PHYS 205 Chapter 31, Fall 2017

Chapter 31 Nuclear Physics and Radioactivity 31.1 Nuclear Structure The atomic nucleus consists of positively charged protons and neutral neutrons. 31.1 Nuclear Structure           neutrons of Number protons of Number neutrons and of protons Number N Z A   atomic number atomic mass number 31.1 Nuclear Structure Nuclei that contain the same number of protons but a different number of neutrons are known as isotopes. 31.1 Nuclear Structure   1 3 10 15m 2.1 A r    31.1 Nuclear Structure   1 3 10 15m 2.1 A r    Conceptual Example 1 Nuclear Density It is well known that lead and oxygen contain different atoms and that the density of solid lead is much greater than gaseous oxygen. Using the equation, decide whether the density of the nucleus in a lead atom is greater than, approximately equal to, or less than that in an oxygen atom. Decide whether the nucleus of the lead is (a) greater, (b) Equal or (c) smaller than the density of oxygen atom? 31.2 The Strong Nuclear Force and the Stability of the Nucleus The mutual repulsion of the protons tends to push the nucleus apart. What then, holds the nucleus together? The strong nuclear force. 31.2 The Strong Nuclear Force and the Stability of the Nucleus As nuclei get larger, more neutrons are required for stability. The neutrons act like glue without adding more repulsive force. 31.3 The Mass Deficit of the Nucleus and Nuclear Binding Energy     2 2 Mass deficit Binding energy m c c    31.3 The Mass Deficit of the Nucleus and Nuclear Binding Energy Example 3 The Binding Energy of the Helium Nucleus Revisited The atomic mass of helium is 4.0026u and the atomic mass of hydrogen is 1.0078u. Using atomic mass units, instead of kilograms, obtain the binding energy of the helium nucleus. 31.3 The Mass Deficit of the Nucleus and Nuclear Binding Energy .0 0304 u .4 0026 u .4 0330 u    m 931 5. MeV u 1  28 3. MeV Binding energy  31.3 The Mass Deficit of the Nucleus and Nuclear Binding Energy 31.4 Radioactivity A magnetic field separates three types of particles emitted by radioactive nuclei. 31.4 Radioactivity α DECAY D He P 2 2 4 2     A Z A Z 31.4 Radioactivity A smoke detector 31.4 Radioactivity β DECAY D e P 0 1 1     A Z A Z 31.4 Radioactivity γ DECAY P  P    A Z A Z excited energy state lower energy state 31.4 Radioactivity Gamma knife 31.5 The Neutrino During beta decay, energy is released. However, it is found that most beta particles do not have enough kinetic energy to account for all of the energy released. The additional energy is carried away by a neutrino.  Pa e Th 0 1 234 91 234 90     31.6 Radioactive Decay and Activity The half-life of a radioactive decay is the time in which ½ of the radioactive nuclei disintegrate. t Noe N     2 ln 2 T1  31.6 Radioactive Decay and Activity 31.7 Radioactive Dating 31.7 Radioactive Dating Conceptual Example 12 Dating a Bottle of Wine A bottle of red wine is thought to have been sealed about 5 years ago. The wine contains a number of different atoms, including carbon, oxygen, and hydrogen. The radioactive isotope of carbon is the familiar C-14 with ½ life 5730 yr. The radioactive isotope of oxygen is O-15 with a ½ life of 122.2 s. The radioactive isotope of hydrogen is called tritium and has a ½ life of 12.33 yr. The activity of each of these isotopes is known at the time the bottle was sealed. However, only one of the isotopes is useful for determining the age of the wine. Which is it? 31.8 Radioactive Decay Series The sequential decay of one nucleus after another is called a radioactive decay series. 31.8 Radioactive Decay Series 31.9 Radiation Detectors A Geiger counter 31.9 Radiation Detectors A scintillation counter