Measurement and Calibration of Large-area Alpha-particle Sources at NIST

Abstract Work has been performed at NIST to develop and calibrate large-area 238 Pu alpha-particle sources and related measuring equipment. A large-area-source X-ray counting system and a 2πα internal gas-proportional counting system were used for the primary calibration of these sources. Studies have been performed to relate these calibrations to the measurement of thin, large area 239 Pu sources, which are of primary interest but are generally unavailable in the necessary high specific activities required. Two systems have been characterized for the monitoring and measurement of large-area alpha sources: 1. (i) a source-detector mounting unit developed at NIST to calibrate monitoring systems by mounting a standard source of plutonium at an accurately calibrated distance from the user's alpha-monitoring detector; 2. (ii) a diffused-junction silicon semiconductor detector which is used as a transfer standard from low to high activity of large-area alpha-particle sources. A determination of the L X-ray emission probability for 239 Pu has been made.