Radioactivity Polonium and radium X-Rays Marie Curie 1867 - 1934 Wilhelm Roentgen 1845 - 1923 Radioactivity Spontaneous emission of radiation from the nucleus of an unstable isotope. Antoine Henri Becquerel 1852 - 1908

Radioactive Decay Unstable nucleus emits particle and energy Most common decays involve the emission of  - particle ( Decay)  - particle ( Decay)  - particle ( Decay)

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:

The Numbers Atomic mass number: number of nucleons Atomic number: number of protons Neutrons = number of nucleons – number of protons

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

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

Sources of Radiation

Penetrating Power

Shielding

Radioactive Decay 4.5 x 109 yr half-life 24 day half-life 1.17 min half-life 250,000 yr half-life

U 238 Decay Decay Series

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

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

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