Gamma Detection Method for Pb210 in Antarctic Fiord Sediments: Measuring Accumulation Rates in the 20th Century

J. W. Ring and Ryan Cleary Physics Department Hamilton College, Clinton, New York

A method for analyzing core samples by means of a NaI(Tl) detector and a multichannel analyzer has been developed. The 46 keV gamma from the decay of Pb²¹⁰ is the signal that is counted with this equipment. The 22.3-year half-life of Pb²¹⁰ provides an interesting window on the recent history of sedimentation in core samples. This is so because the last century is the period in which global warming is expected to show ever increasing effects as the carbon dioxide content of the atmosphere has been increasing in an accelerating fashion over this time. This method has some advantages over alpha or beta detection for these purposes as for gammas there is less absorption in the sample or its container, no vacuum chamber is necessary and no special sample preparation is required. The activity of these samples is generally low so that it is important to reduce the background as far as is practically possible. Shields of Pb, Al and Cd have been tried with the best results coming with 4" of Pb lined with sheets of Cd on the inner surfaces of the detector cavity. In this case the background counting rate has been reduced to about two thirds of that of the most active samples, i.e. those near the surface. Twenty-five samples of a core taken in Andvord Bay in the Antarctic Peninsula have been analyzed in this fashion. The depth varied from 0 to 50 cm with each sample being 2 cm thick. A support level of Pb²¹⁰ activity was found at depths of 30 cm and greater. With this level subtracted the excess Pb²¹⁰ activity revealed a sedimentation rate of 5.0 +/- 1.0 mm per year.