About 75 years ago, Williard F. Libby, a Professor of Chemistry at the University of Chicago, predicted that a radioactive isotope of carbon, known as carbon, would be found to occur in nature. Since carbon is fundamental to life, occurring along with hydrogen in all organic compounds, the detection of such an isotope might form the basis for a method to establish the age of ancient materials. Working with several collaboraters, Libby the natural occurrence of radiocarbon by detecting its radioactivity in methane from the Baltimore sewer.
Dating c14 contrast, methane made from petroleum products had no measurable radioactivity. Carbon is produced in the upper atmosphere when cosmic rays bombard nitrogen atoms. The ensuing atomic interactions create a steady supply of c14 that rapidly diffuses throughout the atmosphere. Plants take up c14 along with other carbon isotopes photosynthesis in the proportions that occur dating c14 the atmosphere; animals acquire c14 by eating the plants or other animals.
During the lifetime of an organism, the amount of c14 in the tissues remains at an equilibrium since the loss through radioactive decay is balanced by the gain through uptake via photosynthesis or consumption of organically fixed carbon. Dating c14, when the organism dies, the amount of c14 declines such that the longer the time since death the lower the levels of c14 in organic tissue.
This is the dating c14 that permits levels of c14 in organic archaeological, geological, and paleontological samples to be converted into an estimate of time. The measurement of the rate of radioactive decay is known as its dating c14, the time it takes for half of a sample to decay. This means that half of the c14 has decayed by the time an organism has been dead for years, and half of dating c14 remainder has decayed by 11, years after death, etc.
The diminishing levels via decay means that the effective limit for using c14 to estimate time is about 50, dating c14. After this time, there is little if any c14 left. However, to avoid confusion all radiocarbon laboratories continue dating c14 use the half-life calculated by Libby, sometimes rounding it to dating c14. Any organic material that is available in sufficient quantity can be prepared for radiocarbon dating.
Modern AMS accelerator mass spectroscopy methods require tiny amounts, about 50 mg. AMS technology has allowed us to date very small samples such as seeds that were previously undatable. Since there are practical limits to the age range of the method, most samples must be younger than 50, years and older than years. Most samples require chemical pre-treatment to ensure their purity or to recover particular components of the material.
The objective of pre-treatment is to ensure that the carbon being analyzed is native to the sample submitted for dating. Pre-treatment seeks to remove from the sample any contaminating carbon that could yield an inaccurate date. Acids may be used dating c14 eliminate contaminating carbonates. Bases may be used to remove contaminating humic acids.
Some types of samples require more extensive pre-treatment than others, and these methods have evolved over the first 50 years of radiocarbon dating. For example, it was once standard practice to simply burn whole bones, dating c14 the results were eventually seen to be unreliable. Chemical methods for separating the organic collagen from the inorganic apatite components of bone created the opportunity to date both components and compare the results.
The collagen fraction usually yields more reliable dates than the apatite fraction see Dates on bones. In addition to various pre-treatments, the sample must be burned and converted to a form suitable for the counter. The sample must be destroyed in order to measure its c14 content. The first measurements of radiocarbon were made in screen-walled Geiger counters with the sample prepared for measurement in a solid form. These so-called "solid-carbon" dates were soon found to yield ages somewhat younger than expected, and there were many other technical problems associated with sample preparation and the operation of the counters.
Gas proportional counters soon replaced the solid-carbon method in dating c14 laboratories, with the samples being converted to gases such as carbon dioxide, carbon disulfide, methane, or acetylene. Many laboratories now use liquid scintillation counters with the samples being converted to benzene. All of these counter types measure the C content by monitering the rate of decay per unit time.
A more recent dating c14 is the direct counting of c14 atoms by accelerator mass spectrometers AMS. The sample is converted to graphite and mounted in an ion source from which dating c14 is sputtered and dating c14 through a magnetic field. Targets tuned to different atomic weights count dating c14 number of c12, c13, and c 14 atoms in a sample. Many samples reported as "modern" have levels of radioactivity that are indistinguishable from modern standards such as oxalic acid.
Due to contamination from bomb testing, some samples are even more radioactive than the modern standards. Other very young samples may be given maximum limits, such as 40, years. The very old samples have such low radioactivity that they cannot be distinguished reliably from dating c14 background radiation.
Very few laboratories are able to measure ages of more than 40, years. Several aspects of radiocarbon measurement have built-in uncertainties. Every laboratory must factor out background radiation that varies geographically and through time. variation in background radiation is monitered by routinely measuring standards such as anthracite coaloxalic acid, and certain materials of well-known age. The standards offer dating c14 basis for interpreting the radioactivity of the unknown sample, but there is always a degree of uncertainty in any measurement.
Since decay-counting records random events per unit time, uncertainty is an aspect of the method. Most laboratories consider only the counting statistics, i. However, some laboratories factor in other variables such as the uncertainty in the measurement of the half-life.