom of a different element, always.

Alpha, Beta, and Gamma Rays Food irradiation kills microbes doesn’t make the food radioactive there is no diarrhea with astronauts in space (their food is first irradiated).

Which of these is the nucleus of the helium atom? Alpha, Beta, and Gamma Rays CHECK YOUR NEIGHBOR Which of these is the nucleus of the helium atom? A. alpha beta gamma all are different forms of helium A. Alpha.

Which of these is the nucleus of the helium atom? Alpha, Beta, and Gamma Rays CHECK YOUR ANSWER Which of these is the nucleus of the helium atom? A. alpha beta gamma all are different forms of helium A. Alpha.

Which of these is actually a high-speed electron? Alpha, Beta, and Gamma Rays CHECK YOUR NEIGHBOR Which of these is actually a high-speed electron? A. alpha beta gamma all are high speed B. Beta.

Which of these is actually a high-speed electron? Alpha, Beta, and Gamma Rays CHECK YOUR ANSWER Which of these is actually a high-speed electron? A. alpha beta gamma all are high speed Explanation: Choice D may be true, but doesn’t directly answer the question. B. Beta.

Environmental Radiation Radon, a common environmental hazard Most radiation from natural background About 1/5 from non-natural sources.

Environmental Radiation Units of radiation Particle Radiation Dosage Factor Health effect alpha 1 rad  10 = 10 rems beta 10 rad  1 = 10 rems Doses of radiation Lethal doses of radiation begin at 500 rems.

Environmental Radiation Source received annually Typical dose (mrem) Natural origin Cosmic radiation 26 Ground 33 Air (Radon-222) 198 Human tissues (K-40; Ra-226) 35

Environmental Radiation Doses of radiation Typical dose (mrem) Human origin Medical procedures Diagnostic X-rays 40 Nuclear diagnostics 15 TV tubes, other consumer products 11 Weapons-test fallout 1 Commercial fossil-fuel power plants <1 Commercial nuclear power plants <<1

Environmental Radiation Radioactive tracers Radioactive isotopes used to trace such pathways are called tracers.

The Atomic Nucleus and the Strong Force The strong force holds nucleons together.

The Atomic Nucleus and the Strong Force The strong force is more effective with smaller nuclei.

The Atomic Nucleus and the Strong Force A lone neutron is radioactive, and spontaneously transforms to a proton and an electron. A neutron needs protons around to keep this from happening.

The Atomic Nucleus and the Strong Force Alpha emission

The strong force is a force in the The Atomic Nucleus and the Strong Force CHECK YOUR NEIGHBOR The strong force is a force in the A. atom that holds electrons in orbit. nucleus that holds nucleons together. both A and B neither A nor B B. nucleus that holds nucleons together.

The strong force is a force in the The Atomic Nucleus and the Strong Force CHECK YOUR ANSWER The strong force is a force in the A. atom that holds electrons in orbit. nucleus that holds nucleons together. both A and B neither A nor B B. nucleus that holds nucleons together.

In the nucleus of an atom, the strong force is a relatively The Atomic Nucleus and the Strong Force CHECK YOUR NEIGHBOR In the nucleus of an atom, the strong force is a relatively A. short-range force. long-range force. unstable force. neutralizing force. A. short-range force.

In the nucleus of an atom, the strong force is a relatively The Atomic Nucleus and the Strong Force CHECK YOUR ANSWER In the nucleus of an atom, the strong force is a relatively A. short-range force. long-range force. unstable force. neutralizing force. A. short-range force.

Radioactive Half-Life The rate of decay for a radioactive isotope is measured in terms of a characteristic time, the half-life. The time for half of an original quantity of an element to decay.

Radioactive Half-Life Uranium-238 to lead-206 through a series of alpha and beta decays. In 4.5 billion years, half the uranium presently in Earth will be lead.

Radioactive Half-Life Some radiation detectors (a) a Geiger counter (b) a scintillation counter

Radioactive Half-Life CHECK YOUR NEIGHBOR A certain isotope has a half-life of 10 years. This means the amount of that isotope remaining at the end of 10 years will be A. zero. one quarter. half. the same. C. half.

