Sediment plutonium dating
This logic can be extended to calculate the age of sediments at other depths in the sediment column and/or the rate of sediment accumulation.
In a 'perfect' core, if log [excess Po-210 activity] is plotted as a function of accumulated dry weight of sediment, the line through the data should be a straight line.
An iterative best fit computer model was designed to process the data in cases where the Po-210 background activity is not known. Since the excess Po-210 activities depend directly on the value of the background Po-210 activity, it is evident that only one level of background will yield a perfectly linear fit of the data.
It is based on the assumption that a 'perfect' core will exhibit a linear decrease in log [excess Po-210 activity] vs. The computer model performs a number of linear regressions, each time using a slightly different value of background Po-210.
After several days residence time in the atmosphere, the Rn-222 naturally decays to polonium-218, a metallic radionuclide which, over a period of hours/days, falls to the earth with dust and rain.
A number of subsequent radioactive decays occur over a period of minutes and Pb-210 (half-life = 22.3 ) is produced.
The overall result is that radium-226 is found at low and essentially unchanging levels in soils everywhere.It is possible to indirectly estimate the background Po-210 by measuring the Ra-226 (via Rn-222) in the sediments but this is often omitted because of the additional analytical costs.In lieu of these analyses, it is necessary to make an assumption that the background level is less than the lowest activity measured in the core but greater than zero.When applying the Pb-210 technique, we assume that lake and ocean sediments are receiving a constant input of Pb-210 from the atmosphere.
Pb-210 that was incorporated into the sediments 22.3 years ago will be only one half as radioactive as when initially deposited.
In a typical application, the average accumulation rate over a period of 100 - 200 years is obtained.