A low-level laboratory for alpha and gamma counting of environmental samples

The development of a low-level counting laboratory for the measurement of alpha- and gamma-ray emitting radionuclides in environmental samples is presented. A microcomputer-based counting system has been set up which consists of 12 Si-surface barrier detectors and four Ge(Li) detectors and associated analog-to-digital converters interfaced to a Nuclear Data Corporation ND6620 data acquisition and processing system. Sample preparation includes Ge(Li) detector calibration of a few container sizes and several counting matrix densities. A procedure for 241Pu analysis has been adapted for a low-level liquid scintillation counter and used to measure samples collected at several atolls in the Marshall Islands. The 241Pu/sup(239,240)Pu ratio is generally approximately 20 when decay-corrected back to 1956. However, ratios of from 10 to 26 were measured, indicating that the 241Pu/sup(239,240)Pu ratios may be used to detect particular sources of plutonium isotopes, i.e. from different nuclear detonations or from nuclear waste disposal. Measurements of plutonium and 210Pb have been made of a nuclear waste disposal site 4000 m deep located off the coast of New York, USA. Because of the low-level activity of the plutonium present, measurement times often exceeded 10,000 min. The long-term reliability of the system is proved by these measurements and by the results of the interlaboratory calibration. (author)