Nuclear Physics and Radioactivity PDF

Chapter 30 Nuclear Physics and Radioactivity © 2014 Pearson Education, Inc.  30-1 Structure and Properties of the Nucleus Nucleus is made of protons and neutrons Proton has positive charge; here is its mass: mp = 1.67262 × 10−27 kg Neutron is...

Chapter 30 Nuclear Physics and Radioactivity © 2014 Pearson Education, Inc.  30-1 Structure and Properties of the Nucleus Nucleus is made of protons and neutrons Proton has positive charge; here is its mass: mp = 1.67262 × 10−27 kg Neutron is electrically neutral, and slightly more massive than the proton: mn = 1.67493 × 10−27 kg me = 9.11 × 10−31 kg mp ˷ 2000 me © 2014 Pearson Education, Inc.  30-1 Structure and Properties of the Nucleus Neutrons and protons are collectively called nucleons. The different nuclei are referred to as nuclides. Number of protons: atomic number, Z Number of nucleons: atomic mass number, A Neutron number: N = A − Z A and Z are sufficient to specify a nuclide. Nuclides are symbolized as follows: X is the chemical symbol for the element; it contains the same information as Z but in a more easily recognizable form. © 2014 Pearson Education, Inc.  30-1 Structure and Properties of the Nucleus Nuclei with the same Z—so they are the same element—but different N are called isotopes. As carbon For many elements, several different isotopes exist in nature. the size of the nucleus Measure by:  30-1 Structure and Properties of the Nucleus Masses of atoms are measured with reference to the carbon-12 atom, which is assigned a mass of exactly 12u. A u is a unified atomic mass unit. 1 u = 1.6605 × 10−27 kg = 931.5 MeV/c2 © 2014 Pearson Education, Inc.  30-1 Structure and Properties of the Nucleus From the following table, you can see that the electron is considerably less massive than a nucleon.  30-3 Radioactivity Towards the end of the 19th century, minerals were found that would darken a photographic plate even in the absence of light. This phenomenon is now called radioactivity. Marie and Pierre Curie isolated two new elements that were highly radioactive; they are now called polonium and radium. Radioactivity is the result of the disintegration or decay of an unstable nucleus.  30-3 Radioactivity Radioactive rays were observed to be of three types: 1. Alpha rays, which could barely penetrate a piece of paper 2. Beta rays, which could penetrate 3 mm of aluminum 3. Gamma rays, which could penetrate several centimeters of lead We now know that alpha rays are helium nuclei, beta rays are electrons, and gamma rays are electromagnetic radiation. © 2014 Pearson Education, Inc.  30-3 Radioactivity Alpha and beta rays are bent in opposite directions in a magnetic field, while gamma rays are not bent at all.  30-6 Gamma Decay Gamma rays are very high-energy photons. They are emitted when a nucleus decays from an excited state to a lower state, just as photons are emitted by electrons returning to a lower state. In gamma decay, A and Z remain the same, so after it we will have the same chemical element.  30-8 Half-Life and Rate of Decay Nuclear decay is a random process; the decay of any nucleus is not influenced by the decay of any other. © 2014 Pearson Education, Inc.  30-8 Half-Life and Rate of Decay This equation can be solved, using calculus, for N as a function of time: (30-4)  30-8 Half-Life and Rate of Decay The half-life is the time it takes for half the nuclei in a given sample to decay. It is related to the decay constant: (30-6) © 2014 Pearson Education, Inc.  30-8 Half-Life and Rate of Decay Therefore, the number of decays in a short time interval is proportional to the number of nuclei present and to the time: (30-3a) Here, λ is a constant characteristic of that particular nuclide, called the decay constant. © 2014 Pearson Education, Inc.  Example 3 The activity of radioactive source decreases by 5.5% in 31 hours. What is the half-life of this source? Solution: The original activity is λ N0 , so the activity 31.0 hours later is 0.945 λN0. ln 2 0.945 N 0 = N 0e − t  ln 0.945 = − t = − t T1 2 ln 2 T1 2 = −  31 = 379.84 h or 15.8 days ln 0.945 © 2014 Pearson Education, Inc.  Summary Nuclei contain protons and neutrons—nucleons Total number of nucleons, A, is atomic mass number Number of protons, Z, is atomic number Isotope notation: Nuclear masses are measured in u; carbon-12 is defined as having a mass of 12 u 1 u = 1.6605 × 10−27 kg = 931.5 MeV/c2 © 2014 Pearson Education, Inc.  Summary Unstable nuclei decay through alpha, beta, or gamma emission An alpha particle is a helium nucleus; a beta particle is an electron or positron; a gamma ray is a highly energetic photon  Summary Electric charge, linear and angular momentum, mass-energy, and nucleon number are all conserved Radioactive decay is a statistical process The number of decays per unit time is proportional to the number of nuclei present: The half-life is the time it takes for half the nuclei to decay