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RADIOACTIVITY Powered By Docstoc
 Almost all atoms we’ve encountered
  have stable nuclei
    – Not radioactive
 Radioactive atoms are relatively rare,
  which is fortunate since radiation can
  be harmful
 Radioactive atoms have nuclei that
  are disintegrating
 So why are some nuclei stable and
  others unstable?
 The fundamental force that holds
  the nucleus together is called
  the Strong Nuclear Force.
   ̵    If the force is strong enough
        the nucleus will be stable
 One reason the force might not be
  strong enough is the number of
  protons and neutrons in the nucleus.
       – Not all combinations of protons and
         neutrons are stable.
       – There must be a “magic” number of
         each particle to ensure stability
 The first 20 stable nuclei follow a
  distinct pattern.
   – For elements with atomic numbers
     between 1 and 20 stable nuclei have
     almost equal numbers of protons
     and neutrons.
   – Beyond 20 protons, nuclei need
     increasingly more neutrons than
     protons to be stable.
 Nuclei are unstable not only if they
  contain too few neutrons, but also if
  they contain too many.
 Radioactive elements/isotopes are
  responsible for producing what we
  think of as radiation.
 There are three different forms of
  nuclear radiation
   – Alpha       Particles emitted
   – Beta          from nucleus
   – Gamma     AKA: ionizing radiation
 Spontaneous emission of radiation
  from an atom is known as
           Alpha Particles
 Alpha radiation consists of a stream
  of high-energy alpha particles
 Consists of 2 protons and 2 neutrons
  and is identical to a helium-4 nucleus
 Can be represented by the symbol
           Alpha Particles
 Alpha particles do not have much
  penetrating power.
 They are able to travel only a few
  centimeters through air and are
  easily stopped by paper or clothing
 Not normally harmful to humans
  − Can be very dangerous if source is
             Beta Particles
 Beta radiation consists of a stream of
  high-speed electrons
 A neutron changes into a proton and
  an electron
   – The proton remains in the nucleus
     and the electron is propelled out of
     the nucleus
 Beta radiation is represented by the
            Beta Particles
 The mass number is zero
 100 times more penetrating then
  alpha radiation
 Can damage the skin and tissues
  − Ionizing radiation can cause damage
    to DNA, which might lead to…
    • Apoptosis (cell death)
    • Mutation
            Gamma Rays
 A Gamma ray is highly energetic
  light, similar to x-rays
 Does not consist of particles
 Gamma radiation accompanies
  alpha and beta radiation
 Much more penetrating than either
  of a or b
 It is able to penetrate deeply into
  solid material, including body tissue
 Symbolized by:
    Properties of Some Radiations
            Alpha        Beta       Gamma
           Radiation   Radiation    Radiation

Comp-       Alpha       Beta       High-energy
osition    Particle    Particle      photon
              4             0
Symbol     a, 2 He       b, e