Radioactive Half-Life CHECK YOUR ANSWER A certain isotope has a half-life of 10 years. This means the amount of that isotope remaining at the end of 10 years will be A. zero. one quarter. half. the same. C. half.

Radioactive Half-Life A challenge… Suppose the number of neutrons in a reactor that is starting up doubles each minute, reaching 1 billion neutrons in 10 minutes. When did the number of neutrons reach half a billion? A. 1 minute 2 minutes 5 minutes 9 minutes D. 9 minutes.

Radioactive Half-life CHECK YOUR ANSWER Suppose the number of neutrons in a reactor that is starting up doubles each minute, reaching 1 billion neutrons in 10 minutes. When did the number of neutrons reach half a billion? A. 1 minute 2 minutes 5 minutes 9 minutes Explanation: This question would be appropriate with Appendix D, Exponential Growth and Doubling Time. Can you see that working backward, each minute has half the number of neutrons? D. 9 minutes.

Transmutation of the Elements With alpha or beta particle, a different element is formed. This is transmutation, which occurs in natural events, and also initiated artificially in the laboratory. Uranium naturally transmutes to thorium when an alpha particle is emitted.

Transmutation of the Elements Natural transmutation Thorium naturally transmutes to protactinium when a beta particle is emitted. An electron is e. Superscript 0 indicates electron’s mass is insignificant compared with nucleons. Superscript -1 is the electric charge of the electron.

Transmutation of the Elements CHECK YOUR NEIGHBOR When an element ejects an alpha particle and a beta particle, the atomic number of the resulting element A. reduces by 2. reduces by 4. increases by 2. increases by 4. A. reduces by 2.

Transmutation of the Elements CHECK YOUR ANSWER When an element ejects an alpha particle and a beta particle, the atomic number of the resulting element A. reduces by 2. reduces by 4. increases by 2. increases by 4. Explanation: An alpha particle (a helium nucleus) has atomic number 2. So ejection of an alpha particle means a loss of 2 protons. So the atomic number of the element is lowered by 2. A. reduces by 2.

Transmutation of the Elements CHECK YOUR NEIGHBOR When an element ejects an alpha particle and a beta particle, the atomic number of that element A. reduces by 1. increases by 1. reduces by 2. increases by 2. A. reduces by 1.

Transmutation of the Elements CHECK YOUR ANSWER When an element ejects an alpha particle and a beta particle, the atomic number of that element A. reduces by 1. increases by 1. reduces by 2. increases by 2. Explanation: Alpha emission reduces atomic number by 2, and beta emission increases atomic number by 1, so net result is 1. A. reduces by 1.

Transmutation of the Element Artificial transmutation an alpha particle fired at and impacting on a nitrogen atom, which transmutes to oxygen and hydrogen

Atoms can transmute into completely different atoms in Transmutation of the Elements CHECK YOUR NEIGHBOR Atoms can transmute into completely different atoms in A. nature. laboratories. both A and B neither A nor B C. both of these.

Atoms can transmute into completely different atoms in Transmutation of the Elements CHECK YOUR ANSWER Atoms can transmute into completely different atoms in A. nature. laboratories. both A and B neither A nor B Explanation: Atomic transmutation occurs in nature, in laboratories, and as far as we know, throughout the cosmos. C. both of these.

Transmutation of the Elements CHECK YOUR NEIGHBOR An element emits 1 beta particle, and its product then emits 1 alpha particle. The atomic number of the resulting element is changed by A. zero. −1. −2. none of the above B. minus 1.

Transmutation of the Elements CHECK YOUR ANSWER An element emits 1 beta particle, and its product then emits 1 alpha particle. The atomic number of the resulting element is changed by A. zero. −1. −2. none of the above Explanation: Beta emission increases atomic number by 1, then alpha emission decreases atomic number by 2, so the net change is –1. B. minus 1.

Radiometric Dating Earth’s atmosphere is continuously bombarded by cosmic rays, which causes many atoms in the upper atmosphere to transmute. These transmutations result in many protons. a nitrogen that captures a neutron and becomes an isotope of carbon by emitting a proton:

Radiometric Dating Carbon-14 is a beta emitter and decays back to nitrogen. Because living plants take in carbon dioxide, any C-14 lost by decay is immediately replenished with fresh C-14 from the atmosphere. Dead plants continue emitting C-14 without replenishment.

