Improved counting efficiencies for measuring 239Pu in the lung in the sitting position.

Abstract

For a study sponsored by the International Atomic Energy Agency (IAEA), Pacific Northwest Laboratory (PNL) made lung counting measurements of lung deposition in 10 women who had inhaled 92mNb in England. These women were measured in both the supine and sitting (torso 45 degrees from horizontal) positions with a counting system of Ge planar detectors and another counting system of Phoswich detectors. The average increase in counting efficiency from the supine to the sitting position was 108% for the Ge detectors and 310% for the Phoswich detectors. Individual counting efficiency increases ranged from 15-260% for the Ge detectors and from 40-480% for the Phoswich detectors. The increase in counting efficiency was roughly proportional to the total chest wall thickness of the measured individuals. By using the equivalent muscle tissue thickness of the individual's chest wall, the increase in counting efficiency could be attributed to the decrease in the tissue absorption in the sitting position. The decreased distance between the lungs and the detector achieved in the sitting positions may also account for some of the improvement in counting efficiency. In addition, some undetermined internal change in activity distribution (source-to-detector geometry) and tissue absorption may possibly contribute to the improved counting efficiency of the sitting vs. the supine position.