A Talk by Experts on Luminescence Dating
Today I felt delighted to have attended a talk on the topic: Luminescence dating for dating terracotta.
Speakers of the Talk
The talk was delivered by Dr. Morthkai, Scientist-D, Birbal Sahni Institute of Palaeoscience, Lucknow, India.
I hereby gist the valuable information delivered by Dr. P Morthkai, the speaker. Dr. P Morthkai is a scientist at the distinguished Birbal Sahni Institute of Palaeoscience. The talk provided insights into the scientific underpinning Luminescence Dating - a geochronological method, like carbon dating that can be used for studying the age of paleontological and archaeological materials.
How does Carbon-14 dating differ from Luminescence Dating
Two important graphs we need to understand that distinguish the Luminescence dating from the carbon-14 dating.
Carbon Dating
In carbon dating, as you see, we measure or quantify the carbon-14, the radioactive isotope of carbon as time passes. It takes 5730 years for a given amount of carbon to become half of its initial quantity. Carbon dating is based on the carbon-14 that is absorbed into living beings while they are alive. To speak of the formation of carbon-14 and its entry into the living body, first of all, the carbon-14 is produced in the lower stratosphere and upper troposphere by the activity of the cosmic rays. Cosmic rays generate neutrons, and those when travel through the atmosphere strike with the nitrogen-14 and turn them into carbon-14. These carbon-14, when they combine with oxygen, form carbon dioxide. Plants use these carbon dioxide for photosynthesis and, as consumers, when animals including humans ingest plant foods they take in carbon-14. These carbon-14 are exchanged with the atmosphere and therefore maintaining a constant ratio of C-14 to C-12. When an organism dies, there is no more addition of C-14 into the system and the existing C-14 starts to decay to N-14. Studies show that the ratio of C-14 to C-12 is 1.25 parts of C-14 to 10^12 parts of C-12, and this ratio remains almost constant along the lifespan of animals. If the body mass of an animal is known, it is possible to estimate the initial number of carbon atoms and Carbon-14 atoms. Since the half-life of Carbon-14 is 5730, after 10 half-lives, almost 99% of the original Carbon-14 would have been decayed, and therefore fossils that are at most 50,000 years old can be dated accurately.
Luminescence Dating
Whereas C-14 dating involves the calculation of age of an archaeological object by measuring the number of C-14 atoms after decay, luminescence dating involves determining the age of archaeological and paleontological objects by measuring optically stimulated luminescence (OSL). The OSL is due to the accumulation of the trapped electrons between the valence and the conduction bands. The brightness or the intensity of the OSL is based on the number of electrons that are trapped between valence and the conduction band. When the material is optically stimulated, the electrons are ejected from the hole-traps (between the valence and the conduction band) and fall to the valence band, thus emitting light. The intensity of luminescence depends on the number of electrons that were thrown out from the valence band upon bombardment by high eV (electron volt) radiation energy (alpha, beta particles, gamma rays, etc.) . In the image, there is a reference to a point in time at which the signal is reset to 0. At this point, all the trapped electrons have been evacuated from the hole-traps and they are now in the valence band. This happens when the material is subjected to intense heat - for example in the case of preparation of terracotta materials - or exposure to sunlight.
Here, we need to understand that sediments and archaeological artifacts contain dosimeters - quarts and feldspars - and they can trap electrons when irradiated. These electron traps are nothing but holes or impurities that can accept electrons. If there are radiation energy in the surrounding environment of a dosimeter, the electrons get expelled from the electron-dense valence band to the electron-free conduction band overcoming a binding energy. If the binding energy needed for electrons in the valence band of Silicon atom (of SiO2) is 6 eV, the radiation energy of alpha, beta particles, and gamma rays is in the order of millions. This would result in the mass ejection of electrons from the valence band of the Silicon and other atoms to the respective conductions bands. The electrons stay only temporarily in the conduction band and fall back only to get tapped in the holes between the valence and the conduction band. When the dosimeters are exposed to sunlight or any thermal source, the electrons that are trapped come back to the valence band, emitting light.
Calculation of Age of a paleontological/ archaeological object
If we know the total electrons that were trapped and the rate at which the electrons trapped, we can calculate the time it took for the electrons to become trapped; and that is the time we refer to as the age of the material concerned.
Time = Dose (or quantity of the trapped electrons in Gy)/ Dose Rate (Gy/Ka)
We will get the time in kilo year or 1000 years. Here, the rate at which electrons trapped is calculated indirectly by the calculating the rate at which radiation energy is emitted/ dose rate (expressed per 1000 years)
Singhvi's beaker model
Singhvi’s beaker model can be used to understand the applicability of Luminescence dating in calculating the age of paleontological and archaeological objects.
Here, how do we calculate the time taken to fill the water? Obviously by dividing the volume by drop rate. Here, the value is affected by the size of the beaker, and variability in the drop rate. So, assumptions are made that beakers are of comparable size and drop rate is uniform.
In the same sense, when we measure time/ age of an archaeological object, the dosimeter is like a beaker, the electrons that are trapped in the hole-trap is the water that is collected in a beaker, and the dose rate is like drop rate. Here, we make the assumption that the dosimeter is irradiated at a constant rate.
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