Radioactivity

1
Radioactivity
Polonium and radium
X-Rays
Marie Curie 1867 - 1934
Wilhelm Roentgen 1845 - 1923

• Spontaneous emission of radiation from the
  nucleus of an unstable isotope.

Antoine Henri Becquerel 1852 - 1908
2
Radioactive Decay

• Unstable nucleus emits particle and energy
• Most common decays involve the emission of
• ? - particle (? Decay)
• ? - particle (? Decay)
• ? - particle (? Decay)

3
Alpha Decay

• Alpha decay occurs when there are too many
  protons in the nucleus which cause excessive
  electrostatic repulsion.
• An alpha particle is ejected from the nucleus.
• An alpha particle is 2 protons and 2 neutrons.
• An alpha particle is also a helium nucleus.
• Alpha particle symbol

4
The Numbers
Atomic mass number number of nucleons
Atomic number number of protons
Neutrons number of nucleons number of protons
5
Beta Decay

• Beta decay occurs when neutron to proton ratio is
  too big
• A neutron is turned into a proton and electron
  and an antineutrino
• The electron and the antineutrino are emitted

6
Gamma Decay

• Gamma decay occurs when the nucleus is at too
  high an energy
• Nucleus falls down to a lower energy level
• High energy photon gamma ray - is emitted

7
Sources of Radiation
8
Penetrating Power
9
Shielding
10
Radioactive Decay
4.5 x 109 yr half-life
24 day half-life
1.17 min half-life
250,000 yr half-life
11
U 238 Decay

• Decay Series

12
Half-Life

• Half-life is the time required for half of the
  element to decay to another element.
• For Thorium-234, the half-life is 24 days.
• That means that every 24 days half of the
  thorium-234 in a sample has decayed.
• If you begin with a 1 kg chunk of Thorium-234,
  after 24 days, you will have ½ kg of Thorium-234.
• After 48 days?
• ¼ kg
• After 72 days?
• 1/8 kg
• After 96 days?
• 1/16 kg
• After 120 days?
• 1/32 kg

13
Carbon Dating

• Cosmic rays cause transmutation of Nitrogen to
  Carbon-14
• C-14 is radioactive with a half-life of 5730
  years
• It decays back to Nitrogen by beta decay
• The ratio of C-12 (stable) atoms to C-14 atoms in
  our atmosphere is fairly constant about 1012/1
• This ratio is the same in living things that
  obtain their carbon from the atmosphere

14
Carbon Dating
The radioactive carbon isotopes in the skeleton
diminish by one-half every 5730 years.