VIDEO
The Discovery of Radioactivity
Absolute Time
The second method to study geological time is done by the chemical and radiological testing of different isotopes (forms of the same element) within rock and mineral samples. This is called absolute or chronometric time measurements.
By using rock and fossil samples that have been classified as to their relationship to one another, laboratory testing can then determine a sample’s age and time placement. The two methods work together to give scientists an accurate time picture of a sample’s age.
Radioactivity
The radioactive properties of different elements were discovered in 1896, by Antoine Becquerel when he discovered that a photographic plate in his lab, never exposed to sunlight, had somehow become exposed. The only possible culprit was a nearby uranium salt sitting on the laboratory bench top. The term, radioactivity, was first used by French scientist, Marie Curie, in 1898. Marie Curie and her physicist husband, Pierre, found that radioactive particles were emitted as either electrically negative (-) called beta (ß) particles, or positive (+) called alpha (α) charged particles.
Radioactivity is the characteristic of an element to change into another element through the loss of charged particles from its nuclei.
Following the further understanding and discovery of radioactive breakdown products, researchers began to see a use for radioactivity and radioactive elements, in the study of rock, mineral, and fossil samples.
Nuclear Reactions
Most chemical reactions are focused on the outer electrons of an element, sharing, swapping, and bumping electrons into and out of the joining elements of a reaction. Nuclear reactions are different. They take place within the nucleus.
There are two types of nuclear reactions. The first is the radioactive decay of bonds within the nucleus that emit radiation when broken. The second is the ‘‘billiard ball’’ type of reactions, where the nucleus or a nuclear particle (like a proton) is slammed into by another nucleus or nuclear particle.
Radioactive Decay
A radioactive element, like everything else in life, decays or ages. When uranium decays over billions of years, it goes through a process of degrading into lower and lower energy forms until it settles into one that is stable. The ages of the most ancient rocks can be found by measuring the decay of specific isotopes that are not stable, but break down to other element forms.
The sample is dated using testing techniques known as radiometric dating. This considers all the various melting and environmental influences that have affected the sample.
When a radioactive element decays, different nuclear particles are given off. These speeding radiation particles can be separated by an electric (magnetic field) and detected in the laboratory:
Beta (ß) particles (-) = negatively charged particles
Alpha (α) particles = positively charged particles
Gamma (τ) particles = electromagnetic radiation with no overall charge (similar to x-rays)
The age of geological samples is found by measuring radioisotope decay. Decay of radioactive isotopes is affected by the stability of an element at a certain energy level (where its electrons are stable and bonded). Bismuth (Bi) is the heaviest element in the Periodic Table with a minimum of one stable isotope. All other heavier elements are radioactive. Geologists study isotopes of different chemical elements to find the rate of
decay over time. Depending on the rate of decay of a sample, an estimate of its age can be done. It is also possible to find and compare the radioactive decay of a sample to the rock in which it was found. This gives geologists another clue as to the life cycle and history of the specimen, as well as a hint as to how it was deposited.
Isotopes are chemically identical atoms of the same element that have different numbers of neutrons and mass numbers.
A rock sample’s age can be found by comparing three pieces of information:
1. The amount of the original element or parent isotope,
2. The amount of the new element or daughter isotope formed, and
3. The rate of decay of a specific radioactive isotope present in the rock.
A mass spectrometer is an instrument that measures the ratios of isotopes in samples. Uranium has all radioactive isotopes while potassium has only one. By noting the rate of decay that uranium-238 displays while losing electrons and alpha particles and trying to become the more stable lead-206, scientists then test other radioactive samples with a similar rate of decay to the stable lead form. Meteorites have been found to be as old as the Earth and older by using this method.
Half-Life
All radioactive isotopes have a specific set, half-life. These time periods are not dependent on pressure, temperature, or bonding properties.
The half-life of a radioactive isotope is the time needed for ½ of a specific element sample to decay.
For example, the half-life of 238 U92 is 4.5×10 (9) years. It is amazing to think that the uranium found today will be around for another four billion years. In 1953, Clair Patterson and Friedrich Houtermans separately determined the age of the Earth and the solar system as being around 4.6 billion years old by finding and comparing the radioactive decay rates of isotopes of lead in the earliest rocks known to exist.
Through the radiometric dating of ancient rocks, the Earth was calculated to be over four billion years old. Zircon crystals found in Western Australia have radiometric ages of over 4.3 billion years. By figuring out how long the oldest lead ores took to change from their earliest formation (nebular gas) to later compression and inclusion in the Earth’s crust, scientists were then able to estimate the age of lead-containing meteorites. These meteorites have been dated at nearly 4.6 million years using the radioactive decay of uranium-235 to lead-207.
In this same way, Patterson and Houtermans were able to estimate the age of our solar system to be about 4.54 billion years. The age of our Milky Way galaxy was judged to be between 11 and 13 billion years. In a study published in Science in January 2003, a team of researchers estimated that the Universe was between 11.2 and 20 billion years old. Most estimates of the Universe’s age, in recent years, have ranged between 10 and 15 billion years. Data supplied by the Hubble Space Telescope in 2003 led to a refined estimate of 13-14 billion years. The new calculations, by Lawrence Krauss of Case Western Reserve University and Brian Chaboyer at Dartmouth College, involved new information about old star clusters in our galaxy and a better understanding of how stars evolve. It was based on when stars are thought to end the main sequence of their lives, a point at which they’ve used up the hydrogen that fuels thermonuclear fusion and therefore begin to dim.
