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Nuclear fission and fusion. Types of decay process Rates of decay Nuclear stability Energy changes Fission and fusion. . . . . Forces at work in the nucleus. Electrostatic repulsion: pushes protons apart Strong nuclear force: pulls protons together

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Nuclear fission and fusion

Nuclear fission and fusion

Types of decay process

Rates of decay

Nuclear stability

Energy changes

Fission and fusion


Forces at work in the nucleus

Forces at work in the nucleus

  • Electrostatic repulsion: pushes protons apart

  • Strong nuclear force: pulls protons together

  • Nuclear force is much shorter range: protons must be close together


Neutrons only experience the strong nuclear force

Neutrons only experience the strong nuclear force

  • Proton pair experiences both forces

  • Neutrons experience only the strong nuclear force

  • But: neutrons alone are unstable


  • Neutrons act like nuclear glue

    Neutrons act like nuclear glue

    • Helium nucleus contains 2 protons and 2 neutrons – increase attractive forces

      • Overall nucleus is stable


    As nuclear size increases electrostatic repulsion builds up
    As nuclear size increases, electrostatic repulsion builds up

    • There are electrostatic repulsions between protons that don’t have attractive forces

    • More neutrons required

    Long range repulsive force with no compensation from attraction



    Upper limit to nuclear stability
    Upper limit to nuclear stability

    • Beyond atomic number 83, all nuclei are unstable and decay via radioactivity

    • Radioactive decay (Transmutation) – formation of new element

    Mass number

    Atomic number decreases

    Alpha particle emitted

    Atomic number


    Odds and sods
    Odds and sods

    • All elements have a radioactive isotope

    • Only H has fewer neutrons than protons

    • The neutron:proton ratio increases with Z

    • All isotopes heavier than bismuth-209 are radioactive

    • Most nonradioactive isotopes contain an even number of neutrons (207 out of 264). 156 have even protons and neutrons; 51 have even protons and odd neutrons; 4 have odd protons and neutrons


    Nuclear processes relieve instability
    Nuclear processes relieve instability

    • Chemical reactions involve electrons; nuclear reactions involve the nucleus

    • Isotopes behave the same in chemical reactions but differently in nuclear ones

    • Rate of nuclear process independent of T,P, catalyst

    • Nuclear process independent of state of the atom – element, compound

    • Energy changes are massive



    Alpha particle emission
    Alpha particle emission

    92 protons

    146 neutrons

    238 nucleons

    2 protons

    2 neutrons

    4 nucleons

    90 protons

    144 neutrons

    234 nucleons


    Beta particle emission
    Beta particle emission

    53 protons

    78 neutrons

    131 nucleons

    0 nucleons

    -1 charge

    54 protons

    77 neutrons

    131 nucleons


    Other decay processes
    Other decay processes

    • Positron emission: the conversion of a proton into a neutron plus positive electron

      • Decrease in z with no decrease in m

    • Electron capture: the capture of an electron by a proton to create a neutron

      • Decrease in z with no decrease in m

    19 protons

    21 neutrons

    40 nucleons

    18 protons

    22 neutrons

    40 nucleons

    0 nucleons

    +1 charge

    80 protons

    117 neutrons

    197 nucleons

    0 nucleons

    -1 charge

    79 protons

    118 neutrons

    197 nucleons



    Measuring decay
    Measuring decay

    • Rates of radioactive decay vary enormously – from fractions of a second to billions of years

    • The rate equation is the same first order process

      Rate = k x N


    Half life measures rate of decay
    Half-life measures rate of decay

    • Concentration of nuclide is halved after the same time interval regardless of the initial amount – Half-life

    • Can range from fractions of a second to millions of years


    Mathematical jiggery pokery
    Mathematical jiggery pokery

    • Calculating half life from decay rate

      t = 0, N = No; t = t1/2, N = No/2

    • Calculating residual amounts from half life


    Magic numbers
    Magic numbers

    • Certain numbers of protons and/or neutrons convey unusual stability on the nucleus

      2, 8, 20, 28, 50, 82, 126

    • There are ten isotopes of Sn (Z=50); but only two of In (Z=49) and Sb (Z=51)

    • Magic numbers are associated with the nuclear structure, which is analogous to the electronic structure of atoms



    Stability is not achieved in one step products also decay
    Stability is not achieved in one step: products also decay

    • Here atomic number actually increases, but serves to reduce the neutron:proton ratio

    • Beta particle emission occurs with neutron-excess nuclei

    • Alpha particle emission occurs with proton-heavy nuclei


    Radioactive series are complex
    Radioactive series are complex

    The decay series from uranium-238 to lead-206. Each nuclide except for the last is radioactive and undergoes nuclear decay. The left-pointing, longer arrows (red) represent alpha emissions, and the right-pointing, shorter arrows (blue) represent beta emissions.


