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.
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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