Research Professors - Dr. Paul DeYoung, Graham Peaslee, Peter Jolivette
Supported by the NSF-REU
The majority of my research this summer involved 210Pb dating of sediment core samples from White Lake, MI. 210Pb dating is a relatively new and very useful method in radiolimnology. It utilizes the accumulation of 210Pb from the decay of 222Rn in the air. As 222Rn decays, its daughter products settle all over the Earth’s surface, quickly decaying to 210Pb. Once a layer of sediment is covered by other sediment, it is no longer exposed to the settling of these daughter products and ceases to accumulate 210Pb from the air. A measurement of the amount of 210Pb remaining in the sample can give an age, in much the same way as 14C dating is used. To measure the amount of 210Pb, one must count the number of characteristic gamma rays emitted from the sample. For 210Pb, the characteristic gamma rays are 46 keV.
While the sediment no longer accumulates 210Pb from the air once it has been covered, it continues to accumulate 210Pb from the decay of 238U that is everywhere in the rock itself. Thus, a distinction must be drawn between 210Pb from the rock, or supported 210Pb, and that from the air, unsupported 210Pb. We measure 214Bi as an indicator of supported 210Pb, because it comes right before 210Pb in the decay chain and has a very short half-life (about 20 minutes). Any 214Bi in the sample therefore comes from the rock itself, and a measurement can give the amount of supported 210Pb. By subtracting supported 210Pb from total 210Pb, measured using the 45 keV peak, we can get unsupported 210Pb and the date. The characteristic gamma rays for 214Bi are at 609 keV.
We used an intrinsic Ge detector to make the measurements because of its sensitivity to the wide range of gamma energies required and because it has very sharp resolution, so different energy gamma rays can be distinguished from one another.
There were several concerns we had with the experiment design. First, how high would the radioactive background be and would we be able to see the peaks over it. The background in the room turned out to be quite high, so it was necessary to reduce it if possible. I built a lead house around the detector to act as a shield from the background and it worked quite well. We managed to cut the low-energy background by two orders of magnitude. Future plans call for a shield constructed of several different materials covering a range of atomic numbers.
Another concern we had was what would the impact of self-attenuation within the sample itself have on the efficiency of our setup. Our samples were 15 mm thick, which makes it likely that a gamma ray emitted from the back of the sample will be absorbed within the sample, never making it to the detector. We ran a series of tests to determine if there was in fact a significant self-attenuation effect, and are in the process of measuring the attenuation coefficient, m, in the equation:
We were also concerned about the effect of Rn gas escaping from our plastic sample cups, which would alter the results for the supported 210Pb. We are still in the process of determining the effect of this.
The work I did this summer gives us an excellent understanding of our experimental setup, how to account for a number of complications, and readies us to do radiodating of sediment samples.
For a much smaller percentage of my time this summer, I worked on modeling data taken at Berkeley in Fall, 1999. The experiment measured fusion of 11C and 12C with 197Au (making 208At and 209At) and then observing the fission. We are looking to find out if there is a proton "shell effect" in 11C . Using the PACE statistical model, we are trying to determine if there is a fusion-suppressing effect, which would give evidence for a proton shell. I have been working to fit the model to the data that was accumulated at Berkeley last fall.Click to see a slide show of Kyle Helland's work.