MEASUREMENT AND CALIBRATION OF LARGE-AREA ALPHA-PARTICLE SOURCES AT NIST

Work has been performed at NIST to develop and calibrate large-area Pu -238 alpha-particle sources and related measuring equipment. A large-a rea-source X-ray counting system and a 2 pi alpha internal gas-proport ional counting system were used for the primary calibration of these s ources. Studies have been performed to relate these calibrations to th e measurement of thin, large area Pu-239 sources, which are of primary interest but are generally unavailable in the necessary high specific activities required. Two systems have been characterized for the moni toring and measurement of large-area alpha sources: (i) a source-detec tor mounting unit developed at MST to calibrate monitoring systems by mounting a standard source of plutonium at an accurately calibrated di stance from the user's alpha-monitoring detector; (ii) a diffused-junc tion silicon semiconductor detector which is used as a transfer standa rd from low to high activity of large-area alpha-particle sources. A d etermination of the L X-ray emission probability for Pu-239 has been m ade.