S17-GammaSpectroscopy

A Heuristic Analysis of the Time Dependence of Radioisotope Abundances in Sediments from Lake McCarrons

Evan Meeker & Tony Holm

University of Minnesota Twin Cities

Department of Physics and Astronomy

Abstract

Using a high-resolution, high-purity germanium (HPGe) detector, spectra of gamma ray emissions from radioactive isotopes in sediment samples from Lake McCarrons were taken. By comparing the spectra of sediments from different depths, it was found that there is no significant time dependence of natural radiation over the time frame the sample covers. In addition, no (unnatural) radiation from the fallout of nuclear testing could be detected.

Introduction

Gamma spectroscopy is a common method of determining the abundance of radioactive materials in a substance. These rays are released during the spontaneous decay of unstable materials, either directly (γ-decay) or from the annihilation of a released positron (β-decay). Of particular interest to us in dating of sediment taken from Lake McCarrons in Roseville, MN. was Cesium-137 because it's anthropogenic, being created only by nuclear explosions, and has a relatively long half life of about 30 years. In addition we were also interested in potassium-40 and thallium-208 as these are very common natural sources of radioactivity.

Experimental Setup

The bulk of this experiment’s setup is simply the detector and a “castle” made of lead bricks to shield the detector from background radiation. This is a fairly simple arrangement, only requiring lead bricks to be built around a cavity that can fit both the detector’s detecting arm and a soil sample.

Photo of the detector with the lead castle around the sample

Data

Below are spectra taken from 18, 48, and 78cm from the top of the sample, respectively. The age of the sample at those points corresponds roughly to 2004, 1940, and 1850.

The Cs-137 peak occurs at approximately 667keV and cannot be seen in any of the three graphs. However both the K-40 peak, just below 1500keV and the Tl-208 peak, just above 2500keV can be seen in all three graphs.

Analysis

None of the three spectra differed in any significant way from any of the others. Although it could be seen there was slightly more potassium in the 1850's followed by a dip in the 1940's and a rise in 2004 but not quite to 1850's level, the change was not significant. There was also similar distribution with thallium although in 2004 it was higher than in the 1850's rather than vice-versa, but again the changes were not significant. And the Cs-137 peak could not be distinguished in any of the three spectra. Without further analysis and more time, it is difficult if not impossible to draw any further conclusions from this data