Explainer: what is radiocarbon dating and how does it work?
Carbon dating, or radiocarbon dating, like any other laboratory testing technique, can be extremely reliable, so long as all of the variables involved are controlled. Radiocarbon dating is a method for determining the age of an object containing organic material by using the properties of radiocarbon, a radioactive isotope of carbon. The method was developed in the late s by Willard Libby, who received the Nobel Prize in Chemistry for his work in . works in Baltimore, and after isotopically enriching their samples they were. Radiocarbon dating works by comparing the three different isotopes of carbon. Isotopes of a particular element have the same number of.
Accelerator mass spectrometry AMS is a modern radiocarbon dating method that is considered to be the more efficient way to measure radiocarbon content of a sample. In this method, the carbon 14 content is directly measured relative to the carbon 12 and carbon 13 present. The method does not count beta particles but the number of carbon atoms present in the sample and the proportion of the isotopes.
Carbon Datable Materials Not all materials can be radiocarbon dated.
How Does Carbon Dating Work
Most, if not all, organic compounds can be dated. Samples that have been radiocarbon dated since the inception of the method include charcoalwoodtwigs, seedsbonesshellsleather, peatlake mud, soilhair, potterypollenwall paintings, corals, blood residues, fabricspaper or parchment, resins, and wateramong others.
Physical and chemical pretreatments are done on these materials to remove possible contaminants before they are analyzed for their radiocarbon content.
Carbon Dating Standards The radiocarbon age of a certain sample of unknown age can be determined by measuring its carbon 14 content and comparing the result to the carbon 14 activity in modern and background samples.
The principal modern standard used by radiocarbon dating labs was the Oxalic Acid I obtained from the National Institute of Standards and Technology in Maryland. This oxalic acid came from sugar beets in When the stocks of Oxalic Acid I were almost fully consumed, another standard was made from a crop of French beet molasses.
Over the years, other secondary radiocarbon standards have been made. Radiocarbon activity of materials in the background is also determined to remove its contribution from results obtained during a sample analysis. Background samples analyzed are usually geological in origin of infinite age such as coal, lignite, and limestone. The CRA conventions include a usage of the Libby half-life, b usage of Oxalic Acid I or II or any appropriate secondary standard as the modern radiocarbon standard, c correction for sample isotopic fractionation to a normalized or base value of These values have been derived through statistical means.
The first part involves accelerating the ions to extraordinarily high kinetic energies, and the subsequent step involves mass analysis. There are two accelerator systems commonly used for radiocarbon dating through accelerator mass spectrometry.
One is the cyclotron, and the other is a tandem electrostatic accelerator. AMS Analysis via Tandem Accelerator After pretreatment, samples for radiocarbon dating are prepared for use in an accelerator mass spectrometer by converting them into a solid graphite form.
This is done by conversion to carbon dioxide with subsequent graphitization in the presence of a metal catalyst. Burning the samples to convert them into graphite, however, also introduces other elements into the sample like nitrogen When the samples have finally been converted into few milligrams of graphite, they are pressed on to a metal disc.
Reference materials are also pressed on metal discs. These metal discs are then mounted on a target wheel so they can be analyzed in sequence. Ions from a cesium gun are then fired at the target wheel, producing negatively ionized carbon atoms.
These negatively ionized carbon atoms pass through focusing devices and an injection magnet before reaching the tandem accelerator where they are accelerated to the positive terminal by a voltage difference of two million volts.
At this stage, other negatively charged atoms are unstable and cannot reach the detector. The negatively charged carbon atoms, however, move on to the stripper a gas or a metal foil where they lose the electrons and emerge as the triple, positively charged carbon atoms.
Accelerator Mass Spectrometry, C14 Dating, What is AMS?
At this stage, molecules that may be present are eliminated because they cannot exist in this triple charged state. The carbon atoms with triple positive charge further accelerate away from the positive terminal and pass through another set of focusing devices where mass analysis occurs.
In mass analysis, a magnetic field is applied to these moving charged particles, which causes the particles to deflect from the path they are traveling. For instance, the amount varies according to how many cosmic rays reach Earth. Luckily, we can measure these fluctuations in samples that are dated by other methods.
Tree rings can be counted and their radiocarbon content measured. A huge amount of work is currently underway to extend and improve the calibration curve. In we could only calibrate radiocarbon dates until 26, years.
Now the curve extends tentatively to 50, years. Dating advances Radiocarbon dates are presented in two ways because of this complication. The uncalibrated date is given with the unit BP radiocarbon years before The calibrated date is also presented, either in BC or AD or with the unit calBP calibrated before present - before The second difficulty arises from the extremely low abundance of 14C. Many labs now use an Accelerator Mass Spectrometer AMSa machine that can detect and measure the presence of different isotopes, to count the individual 14C atoms in a sample.
Australia has two machines dedicated to radiocarbon analysis, and they are out of reach for much of the developing world. In addition, samples need to be thoroughly cleaned to remove carbon contamination from glues and soil before dating. This is particularly important for very old samples. Because of this, radiocarbon chemists are continually developing new methods to more effectively clean materials. These new techniques can have a dramatic effect on chronologies.