Nuclear Physics

1. Nuclear Physics

2. The Atom • An atom consists of negatively charged electrons, positively charged protons, and neutrons (particles consisting of no electrical charge). • The protons and neutrons are found in the nucleus. • The electrons occupy the electron cloud-the space surrounding the positively charged nucleus.

3. What holds the nucleus together? • The negatively charged electrons that surround the positively charged nucleus are held in place by an attractive electromagnetic force. • There is an electromagnetic repulsive force between the protons (like charges repel). • They do not fly apart however, due to the strong nuclear force (also called the strong force) which acts between the protons and neutrons in the nucleus holding them together. • This strong force is 100 times stronger than the repulsive force between protons. • The range of the strong force is short-only about the radius of a proton. It is attractive and is of the same strength between protons and neutrons.

4. The strong force • In order for a particle to be pulled from the nucleus, work must be done to overcome the attractive force. Doing work adds energy to the system. • Therefore, the assembled nucleus has less energy than the separate protons and neutrons that make it up. • The difference is the binding energy of the nucleus. • Because the nucleus has less energy, all binding energies are negative.

5. E = mc2 • Einstein showed that mass and energy are equivalent, so the binding energy can be expressed in the form of an equivalent amount of mass. E=mc2 (c is the speed of light) • c=3.00 x108 m/s • Example: the mass of a proton is 1.007276 u and the mass of a neutron is 1.008665 u. What do you expect the mass of a helium nucleus to be? (note: 1 u = 1.6605 x 10-27 kg) • If the mass of the helium nucleus were equal to the mass of 2 protons and 2 neutrons, the mass would be 4.031882 u. • Careful measurement shows that the mass of the nucleus is only 4.002603 u. • The actual mass of the nucleus is less than the individual components by 0.029279 u. This difference in mass is called the mass defect. • Binding energy can be determined through the use of the equation E=mc2.

6. Calculating mass defect and binding energy • 1 u = 931.49 MeV of energy. • E=mc2=(1.6605x10-27)(2.9979 x 108)2 E=(1.4294x10-10 J) (1 eV/1.60217 x 10-19J) =9.3149x108eV E=931.49 MeV • Find the mass defect and the binding energy of hydrogen-3. The mass of the hydrogen-3 isotope is 3.016049 u. The mass of a hydrogen atom is 1.007825 u, and the mass of a neutron is 1.008665 u.

7. Which weighs more? • What weighs more, an electron and a proton, or a hydrogen atom? • On a two-pan balance, you put 2 neutrons and 1 proton on one side and you put a hydrogen-3 nucleus on the other side. Which side weighs more?

8. Radioactivity • Radioisotopes are unstable isotopes whose nuclei gain stability by spontaneously undergoing changes. • Because the range of the strong force is so short that in large nuclei the repulsive force is greater resulting in an unstable nuclei. • When a radioactive isotope decays, some of the energy holding the nucleus together is released in the form of a radioactive particle with mass and kinetic energy. • These changes are accompanied by the emission of large amounts of energy. • Radioactive decay is the process by which materials give off this energy. • The penetrating rays and particles that are emitted during these changes are called radiation. • Eventually unstable radioactive isotopes are transformed into stable isotopes of a different element.

9. Radioactive Isotopes • All elements consist of at least one radioactive isotope. • Isotopes that have too many or too few neutrons (atomic mass larger or smaller than the average) tend to be radioactive. • All isotopes with an atomic number greater than 83 are radioactive.

10. 10 Identify the radioactive isotope • 11H • 614C • 816O • 714N

11. 10 Identify the radioactive isotope • Chlorine-35 • Carbon-12 • Lead-207 • Potassium-40

12. Types of Radiation • Alpha radiation • Beta radiation • Gamma radiation

13. Alpha Radiation • Alpha radiation consists of helium nuclei that are emitted from a radioactive isotope. • Alpha particles consist of two protons and two neutrons. • Alpha particles have a 2+ charge. • The symbol for an alpha particle is 42He or α. • Alpha particles are the most massive of the radioactive particles (4 amu), the most damaging, and are the least penetrating (easily stopped by a piece of paper).

14. 10 What is the product when plutonium-238 undergoes alpha decay? • Uranium-234 • Thallium-206 • Lead-206 • Radium-226

15. Explanation of Answer • 23894Pu  42He + 23492U

16. 10 What is the product when bismuth-210 undergoes alpha decay? • Radium-226 • Lead-206 • Thallium-206 • Uranium-232

17. Explanation of Answer • 21083Bi  42He + 20681Tl

18. 10 What is the product when polonium-210 undergoes alpha decay? • Bismuth-206 • Lead-206 • Radium-206 • Thorium-206

19. Beta Particles • Beta particles consist of fast moving electrons formed by the decomposition of a neutron in an atom. • The neutron decomposes into a proton and an electron-the proton remains in the nucleus and the electron is emitted. • Beta particles have a 1- charge. • The symbol for a beta particle is o-1e or β. • Beta particles are 8000 x lighter than an alpha particle, are less damaging, but are more penetrating (stopped by aluminum foil or thin pieces of wood).

20. 10 What is the product when carbon-14 undergoes beta decay? • Carbon-13 • Nitrogen-14 • Oxygen-14 • Boron-10

21. Explanation of Answer • 146C  0-1e + 147N

22. 10 What is the product when strontium-90 undergoes beta decay? • Rubidium-90 • Krypton-91 • Strontium-91 • Yttrium-90

23. 10 What is the product when potassium-40 undergoes beta decay? • Calcium-40 • Scandium-40 • Argon-40 • Chlorine 40

24. Gamma Radiation • Gamma radiation is high energy electromagnetic radiation. • Gamma rays are emitted along with alpha or beta particles. • Gamma rays have no mass or charge. • The symbol for gamma rays is ooγ • Gamma rays are extremely penetrating and potentially dangerous (stopped only by several meters of concrete or several centimeters of lead).

25. Other Nuclear Reactions • Fission-when a large nucleus is bombarded with neutrons, a division of the nucleus into 2 smaller nuclei occurs resulting in a large release of energy. • This energy is used in nuclear power plants and in atomic bombs. • Fusion-nuclei with small masses combine to form a nucleus with a larger mass. • This type of reaction occurs in the sun and in hydrogen bombs. • The high temperature needed to start a fusion reaction is produced by a fission reaction.

26. Half-Life • A half-life is the time it takes for one -half of the nuclei of a sample of radioactive isotopes to undergo radioactive decay. • Half-lives may be as short as a fraction of a second or as long as billions of years. • For examples, see the chart on page 810.

27. Determining Half-Lives • In order to solve problems involving half-lives, the following equation may be used: # of half-lives = total time/time of one half-life • To determine the amount of sample left, the following equation may be used: amount left = starting amount/ 2# of half-lives

28. 0 The half-life of carbon-14 is 5700 years. If a 10 gram sample undergoes decay for 17,100 years, how many half-lives has the sample undergone? • 10 • 5 • 3 • 1

29. 0 Cobalt-60 is a radioactive element used as a source of radiation in the treatment of cancer. Cobalt-60 has a half-life of five years. If a hospital starts with a 1000-mg supply, how much will remain after 10 years? • 1000 mg • 750 mg • 500 mg • 250 mg

30. 10 Polonium-210 has a half-life of 138 days. How much of a 2.34 kg sample will remain after four years? • 0.644 mg • 1.50 mg • 1.51 g • 10.6 g