Carbon Dating

Carbon dating is a variety of radioactive dating which is applicable only to matter which was once living and presumed to be in equilibrium with the atmosphere, taking in carbon dioxide from the air for photosynthesis. 碳测定是一种放射性鉴年法，仅适用于曾是生物体的物质，并假设其与大气处于平衡状态，能够从空气中吸收二氧化碳进行光合作用。 Cosmic ray protons blast nuclei in the upper atmosphere, producing neutrons which in turn bombard nitrogen, the major constituent of the atmosphere . This neutron bombardment produces the radioactive isotope carbon-14. The radioactive carbon-14 combines with oxygen to form carbon dioxide and is incorporated into the cycle of living things. 宇宙射线中的质子撞击大气层中的核，产生中子，这些中子再轰击大气中的主要成分氮，产生放射性同位素碳-14。放射性碳-14与氧结合形成二氧化碳，并被生物圈循环所吸收。 The carbon-14 forms at a rate which appears to be constant, so that by measuring the radioactive emissions from once-living matter and comparing its activity with the equilibrium level of living things, a measurement of the time elapsed can be made. 碳-14形成的速度似乎是一个常数，因此通过测量已故物质的放射性发射并将其活动性与活生物体的平衡电平进行比较，可以测定经过的时间。

Carbon Dating

Presuming the rate of production of carbon-14 to be constant, the activity of a sample can be directly compared to the equilibrium activity of living matter and the age calculated. Various tests of reliability have confirmed the value of carbon data, and many examples provide an interesting range of application. 假定碳-14的产生速率保持恒定，样品的活度可以直接与生物体的平衡活度进行比较，从而计算出其年龄。各种可靠性测试已证实碳数据的准确性，许多实例展示了其广泛的应用范围。 Carbon-14 decays with a halflife of about 5730 years by the emission of an electron of energy 0.016 MeV. This changes the atomic number of the nucleus to 7, producing a nucleus of nitrogen-14. At equilibrium with the atmosphere, a gram of carbon shows an activity of about 15 decays per minute. 碳-14通过发射能量为0.016 MeV的电子衰变，其半衰期约为5730年。这使原子核的原子序数变为7，生成氮-14的原子核。在与大气平衡时，1克碳的活度约为每分钟15次衰变。 The low activity of the carbon-14 limits age determinations to the order of 50,000 years by counting techniques. That can be extended to perhaps 100,000 years by accelerator techniques for counting the carbon-14 concentration. 碳-14的低活性限制了用计数技术测定的年代仅限于约50,000年。通过加速器技术测定碳-14浓度，这一范围可以扩展到可能的100,000年。

Carbon-14 Equilibrium Activity

Since living organisms continually exchange carbon with the atmosphere in the form of carbon dioxide, the ratio of C-14 to C-12 approaches that of the atmosphere.

From the known half-life of carbon-14 and the number of carbon atoms in a gram of carbon, you can calculate the number of radioactive decays to be about 15 decays per minute per gram of carbon in a living organism.

Radioactive carbon is being created by this process at the rate of about two atoms per second for every square centimeter of the earth's surface." Levin

The rate of production of carbon-14 in the atmosphere seems to be fairly constant. Carbon dating of ancient bristlecone pine trees of ages around 6000 years have provided general corroboration of carbon dating and have provided some corrections to the data.

Reliability of Carbon Dating

From the dating of ancient bristlecone pine trees from the western U.S., a correction curve for the carbon dating over the range back to 5000 BC has been developed. Trees dated at 4000 BC show the maximum deviation of between 600 and 700 years too young by carbon dating.

Prior to carbon dating methods, the age of sediments deposited by the last ice age was surmised to be about 25000 years. "Radiocarbon dates of a layer of peat beneath the glacial sediments provided an age of only 11,400 years."

These examples are from The Earth Through Time, 2nd Ed. by Harold L. Levin

Krane points out that future carbon dating will not be so reliable because of changes in the carbon isotopic mix. Fossil fuels have no carbon-14 content, and the burning of those fuels over the past 100 years has diluted the carbon-14 content. On the other hand, atmospheric testing of nuclear weapons in the 1950s and 1960s increased the carbon-14 content of the atmosphere. Krane suggests that this might have doubled the concentration compared to the carbon-14 from cosmic ray production.

Accelerator Techniques for Carbon Dating

Accelerator techniques for carbon dating have extended its range back to about 100,000 years, compared to less than half that for direct counting techniques. One can count atoms of different masses with a mass spectrometer, but that is problematic for carbon dating because of the low concentration of carbon-14 and the existence of nitrogen-14 and CH₂ which have essentially the same mass. Cyclotrons and tandem accelerators have both been used to fashion sensitive new mass spectrometer analyses. The tandem accelerator has been effective in removing the nitrogen-14 and CH₂, and can be followed by a conventional mass spectrometer to separate the C-12 and C-13. A sensitivity of 10^-15 in the ¹⁴C/¹²C ratio has been achieved. These techniques can be applied with a sample as small as a milligram.