SOIL MEASUREMENTS BY NUCLEAR TRACK DETECTORS

In characterization of soil contaminated by transuranics or actinides, some one would be interested in both horizontal and depth distributions. Nuclear Track Detectors (NTD's) are an excellent tool for making inexpensive in-situ measurements. The activity and spatial distribution of such alpha emitting radionuclides as plutonium, americium, and uranium, can be determined. This paper will focus on the methodology for surface mapping and depth profiling of alpha contaminants. The detector material is CR-39. The plastic detectors are placed directly on the soil surface to be measured or inserted into the soil at the measurement point. The detectors may be wedged into a hole punched into the ground or driven into the ground inside of a protective rod. After exposure, the plastic is retrieved, washed with water, and chemically etched overnight in a KOH solution. The lengthwise distribution of etch tracks along the strip is measured to produce a depth profile of the alpha activity. With this methodology it is possible to measure activities below a few Bq per gram, identify hot spots with submillimeter accuracy, and profile distributions to depths of several tens of centimeters. The in-situ measurements produce no radioactive waste and are very low in cost compared to radiochemical methods of analysis.