Radiometric Dating Relative amounts of C-12 to C-14 enable dating of organic materials.

Radiometric Dating CHECK YOUR NEIGHBOR The half-life of carbon-14 is about 5730 years, which means that the present amount in your bones will reduce to zero A. when you die. in about 5730 years. in about twice 5730 years. none of the above D. none of these.

Radiometric Dating CHECK YOUR ANSWER The half-life of carbon-14 is about 5730 years, which means that the present amount in your bones will reduce to zero A. when you die. in about 5730 years. in about twice 5730 years. none of the above Explanation: In theory, the amount never reaches zero. In eons to come, trace amounts of the carbon-14 in your bones, even if completely dissolved, will still exist. D. none of the above.

Nuclear Fission German scientists Otto Hahn and Fritz Strassmann in 1938 accidentally discovered nuclear fission.

Nuclear Fission A typical uranium fission reaction

Nuclear Fission Chain reaction—a self-sustaining reaction in which the products of one reaction event stimulate further reaction events.

Nuclear Fission Chain reaction in uranium small amount, chain reaction fizzles critical amount, chain reaction produces an explosion

The greater the surface area of a piece of fission material, the Nuclear Fission CHECK YOUR NEIGHBOR The greater the surface area of a piece of fission material, the A. less likely an explosion. more likely an explosion. neither A nor B; mass, rather than surface area is significant none of the above A. less likely an explosion.

The greater the surface area of a piece of fission material, the Nuclear Fission CHECK YOUR ANSWER The greater the surface area of a piece of fission material, the A. less likely an explosion. more likely an explosion. neither, A nor B; mass, rather than surface area is significant none of the above Explanation: When a chain reaction occurs, it fizzles out when neutrons escape a surface. Therefore, the greater the surface area, the less likely an explosion will occur. A. less likely an explosion.

Which of these nuclei has the greatest mass? Nuclear Fission CHECK YOUR NEIGHBOR Which of these nuclei has the greatest mass? A. hydrogen iron lead uranium D. uranium.

Which of these nuclei has the greatest mass? Nuclear Fission CHECK YOUR ANSWER Which of these nuclei has the greatest mass? A. hydrogen iron lead uranium D. uranium.

In which of these nuclei does the proton have the greatest mass? Nuclear Fission CHECK YOUR NEIGHBOR In which of these nuclei does the proton have the greatest mass? A. hydrogen iron lead uranium A. hydrogen

In which of these nuclei does the proton have the greatest mass? Nuclear Fission CHECK YOUR ANSWER In which of these nuclei does the proton have the greatest mass? A. hydrogen iron lead uranium A. hydrogen.

In which of these nuclei does the proton have the least mass? Nuclear Fission CHECK YOUR NEIGHBOR In which of these nuclei does the proton have the least mass? A. hydrogen iron lead uranium C. lead.

In which of these nuclei does the proton have the least mass? Nuclear Fission CHECK YOUR ANSWER In which of these nuclei does the proton have the least mass? A. hydrogen iron lead uranium Explanation: A look at the curves of Figures 16.33–16.35 show this. Iron has the least mass per nucleon, but the strongest binding energy. C. lead.

Nuclear Fission CHECK YOUR NEIGHBOR When a uranium nucleus undergoes fission, the energy released is primarily in the form of A. gamma radiation. kinetic energy of fission fragments. kinetic energy of ejected neutrons. all of the above about equally B. kinetic energy of fission fragments.

Nuclear Fission CHECK YOUR ANSWER When a uranium nucleus undergoes fission, the energy released is primarily in the form of A. gamma radiation. kinetic energy of fission fragments. kinetic energy of ejected neutrons. all of the above about equally Explanation: Kinetic energy of fragments is what becomes heat energy. Interestingly, gamma-ray energy is tiny in comparison. Neutrons, although important for the chain reaction, contribute a small part of the energy release. Choice D is likely a guess. B. kinetic energy of fission fragments.

Nuclear Fission Fission bomb A bomb in which pieces of uranium are driven together is a so-called “gun-type” weapon, as opposed to the now more common “implosion weapon.” Constructing a fission bomb is a formidable task. The difficulty is separating enough U-235 fuel.

