Home - Education Resources - NDT Course Material - Radiography
 

-
Radiography

Introduction
History
Present State
Future Direction

Physics of Radiography
Nature of Penetrating Radiation
X-rays
Gamma Rays
Activity
Decay Rate
  -Carbon 14 Dating
Ionization
Inverse Square Law
Interaction of RT/Matter
Attenuation Coefficient
Half-Value Layer
Sources of Attenuation
  -Compton Scattering
Geometric Unsharpness
Filters in Radiography
Scatter/Radiation Control
Radiation Safety

Equipment & Materials
X-ray Generators
Radio Isotope Sources
Radiographic Film
Exposure Vaults

Techniques & Calibrations
Imaging Consideration
Contrast
Definition
Radiographic Density
Characteristic Curves
Exposure Calculations
Controlling Quality

Film Processing
Viewing Radiographs
Radiograph Interp-Welds
Radiograph Interp - Castings

Advanced Techniques
Real-time Radiography
Computed Tomography
XRSIM

References

Quizzes
-

Carbon-14 Dating

Radio-carbon dating is a method of obtaining age estimates on organic materials. The word "estimates" is used because there is a significant amount of uncertainty in these measurements. Each sample type has specific problems associated with its use for dating purposes, including contamination and special environmental effects. More information on the sources of error in carbon dating are presented at the bottom of this page.

The method was developed immediately following World War II by Willard F. Libby and coworkers and has provided age determinations in archeology, geology, geophysics, and other branches of science. Radiocarbon dating estimates can be obtained on wood, charcoal, marine and freshwater shells, bone and antler, and peat and organic-bearing sediments. They can also be obtained from carbonate deposits such as tufa, calcite, marl, dissolved carbon dioxide, and carbonates in ocean, lake and groundwater sources.

Carbon dioxide is distributed on a worldwide basis into various atmospheric, biospheric, and hydrospheric reservoirs on a time scale much shorter than its half-life. Measurements have shown that in recent history, radiocarbon levels have remained relatively constant in most of the biosphere due to the metabolic processes in living organisms and the relatively rapid turnover of carbonates in surface ocean waters. However, changes in the atmosphere over the ages are a source of uncertainty in the measurements.

Carbon (C) has three naturally occurring isotopes. Both C-12 and C-13 are stable, but C-14 decays by very weak beta decay to nitrogen-14 with a half-life of approximately 5,730 years. Naturally occurring radiocarbon is produced as a secondary effect of cosmic-ray bombardment of the upper atmosphere. Plants transpire to take in atmospheric carbon, which is the beginning of absorption of carbon into the food chain. Animals eat the plants and this action introduces carbon into their bodies.

After the organism dies, carbon-14 continues to decay without being replaced. To measure the amount of radiocarbon left in a artifact, scientists burn a small piece to convert it into carbon dioxide gas. Radiation counters are used to detect the electrons given off by decaying C-14 as it turns into nitrogen. The amount of C-14 is compared to the amount of C-12, the stable form of carbon, to determine how much radiocarbon has decayed, thereby dating the artifact.

Exponential Decay Formula: A = A0* 2^(-t/k)

Where "A" is the present amount of the radioactive isotope, "A0" is the original amount of the radioactive isotope that is measured in the same units as "A." The value "t" is the time it takes to reduce the original amount of the isotope to the present amount, and "k" is the half-life of the isotope, measured in the same units as "t."

The applet allows you to choose the C-14 to C-12 ratio, then calculates the age of our skull from the formula above.

Uncertainty in Carbon Dating
As mentioned above, there is significant uncertainty in carbon dating.  There are several variables that contribute to this uncertainty. First, as mentioned previously, the proportions of C-14 in the atmosphere in historic times is unknown. The C-14:C-12 atmospheric ratio is known to vary over time and it is not at all certain that the curve is “well behaved.”

Complicating things further, various plants have differing abilities to exclude significant proportions of the C-14 in their intake. This varies with environmental conditions as well. The varying rates at which C-14 is excluded in plants also means that the apparent age of a living animal may be affected by an animal's diet. An animal that ingested plants with relatively low C-14 proportions would be dated older than their true age.

Attempts are often made to index C-14 proportions using samples of know age. While this may be useful to eliminate the uncertainty of atmospheric proportions of C-14, it does not compensate for local conditions such as which plant species are in the diet. The uncertainty in the measurement leads some to conclude that the method is far less predictive of age than is commonly supposed, especially for older samples.

Last updated on October 28, 2010