Analytical Methods:Radiogenic Isotopes

SASSA Home Page &#8658; Analytical Methods Home Page &#8658; Specialist Techniques &#8658; Radiogenic Isotope Analysis

Questions
Gamma spectroscopy measures the rate and energy of gamma rays given off by radioactive substances in the soil. These gamma rays are produced by potassium-40 (K-40), uranium (U)and the other daughter isotopes formed through its decay, thorium (Th) and its daughters, and caesium-137 (Cs-137) deposited primarily as a result of nuclear weapons testing since the 1950's. Relative concentrations of U, Th and K-40 in soil vary according to the geology of the parent materials, and can be altered by inputs, which may either contain radiogenic materials or dilute the soil with non-radiogenic materials. This provides a means of mapping human influence on soils and this is now being explored for its potential as a geophysical technique (e.g. Ruffell et al., 2006). The concentration of radiocaesium activity in the soil has been used as a means of mapping patterns of modern (post 1950) soil erosion. This has applications in the management of archaeological sites in arable areas (Wilkinson et al., 2006).

Case studies where radiogenic isotope analysis has been used in archaeology include:
 * Climate change and the collapse of the Akkadian empire.
 * Analysis of Metavolcanic Rocks from the Vicinity of Fort Bragg, North Carolina: Artifact Source Quarry Discrimination.

Samples
Laboratory analysis of soils produces a much higher level of resolution than in-situ or remote sensing studies. Samples should always be taken, therefore, to ground-truth, assess and correct the field data. Samples should be of known volume, and corers are ideal. Once in the laboratory cores can be cut into sections corresponding to fixed depth intervals or context / soil horizon depths.

In general relatively large samples are best for gamma spectrometry as this helps to reduce the time each sample has to be left in the machine. However, sample size should not be so large that material from more than one context becomes mixed.

Soils should be dried and ground, and are then packed into special pots so that the density and volume of the sample can be calculated. The pots are sealed from the air and left for a period of time to re-equilibriate before being analysed.

Analysis
Gamma spectrometers consist of detectors composed of High-Purity Germanium (HPGe) or Sodium Iodide (NaI) crystals. The latter has a much lower level of resolution and is generally used in remote sensing and in-situ applications. Gamma rays entering the detector interact with the crystal producing either electrons or photons of light. These are converted into voltage pulses which are analysed and converted into a spectra.

The count time depends on the level of radioactivity in the sample, the sample size and the nature of the detector. However, count times can be in the order of days.

Data and interpretation
Data analysis and interpretation is a specialist task. If field measurements are being used, soil depth, moisture content and clay content can also affect the results so their effects should also be considered.

Data tends to be analysed by comparing ratios of the different isotope groups using bi-plots and ternary diagrams to highlight different groupings of samples.

Related techniques

 * Stable isotope analysis

 &larr; Back to Specialist Techniques