Charge        2+          1-           0

              4        1/1837          0
    Properties of Some Radiations
            Alpha           Beta         Gamma
           Radiation      Radiation      Radiation
Common Radium- Carbon-14
 source  226                            Cobalt-60
Appox.                    0.05 to 1
            5 MeV           MeV           1MeV
              Low         Moderate       Very high
Power       (0.05mm       (4mm body    (penetrates body
           body tissue)     tissue)         easily)
Shield-     Paper,                        Lead,
                          Metal foil
 ing       clothing                      concrete
         Radioactive Decay
 When an atom emits one of these
  kinds of radiation, it is said to be
 An atom may undergo an alpha or
  beta decay.
 The radiation is called decay because
  the nucleus decomposes to form a
  new nucleus, called transmutation
 The best way to understand a decay
  is with a nuclear equation
       Alpha Decay Equations
 An alpha particle is a particle
  composed of 2 protons and 2
 With each expulsion of an alpha
  particle from the atom’s nucleus the
  atom loses 4 units of mass & 2
  protons (+2 charged particle)
 Any change in #’s of protons changes
  the type of atom, this is transmutation.
        Beta Decay Equations
 If you remember a b is an e- that is
  expelled from an atom
 This electron is the result of one of the
  atom’s neutrons decomposing into a
  proton and an electron.
 This results in the atom having one
  more proton which causes it to
  transmutate into a different atom.
   Other Nuclear Decay Particles…
 There are 2 other types of radioactive
  decays observed.
   – The antimatter equivalent to a beta
     particle is called a positron ( +1e )
        22Ti         44
                      21   Sc     0
                                  1   e
   – The absorption of an electron by a
     nucleus, a.k.a. electron capture
        22   Ti  e 
                             21   Sc
 Every radioactive isotope has a unique
  rate of decay or half-life
   – 1 half life is the time required for ½
     of nuclei of a radioactive sample
     to decay
   – After 1 half-life, ½ of the original
     nuclei have undergone transmutation
 Half lives may be as as short as a
  fraction of a second or as long as
  billions of years
  – Most artificially produced radioactive
    isotopes have very short half-lives
  – Rapidly decaying isotopes do not
    pose long term bio-radiation hazards
    to med patients
 A simple half-life equation:
   amount left  Original amount  
     n  thenumber of half - lives
         Half-Lifes & Radiation
   Isotope       Half-life    emitted
 Carbon-14     5730 years       b
Potassium-40    1.25x109       b, 
 Radon-222       3.8 days       a
Radium-226     1600 years      a, 
Thorium-230     75,400yrs      a, 
Thorium-234     24.1 days      b, 
Uranium-235    7.0x108yrs      a, 
Uranium-238    4.46x109yrs      a
 One radioactive isotope that has a
  long half-life is U-238, which decays
  through a complex series of radio-
  active intermediates to the eventual
  stable isotope of Pb-206
   – It’s possible to use this method
     to date rocks nearly as old as the
     solar system
       Other Nuclear Reactions
 Radioactive decay is only one of
  several types of nuclear rxns.
 Two other rxns that involves radio-
  active particles are fission & fusion
   –These rxns deal with the interaction
    of nuclear particles not a decay of a
            Nuclear Fission
 In a nuclear fission reaction, a large
  nucleus is split into two smaller nuclei
  of approximately equal mass
 Fission rxns are used to provide what
  is commonly called nuclear power.
   -carefully controlled fission rxns
 In a nuclear reactor, the fission of 4.5g
  of U-235 will satisfy a person’s energy
  needs for 1 year.
 Fission reactors are used for a clean
  & efficient source of power.
            Nuclear Fission
 One fission rxn produces enough
  neutrons to start 3 more fission rxns,
  which in turn produces the neutrons
  needed to start 3 more rxns, and so
  on, in a series
  called a nuclear
  chain reaction.
            Nuclear Fusion
 In a nuclear fusion rxn, 2 small
  nuclei join to form a larger nucleus.
 Like a fission rxn, a fusion reaction
  converts some of the mass of the
  original nuclei into energy- a great
  deal of energy
   –According to the eqn: E=mc2
            Nuclear Fusion
 Fusion rxns are hard to initiate and to
 So far it takes a tremendous amount
  of heat to start
 Cold fusion is a natural research
   –The goal is to
    harness the
    power of the sun
           Using Radiation
 Although radiation can be harmful &
  should always be handled with care;
  it can be used safely & is important
  in many procedures
   – Radioisotopes called tracers are used
     to study chem rxns and molecular
   – Also they can be used to study the
     inner workings of the body
            Using Radiation
 One of the reactants is labeled with
  a radioisotope and added to the rxn
 After the rxn is complete, the radiation
  of the product is measured to
  determine the uptake of the tracer
   – Using this technique much can be
     learned about the reaction
            Using Radiation
 Tracers are used in agricultural
   – The tracer is introduced to the
     substance being tested
   – Plants are treated with the radioactive
     labeled substance
   – Tracer measured to determine the
     location of the substance
   – Often the tracer is also monitored
     in animals that consume the plants,
     in water, and in soil.
           Using Radiation
 Radioisotopes can even
  be used to diagnose
  - I-131, is used to detect
    thyroid problems
  - Tc-99 is used to detect
    brain tumors & liver
  - P-32 is used to detect
    skin cancer
                               Tl-207 scan of the
           Using Radiation
 Radiation has become a routine part
  of the treatment of some cancer
  - The fast-growing cancer cells are
    more susceptible to damage by high-
    energy radiation killing the cancer
  - Unfortunately if it isn’t used localized
    to the cancer cells it can kill healthy
    cells as well.

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