Hope College Department of Physics and Engineering
Research Experiences for Undergraduates
Project Title: Radiodating of Lake Sediments
Student Name: Utsab Khadka
Student's home institution: Hope College
Research Advisor(s): Dr. P.A. DeYoung, Dr. G.F. Peaslee
Source of Support: NSF-RUI
The aim of this project is to date lake sediments by measuring their radioactivity. The process is similar to carbon-14 dating, but it allows us to date the past 100 years rather than 1000 to 5000 years. At the bottom of lakes, mud gets accumulated continually and mud deposited many years ago is found deeper. Cores dug out from different lake bottoms are divided into 40 to 80 layers and freeze dried. The samples are packaged and their gamma radiations are measured using an intrinsic germanium counter. The radioactive sample is shielded with a thick layer of lead and copper. The amounts of radioactive nuclei present in the samples are known by integrating the peaks at various energies. Amongst the various nuclei detected, 210 Pb and 137 Cs are chosen as the nuclei to be used for the dating process. The occurrence of 210 Pb can be traced back to the decay of 222 Rn, which is abundant in the atmosphere. The amount of 210 Pb decreases exponentially as we go deeper down a core. 137 Cs is present in the atmosphere entirely due to human activity such as atomic bomb testing. The first atomic bomb was tested in 1945, and no 137 Cs is found in the atmosphere before that. 137 Cs in the atmosphere increases until it reaches a peak in 1963, when above ground testing is banned. It has been decreasing steadily since then. The peak at 1963 is a powerful mark for dating. It is seen that sedimentation in two samples to be presented is pretty constant each year, and the correlation of age and depth is approximately linear. The errors in sedimentation rate and age measurements increase with depth. Cores from Lake Macatawa, White Lake, Lake Michigan and Muskegon Lake have been analyzed.