Nuclear Physics • Chapter 29
Fossils • How are scientists able to determine the age of fossils? • The “iceman”
Nuclear Physics • The year 1896 marked the birth of nuclear physics • Henri Becquerel accidentally discovered natural radioactivity in uranium compounds. • Researchers tried to identifythe radiation from atomic nuclei. 1 Thorite ------->
Rutherford’s Discovery • Rutherford showed that there were three types of nuclear radiation.
Alpha (a) • Helium nuclei • Least penetrating • Positively charged
Beta (b) • Electrons or positrons • More penetrating than alpha particles • Negatively or positively charged
Gamma (g) • High-energy photons • Most penetrating • No charge
Rutherford’s Experiment • Rutherford bombarded gold foil with alpha particles and disproved the Thompson model of the atom..
The Strong Nuclear Force • Rutherford and his students discovered the nuclear strong force in 1911 • It explained why the protons don’t go flying off from the nucleus.
Milestones in Nuclear Physics • Nuclear reactions were observed in 1930. • The neutron was discovered in 1932. • Artificial radioactivity was produced in 1933. • Nuclear fission was discovered in 1938. • Nuclear fission was first controlled in 1942.
Atomic Nuclei • All nuclei are composed of two types of particles. • The atomic number, Z, equals the number of protons in the nucleus. • The neutron number, N, equals the number of neutrons n the nucleus. • The mass number, A, equals the number of nucleons. (A = Z + N) • A nucleon can be either a proton or a neutron
Nuclear Symbols • The symbol we use to represent nuclei is • Sometimes Z is not shown when the chemical symbol is obvious.
Isotopes • Isotopes have the same number of protons but different numbers of neutrons. • There are four isotopes of carbon.
Hydrogen Isotopes • Hydrogen has three isotopes. • Protium • Deuterium • Tritium
Symbols for Other Particles • alpha • beta or • gamma • neutron
Artificial Isotopes • Artificial isotopes do not occur naturally and are produced in the laboratory. 17-1, 17-2
Charge and Mass • The proton and electron have charges that are equal in magnitude but opposite in sign. • The neutron has no charge. • The masses of the proton and neutron are nearly equal. (See Table 29.1) on pg. 914. • The masses of selected isotopes are in Appendix B on pg. A.14.
Unified Mass Units • Unified mass units (u) are based upon the carbon-12 atom which has a mass of exactly 12 u. • 1 u = 1.660540 x 10-27 kg • The proton and neutron each have a mass of approximately 1 u. • The mass of the electron is much less.
Particle Masses • Proton mass = 1.007276 u • Neutron mass = 1.008665 u • Electron mass = 0.000549 u
Mass Energy Conversion • The energy equivalent of one atomic mass unit is 931.494 MeV.
Rutherford found that an alpha particle on a head-on collision with a nucleus will stop instantaneously at a distance d from the nucleus because of Coulomb repulsion. 29.1
A neutron has the best chance of causing a nuclear reaction because it has no charge.
Nuclear Radii • The average radius of most atomic nuclei can be found by using ro = 1.2 x 10-15 m A is the mass number 285
Nuclear Density • All nuclei have nearly the same density.
Nuclear Stability • Why don’t the protons in the nucleus fly apart because of the repulsive Coulomb forces?
The Strong Nuclear Force • The strong nuclear force is an attractive force between all nuclear particles. • The strong nuclear force dominates the Coulomb force over short distances.
Nuclear Stability • There are over 260 stable nuclei. • Hundreds more are unstable • Light nuclei are stable when N = Z. • Heavier nuclei are stable when N > Z. • Elements with more than 83 protons are always unstable (radioactive). 29.3
Nuclear Mass • The total mass of the nucleus is always less than the sum of the masses of its nucleons.
Nuclear Energy • The total energy of the bound nucleus is always less than the combined energy of the separated nucleons. • The difference is called the binding energy of the nucleus. 290
Splitting a Nucleus • In order to break apart a nucleus, energy must be added to the system.
Binding Energy • Nuclei with an atomic mass near 60 are the most stable. • The average binding energy per nucleon is about 8 MeV / nucleon. 29.4, 291
Natural Radioactivity • Natural radioactivity was accidentally discovered by Becquerel in 1896. • It was later named “radioactivity” by Marie Curie.
Radioactive Elements • Marie and Pierre Curie discovered radium and polonium after years of separating the radioactive elements from tons of pitchblende, a radioactive ore. • Experiments indicated that the radiation was the result of nuclear decay. • Marie Curie died of leukemia in 1934.
The Weak Nuclear Force • The weak nuclear force is responsible for radioactivity.
Anti Matter • What is a positron? • It is an antiparticle with the mass of an electron and the charge of a proton. • Symbol
Identifying Nuclear Particles • A magnetic field can be used to identify the particles involved in nuclear radiation. 29.5, 292
Rutherford’s Mousetrap • Rutherford’s mousetrap experimentally proved that alpha particles were composed of helium nuclei. T-41
The Decay Constant • The decay constant (l) determines the rate at which isotopes decay • A large value for l indicates a rapid rate of decay.
Activity • The activity (R) is defined as the number of decays per second • 1 Bq = 1 decay / second • 1 Ci = 3.7 x 1010 decays / second = 3.7 x 1010 Bq • This is the approximate activity of 1 gram of radium. • The mCi and the mCi are most commonly used.
Remaining Nuclei • The number of nuclei (N) remaining after a given amount of time can be found by using:
Decay Rate • The decay rate (Ro) can be found by using:
Activity • The activity (R) after a given amount of time can be found by using:
Half Life • Half–life • The time it takes or half of a given number of radioactive nuclei to decay is given by: 6, 294, 29.6
Questions 1, 12 Pg. 933
The Decay Processes • Alpha decay • The parent nucleus emits a helium nucleus. • The remaining nucleus is called the daughter. • The daughter nucleus has two less protons and two less neutrons. • Examples
Transmutation • Transmutation is the spontaneous decay of of one element into another. • In the decay process, excess mass is converted into energy of other forms, mostly into the KE of the nuclei. 39-1
Artificial Transmutation • Rutherford accomplished the first artificial transmutation. • He bombarded nitrogen-14 with alpha particles and produced Oxygen-17.
Beta decay • The parent nucleus emits an electron or a positron. How are these particles produced? • The daughter nucleus has the same number of nucleons as the parent nucleus. • The atomic numberincreases by 1 or decreases by 1. • Examples 32-1