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Radioactivity Chapter 21


Radioactivity Chapter 21 Natural occurring phenomena. In the nucleus of an atom there are protons and neutrons. Protons are positively charged so they naturally want ... – PowerPoint PPT presentation

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Title: Radioactivity Chapter 21

RadioactivityChapter 21
  • Natural occurring phenomena.
  • In the nucleus of an atom there are protons and
  • Protons are positively charged so they naturally
    want to repel each other.
  • A strong nuclear force keeps them inside.
  • Neutrons have no charge so they are neutral and
    help cushion the repulsive force the protons
  • the glue that holds the nucleus together

  • Once the nucleus has 20 protons it needs more
    neutrons than protons to keep the nucleus
  • And once you get to a certain number of protons
    there is no number of neutrons sufficient to keep
    the nucleus stable.
  • Once the nucleus becomes unstable it wants to
    break down or decay.

Band of Stability
  • The ratio between neutrons and protons determine
    the stability of a nucleus.
  • If the number is too large or too small the
    nucleus is unstable.
  • No atoms that have an atomic numbers larger than
    83 and a mass number larger than 209 are stable.

Chemical Symbol
  • U Uranium
  • Mass number protons and neutrons
  • Atomic number number of protons

3 Main Types of Radiation
Nuclear Equations
Alpha Decay
  • Alpha emission(greek symbol alpha - a )
  • emission of a nucleus with a mass number of 4 and
    atomic number of 2.
  • can be stopped by a few sheets of paper.
  • Example

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Beta Decay
  • Beta emission (greek symbol ß )
  • emission of a high speed electron.
  • can be stopped by wood or heavy protecting
  • Example

Gamma Decay
  • Gamma emission(greek symbol ? )
  • emits a high energy non-particle radiation.
  • need several inches of lead to block Gamma.
  • Example

Other Types of Particles
  • Particles in nuclear equations
  • Proton
  • Positron
  • Neutron

Nuclear Bombardment
  • When atoms are bombarded by other particles.
  • Usually take place in a particle accelerator.

Overview of Radioactive Emission
Radiation Exposure
  • Radiation is measure by the non SI unit R
    (roentgen equivalent for man).
  • Average dose received annually120 R
  • 70 of radiation comes from natural resources
    such as minerals and cosmic rays. About 30 come
    from medical procedures like x-rays.

Fission vs. Fusion
  • Fission reactions - a very heavy nucleus,
    spontaneously or after absorbing additional light
    particles (usually neutrons), splits into two or
    sometimes three pieces. (a decay is not usually
    called fission.)
  • Occurs only with a critical mass.
  • Fusion reactions - two light nuclei join to form
    a heavier one, with additional particles (usually
    protons or neutrons) thrown off to conserve

2H 3H ? 4He n
Uses of Radiation
  • Nuclear Powerplants
  • Age Dating
  • Radioactive Tracers
  • Cancer Treatment
  • Sterilizing
  • Smoke Detectors
  • Genetic Studies

Radioactive Dating
  • Animals and plants have a known proportion of
    Carbon-14 (a radioisotope of Carbon) in their
  • When they die they stop taking Carbon in, then
    the amount of Carbon-14 goes down at a known rate
    (Carbon-14 has a half-life of 5700 years).
  • The age of the ancient organic materials can be
    found by measuring the amount of Carbon-14 that
    is left.

Radioactive Tracers
  • The most common tracer is called Technetium-99
    and is very safe because it only emits gamma rays
    and doesn't cause much ionisation.
  • Radioisotopes can be used for medical purposes,
    such as checking for a blocked kidney. To do
    this a small amount of Iodine-123 is injected
    into the patient, after 5 minutes 2 Geiger
    counters are placed over the kidneys.
  • Also radioisotopes are used in industry, to
    detect leaking pipes. To do this, a small amount
    is injected into the pipe. It is then detected
    with a GM counter above ground.

Cancer Treatment
  • Because Gamma rays can kill living cells, they
    are used to kill cancer cells without having to
    resort to difficult surgery. This is called
    "Radiotherapy", and works because cancer cells
    can't repair themselves when damaged by gamma
    rays, as healthy cells can.
  • It's vital to get the dose correct - too much and
    you'll damage too many healthy cells, too little
    and you won't stop the cancer from spreading in
  • Some cancers are easier to treat with
    radiotherapy than others - it's not too difficult
    to aim gamma rays at a breast tumor, but for lung
    cancer it's much harder to avoid damaging healthy
    cells. Also, lungs are more easily damaged by
    gamma rays, therefore other treatments may be

  • Even after it has been packaged, gamma rays can
    be used to kill bacteria, mould and insects in
    food. This process prolongs the shelf-life of the
    food, but sometimes changes the taste.
  • Gamma rays are also used to sterilize hospital
    equipment, especially plastic syringes that would
    be damaged if heated.

Smoke Detectors
  • Smoke alarms contain a weak source made of
    Americium-241.Alpha particles are emitted from
    here, which ionize the air, so that the air
    conducts electricity and a small current flows.
    If smoke enters the alarm, this absorbs the a
    particles, the current reduces, and the alarm
    sounds. Am-241 has a half-life of 460 years.

  • The half-life of a substance is the time it takes
    for a radioactive sample to decay into half.