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  • Nuclear Reactions

  • Decisions about nuclear energy require some understanding of nuclear reactions and the nature of radioactivity. This is one of the three units of Palo Verde Nuclear Generating Station in Arizona. With all three units running, enough power is generated to meet the electrical needs of nearly 4 million people.

  • Natural Radioactivity

  • IntroductionHenri Becquerel discovered radioactivity in 1896Becquerel named the emission of invisible radiation from uranium ore radioactivity.Radioactive materials was the name given to materials that gave off this invisible radiation.

  • Radioactivity was discovered by Henri Becquerel when he exposed a light-tight photographic plate to a radioactive mineral, then developed the plate. (A) A photographic film is exposed to an uranite ore sample. (B) The film, developed normally after a four-day exposure to uranite. Becquerel found an image like this one and deduced that the mineral gave off invisible radiation that he called radioactivity.

  • Ernest Rutherford later discovered that there were three kinds of radioactivity.Alpha particles () is a helium nucleus (2 protons and 2 neutrons)A beta particle () is a high energy electronA gamma ray () is electromagnetic radiation with a very short wavelength.

  • Radiation passing through a magnetic field shows that massive, positively charged alpha particles are deflected one way, and less massive beta particles with their negative charge are greatly deflected in the opposite direction. Gamma rays, like light, are not deflected.

  • Radioactivity is the spontaneous emission of particles or energy from an atomic nucleus as it disintegrates.Radioactive decay is the spontaneous disintegration of decomposition of a nucleus.

  • Nuclear EquationsThe two subatomic particles that occur in the nucleus, the proton and the neutron, are called nucleons.The number of protons is the atomic number which determines the identity of the element.The number of protons and neutrons determines the atomic mass of the element.Different isotopes of an element have the same atomic number (same number of protons) but different atomic masses (different number of neutrons)

  • The three isotopes of hydrogen have the same number of protons but different numbers of neutrons. Hydrogen-1 is the most common isotope. Hydrogen-2, with an additional neutron, is named deuterium, and hydrogen-3 is called tritium. Neutrons and protons are called nucleons because they are in the nucleus.

  • Just like any other chemical reaction, we use symbols to show a nuclear reactionAs an example, when uranium 238 emits an alpha particle, it loses 2 protons and 2 neutrons.

    Nuclear reactions must balance just like any other chemical reaction, but we must also be aware of balancing protons and neutrons

  • The Nature of the Nucleus.Protons and neutrons are held together by a nuclear force when they are very close together.The shell model of the nucleus visualizes the protons and neutrons moving on energy levels or shells, much like the electrons move in shells.

  • We can predict the stability of a nucleus by using some simple rulesAll isotopes heavier than atomic number 83 have an unstable nucleusIsotopes with 2, 8, 20, 28, 50, 82, or 126 protons or neutrons in their nucleus occur in the most stable isotopes.Nuclei are the most stable with pairs of protons and neutrons, so those with all protons and all neutrons paired up are the most stable.Isotopes with an atomic number less that 83 are most stable when the ratio of protons to neutrons is 1:1.

  • The dots indicate stable nuclei, which group in a band of stability according to their neutron-to-proton ratio. As the size of nuclei increases, so does the neutron-to-proton ratio that represents stability. Nuclei outside this band of stability are radioactive.

  • Type of Radioactive DecayAlpha emissionThis is the expulsion of an alpha particleBeta EmissionEmission of a beta particlea beta particle is an electron that is ejected form the nucleusGamma emissionThis is a high energy burst of electromagnetic radiationEmission occurs as nuclei try to obtain a balance between nuclear attractions, electromagnetic repulsions, and a low quantum of nuclear shell energy.

  • Unstable nuclei undergo different types of radioactive decay to obtain a more stable nucleus. The type of decay depends, in general, on the neutron-to-proton ratio, as shown.

  • Radioactive Decay SeriesRadioactive decay produces a simpler and more stable nucleus.A radioactive decay series occurs as a nucleus disintegrates and achieves a more stable nucleiThere are 3 naturally occurring radioactive decay series.Thorium 232 ending in lead 208Uranium 235 ending in lead 207Uranium 238 ending in lead 206

  • The radioactive decay series for uranium-238. This is one of three naturally occurring series.

