ACID LEACHING TO INCREASE DETECTION LIMITS IN GAMMA COUNTING UNSUPPORTED PB-210

Unsupported Pb-210 is a commonly used as both a tracer and geochronometer for fluvial, lacustrine, and pedogenic studies. Complications arise when Pb-210 is measured in detrital, fluvial sediments. Because Pb-210 resides both in the substrate (supported) and in grain coatings (excess or unsupported), measuring Pb-210 requires subtracting a proxy for supported Pb-210 from total Pb-210 measurements to determine unsupported Pb-210 activity. This necessarily increases the relative error in unsupported Pb-210 measurements. Typical methods use Ra-226, Pb-214 and/or Bi-214 as proxies for the activity of supported Pb-210. Samples are sealed within containers and are allowed to reach secular equilibrium before gamma counting, which unlike a natural environment, does not allow for gaseous escape and therefore leads to the overestimation of supported Pb-210.

We propose a new technique to minimize errors in unsupported Pb-210 measurements by physically separating grain coatings from substrate material in samples and counting Pb-210 activity only in the grain coatings; this approach removes the need to subtract estimates of supported Pb-210 activity. To separate grain coatings from substrate material, we leach samples for 24 hours in 6M HCl in a warm sonic bath. We then decant the leachate and dry it down. This dried leachate is counted for at least 24 hours in a low-energy, ultra-low background germanium detector.

Preliminary results on samples from western China suggest that the sum of the total Pb-210 activity in the leachate and residue for samples is within error of the total Pb-210 activity measured in the bulk sample. Furthermore, the errors in measurement of the Pb-210 activity of the leachate are significantly reduced from the errors in unsupported Pb-210 measured in bulk samples. We find that the activity of Pb-210 in the leachate is higher than the unsupported Pb-210 activity estimated from the entire sample, suggesting that traditional techniques of measuring Pb-210 overestimate supported Pb-210. We propose that this technique as a more accurate and precise way to measure activity of unsupported Pb-210.