    Energy changes and nuclear decay
    Energy changes and nuclear decay

    • In principle there will be an energy associated with the binding of nuclear particles to form a nucleus

    • Experimentally demanding!


    Use einstein s relationship
    Use Einstein’s relationship

    E = mc2

    • Consider the He nucleus:

      Mass of individual particles = 4.03188 amu

      Mass of He nucleus = 4.00150 amu

      Mass loss = 0.03038 amu

    • The “lost” mass is converted into energy – the binding energy, which is released during the nuclear process

    • For the example above, the energy is 2.73 x 109 kJ/mol


    Inter changeability of mass and energy
    Inter-changeability of mass and energy

    • Loss in mass equals energy given out

      E = mc2

    • Tiny amount of matter produces masses of energy:

      1 gram  1014 J

    • Energy and mass are conserved, but can be inter-changed

    • Binding energy per nucleon presents the total binding energy as calculated previously per nuclear particle

      • Usually cited in eV, where 1 eV = 1.6x10-19J


    Average mass per nucleon varies with atomic number

    Fe

    He

    Nucleon mass

    U

    H

    Average mass per nucleon varies with atomic number

    The binding energy per nucleon for the most stable isotope of each naturally occurring element. Binding energy reaches a maximum of 8.79 MeV/nucleon at 56Fe. As a result, there is an increase in stability when much lighter elements fuse together to yield heavier elements up to 56Fe and when much heavier elements split apart to yield lighter elements down to 56Fe, as indicated by the arrows.


    Mass changes in chemical reactions
    Mass changes in chemical reactions?

    • Conservation of mass and energy means that energy changes in chemical processes involve concomitant changes in mass

    • Magnitude is so small as to be undetectable

    • A ΔH of -436 kJ/mol corresponds to a weight loss of 4.84 ng/mol


    Fission and fusion ways to harness nuclear energy
    Fission and fusion: ways to harness nuclear energy

    • Attempts to grow larger nuclei by bombardment with neutrons yielded smaller atoms instead.

      • Distorting the nucleus causes the repulsive forces to overwhelm the attractive

    • The foundation of nuclear energy and the atomic bomb


    Nuclear fission
    Nuclear fission

    • Nuclear fission produces nuclei with lower nucleon mass

    • One neutron produces three: the basis for a chain reaction – explosive potential

    • Many fission pathways – 800 fission products from U-235



    Nuclear fusion opposite of fission
    Nuclear fusion: opposite of fission

    • Small nuclei fuse to yield larger ones

    • Nuclear mass is lost

    • Example is the deuterium – tritium reaction

    • About 0.7 % of the mass is converted into energy

    +E


    The sun is a helium factory
    The sun is a helium factory

    • The sun’s energy derives from the fusion of hydrogen atoms to give helium


    Fusion would be the holy grail if
    Fusion would be the holy grail if...

    • The benefits:

      • High energy output (10 x more output than fission)

      • Clean products – no long-lived radioactive waste or toxic heavy metals

    • The challenge:

      • Providing enough energy to start the process – positive charges repel

      • Reproduce the center of the sun in the lab

    • Fusion is demonstrated but currently consumes rather than produces energy



    Radioisotopes have wide range of uses
    Radioisotopes have wide range of uses

    • H-3 Triggering nuclear weapons, luminous paints and gauges, biochemical tracer

    • I-131 Thyroid treatment and medical imaging

    • Co-60Food irradiation, industrial applications, radiotherapy

    • Sr-90 Tracer in medical and agricultural studies

    • U-235/238 Nuclear power generation, depleted U used in weapons and shielding

    • Am-241 Thickness and distance gauges, smoke detectors





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