  • When there are a large number of nuclei the ration of the rate of nuclear decay per unit time to the total number of radioactive nuclei will be a constantk=rate nThe radioactive decay constant is specific for each isotope

  • The rate of radioactive decay is expressed in terms of half-lifeThe half-life of an element is the time required from one-half of its unstable nuclei to decayThe half-life of an element is related to the ration of 0.693 to its radioactive decay constantt = 0.693/kThe decay constant for U238 is 4.87 X 10-18/sThe half life is thereforet = 0.693/4.87 X 10-18/s = 1.42 X 1017s = 4.5 X 109 years The half-life of U238 is 4.5 billion years.

  • Radioactive decay of a hypothetical isotope with a half-life of one day. The sample decays each day by one-half to some other element. Actual half-lives may be in seconds, minutes, or any time unit up to billions of years.

  • The half-life of each step in the uranium-238 radioactive decay series.

  • Measurement of Radiation

  • Measurement MethodsFilm badgesWorkers who are exposed to radioactivity carry film badgesThe film is exposed and the optical density of the film shows the workers exposure levels during the time the film badge was worn.Ionization counter.Measure ions that are produced by radiation

  • Scintillation counter.Measures the flashes of light that occur when radiation strikes a phosphor.Geiger counterMeasures pulses of electrons released from the ionization of gas molecules in a metal cylinderEach pulse of electrons is heard as a pop or click

  • This is a beta-gamma probe, which can measure beta and gamma radiation in millirems per unit of time.

  • The working parts of a Geiger counter.

  • Radiation UnitsCurie (Ci)Measurement of the activity of a radioactive source.Measured as the number of nuclear disintegrations per unit of timeA curie is 3.70 X 1010 nuclear disintegrations per second.

  • RadMeasures the amount of energy released by radiation striking living tissueShort for radiation absorbed doseOne rad releases 1 X 10-2 J/kgRemShort for roetgen equivalent manThis takes into account the possible biological damage to humans of certain types of radiation.

  • Radiation ExposureBackground radiation is constantly present in our environment.Most people are exposed to between 100 to 500 millirems per year.This background radiation comes from many natural source.The harm that radiation does to living organisms is due to the fact that it produces ionization which can:Disrupt chemical bonds in biological macromolecules such as DNAProduce molecular fragments which can interfere with enzyme action and essential cell functions.

  • The linear model of exposure proposes that any exposure above zero is damaging and can produce cancer and genetic damage, mostly through its effect on DNAThe threshold model proposes that there is a threshold limit of exposure up to which the human body can repair damage caused by the exposureIt is not until we reach and exceed this threshold that we begin to see irreversible damage.

  • Graphic representation of the (A) threshold model and (B) linear model of low-level radiation exposure. The threshold model proposes that the human body can repair damage up to a threshold. The linear model proposes that any radiation exposure is damaging.

  • Nuclear Energy

  • Introduction.Albert Einstein showed us that energy and matter are the same thing, both are inter-convertible.E=mc2Using mass losses during nuclear reactions, one can calculate the energy change of a system.E=mc2There is a difference between the mass of the individual nucleons that make up a nucleus and the actual mass of the nucleus.This is called the mass defect of the nucleus.The mass defect occurs as energy is released when nucleons join to form a nucleus.

  • The energy that is released is called the binding energy.This is also the energy that is required to break the nucleus into its individual protons and neutrons.The ratio of the binding energy to the nucleon number is a measure of a nucleus stabilityMassive nuclei can gain stability by breaking into smaller nuclei with a release of energy.Smaller nuclei can gain stability by joining together with the release of energy.

  • The maximum binding energy per nucleon occurs around mass number 56, then decreases in both directions. As one result, fission of massive nuclei and fusion of less massive nuclei both release energy.

  • Splitting massive nuclei apart with the release of energy is called nuclear fission.The joining together of less massive nuclei with the release of energy is called nuclear fusion

  • Nuclear FissionAs a nuclear reaction occurs, it has the ability to produce a chain reactionA chain reaction is a reaction wher


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