Nuclear Fission Nuclear fission reactors About 20% of electric energy in the United States is generated by nuclear fission reactors. more in some other countries—about 75% in France Reactors are simply nuclear furnaces.

Nuclear Fission Diagram of a typical power plant.

Nuclear Fission The benefits are plentiful electricity, conservation of billions of tons of fossil fuels every year that are converted to heat and smoke (which in the long run may be far more precious as sources of organic molecules than as sources of heat), and the elimination of megatons of carbon dioxide, sulfur oxides, and other deleterious substances put into the air each year by the burning of fossil fuels. Drawbacks include risks of release of radioactive isotopes into the atmosphere, by accident or by terrorist activities. Radioactive waste disposal is a problem (although not for some countries that monitor it for potential use later).

Nuclear Fission Plutonium-239, like uranium-235, undergoes fission when it captures a neutron.

Nuclear Fission The breeder reactor A breeder reactor breeds Pu-239 from U-238 while “burning” U-235. occurs in all reactors to some extent. in few years can produce twice as much fissionable fuel as it begins with. a more attractive alternative when U-235 reserves are limited. fuel for a breeder may be today’s radioactive wastes

Mass-Energy Equivalence— E = mc2 Early in the early 1900s, Albert Einstein discovered that mass is actually “congealed” energy. Enormous work is required to pull nucleons from a nucleus. This work is energy added to the nucleon that is pulled out.

Mass-Energy Equivalence— E = mc2 Measurements of atomic mass are made with this device.

Mass-Energy Equivalence— E = mc2 more explanation of the mass spectrometer Electrically charged isotopes directed into the semicircular “drum” are forced into curved paths by a strong magnetic field. Lighter isotopes with less inertia (mass) easily change direction and follow curves of smaller radii. Heavier isotopes with greater inertia (mass) follow larger curves. Mass of an isotope ~ distance from entrance slit.

Mass-Energy Equivalence— E = mc2 The plot shows how nuclear mass increases with increasing atomic number.

Mass-Energy Equivalence— E = mc2 A very important graph results from the plot of nuclear mass per nucleon from hydrogen through uranium.

Mass-Energy Equivalence— E = mc2 The same graph, with emphasis on nuclear fission.

Nuclear Fusion nuclear fusion is the opposite of nuclear fission fission, nuclei “fizz” apart fusion, nuclei fuse together Each releases energy in accord with the graph in Figure 16.33.

Nuclear Fusion

Nuclear Fusion Fission and fusion compared Less mass per nucleon occurs in both processes.

Nuclear Fusion Typical fusion reactions

Nuclear Fusion CHECK YOUR NEIGHBOR When a fusion reaction converts a pair of hydrogen isotopes to an alpha particle and a neutron, most of the energy released is in the form of A. gamma radiation. kinetic energy of the alpha particle. kinetic energy of the neutron. all of the above about equally C. kinetic energy of the neutron.

Nuclear Fusion CHECK YOUR ANSWER When a fusion reaction converts a pair of hydrogen isotopes to an alpha particle and a neutron, most of the energy released is in the form of A. gamma radiation. kinetic energy of the alpha particle. kinetic energy of the neutron. all of the above about equally Explanation: By momentum conservation, the ejected neutrons have a high speed compared with the alpha particle, and therefore much kinetic energy. It is the kinetic energy of the neutrons that becomes the heat needed for power. Gamma rays play a small energy role, as they do in fission. C. kinetic energy of the neutron.

Nuclear Fusion Controlling fusion Carrying out fusion is more difficult than thought when fission succeeded. plasma reactors have not been successful other schemes, including lasers, are being considered deuterium pellets rhythmically dropped into synchronized laser crossfire; heat used to produce steam

Nuclear Fission CHECK YOUR NEIGHBOR In either a fission event or a fusion event, the quantity that remains unchanged is A. energy. the mass of nucleons. the number of nucleons. none of the above C. the number of nucleons.

Kinetic Energy CHECK YOUR ANSWER In either a fission event or a fusion event, the quantity that remains unchanged is A. energy. the mass of nucleons. the number of nucleons. none of the above Explanation: This is a premise of reaction equations, whether nuclear or chemical. Although energy and mass undergo changes, the number of particles and amount of charge remains unchanged. C. the number of nucleons.