Abstract
Commercially available electret ionisation chambers (EIC) and alpha track detectors (AIDS) have been adapted for the in situ measurement of soils contaminated with low levels of plutonium at the Nevada Nuclear Weapons Test Site. The measurements were made especially challenging by vagaries of the weather, blowing soil, moisture condensation, radon in soil gas, and the need to leave the detectors in place for periods of up to several days. The most successful approach made a differential measurement on scrapings of surface soil loaded into two Petri dishes covered with thermal insulators. One dish was covered with a radon-permeable Tyvek sheet while the other was left uncovered. The Tyvek sheet interrupts alpha particles emitted by plutonium and other transuranics and background actinides. The difference in the two measurements is then due to these two groups of alpha emitters. Using this in situ, passive measuring method, background measurements were made down to a few tens pCi.g -1 . Site-specific samples of soil were ground to improve homogeneity, analysed radiochemically for alpha-emitting isotopes and the calibration factors then determined for the EICs and ATDs.
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