Sample Analysis

Soil samples were collected by the U.S. Geological Survey in April, 2003 and April, 2004 and were analyzed with a laboratory gamma-ray spectrometer . The sample locations are shown in Figure 3. The sample locations were determined using a GPS device and were collected from the top 5 cm of soil. The samples were sieved to retain only the size fraction less than 2 mm and were dried in a laboratory at 25 °C.

The soil samples were analyzed for the concentrations of potassium (percent K), equivalent uranium in parts per million (ppm eU), and equivalent thorium (ppm eTh) with laboratory gamma-ray spectrometers. Snyder and Duval (2003) describe the design and operation of the spectrometers. The term "equivalent" is used because the measurements use gamma rays from daughter products of the uranium and thorium decay series and the method assumes that the decay series are in radioactive equilibrium. Because the assumption of radioactive equilibrium may not be true, the results are expressed as equivalent concentrations of uranium and thorium.

The soil samples were placed in cylindrical plastic containers with a diameter of 15 cm and a height of 3.8 cm. The samples were weighed on a balance with an estimated accuracy of 0.1 g, sealed with two layers of plastic electrical tape, and immediately placed in the spectrometers for measurement. The measurement time used was a livetime of 26,000 seconds. Following the initial measurements, the samples were stored for a minimum of 21 days before repeating the measurements. This was done to allow the radon to achieve radioactive equilibrium. Assuming that all of the radon gas that could escape did escape during the transfer of the samples to the containers, the two sets of measurements can be used to estimate the fraction of radon emanation from the samples. The dry bulk densities of the samples were also measured using plastic tubes with known volumes.

The gamma-ray spectrometers were calibrated based on measurements of laboratory standards with known concentrations of potassium, uranium, and thorium. Standards were measured at intervals throughout the period of time during which the soil samples were measured. Table 1 lists the statistics of the observed errors for the measured concentrations of potassium, uranium, and thorium in the standards. The negative mean errors for uranium and thorium indicate that the laboratory results are, on average, less than the standard values. The observed errors for the standards indicate that the absolute accuracies of the soil sample concentrations are approximately 0.1 % K, 0.4 ppm eU, and 0.3 ppm eTh.

Table 1. Mean values and standard deviations of observed errors for measured concentrations of potassium, uranium, and thorium in standards used to calibrate laboratory gamma-ray spectrometers.