Radiometric Measurements of Absolute Time
Earth's Dynamic Systems:  Chapter 9  (pps.180-207)

Radiometric dating provides a method for directly measuring geologic time in terms of a specific number of years (not exact years, but approximate years.)  Most of these techniques are based on the idea of radioactive decay; measuring amounts of radioactive decay.

Radioactivity is the spontaneous disintegration of an atomic nucleus with the emission of energy.  Radioactive isotopes are unstable:  their nuclei spontaneously disintegrate, transforming them into completely different atoms.  In the process, radiation is given off, and heat is liberated.  Can't eliminate radioactivity by chemical means.  Radium carbonate is still radioactive.  Radium nucleus unaffected by
chemical reaction. 

Q.  What makes an element radioactive?
A.  Overcrowded nucleus NOTE:  Elements occurring on the chart beyond this point (83 and above) are all radioactive because their nuclei are overcrowded, unstable and therefore throw out particles.

Q.  What comes out of the nucleus?
A.  Types of radioactive emissions

Q.  What types of particles are tossed out of the nucleus?
A.  Alpha, Beta and Gamma particles

Alpha particle

Beta particle Gamma particle

Q.  How is radioactivity measured?
A.  Geiger counter may be utilized

NUCLEAR FISSION  FISSIONsplitting the nucleus
Uranium-238   (238 U 92)  will not split
Uranium-235   (235 U 92)  will split, a relatively rare isotope
A mass of uranium bombarded with a neutron yields an explosion = a branching chain reaction.  This is a nuclear reaction, not a chemical one:
        1)  nucleus is bombarded with a neutron
        2)  nucleus then fissions, releasing energy and more neutrons.  
        3)  depending on concentration of uranium, we could have a nuclear weapon = fission
        4)  plutonium is used now, since uranium is so rare

NOTE:  WWII came to an end when U.S. dropped two fission bombs on Japan in 1945  (Hiroshima & Nagasaki).  Only time in history nuclear weapons have been used.  Japanese retaliated by selling cars in U.S. and no one knows which one had a more severe impact.  Japan is probably still experiencing by-products from these explosions.  Rubidium and barium are waste products from splitting and these can remain radioactive for thousands of years.

NUCLEAR POWERsimilar to fission; yields a clean source of power, unlike coal
Uranium-235   (235 U 92) fuel rods are surrounded by water in a nuclear reactor, because water slows down the bombarding; water is used to keep things cool.  In the U.S. the reactor controls the rate of fissioning.
A nuclear plant at Chernobyl in the Ukraine (previously a republic within the Soviet Union) experienced what is referred to as a meltdown (cut-off water supplies) which resulted in an explosion.  While this was not a nuclear explosion, pulverized uranium rods did penetrate the atmosphere all the way to England and increased rates of thyroid cancer have been diagnosed in Ukraine and Belorussia.
NOTE:  Most nuclear plants are located in Eastern U.S., California, Europe (France), and Russia.  In addition to concerns with meltdowns, geological factors, like earthquakes, could also affect nuclear power plant safety.

NUCLEAR WASTE:  currently builds up in nuclear power plants throughout the world.

Q.  What is the geologic dating term "half-life"?
A.  It is the time it takes for radioactive material to decay to one half the original amount.   Back to: