Radioactivity 6- Radiochemistry The Counting Room: Special Edition

programs. 1 General coincidence summing problems and specific coincidence summing problems encountered when measuring these two radionuclides on filter papers were discussed in the last issue? In order to perform accurate measurements of 134CS and 60Co in liquid samples using Marinelli beakers, coincidence summing effects should be investigated. Liquid samples of 134Cs and 60Co were prepared in Marinelli beakers and measured by germanium gamma-ray spectroscopy using the efficiency curve from the coincidence-free standards. The measured activity of 134Cs using the 605-keV gamma ray was only 85% of the known value. Obviously, coincidence summing correction factors must be known in order to accurately determine the activity of 134CS in unknown liquid samples in Marinelli beakers. As with the filter-paper geometry discussed in the last issue, coincidence summing correction factors will probably be different for each detector. The measured activity of 6° Co in the Marinellibeaker sample was 96% of the known value when using the 1173-keVgamma ray. The 1332-keVgamma ray was not used because it would have required an extrapolation of 220 keY past the highest energy point on the coincidence-free fficiency curve. Turning from the measurement process to the calibration process, the coincidence-free standards described above suffer from two major deficiencies. First, the energy range is restricted by the highest energy gamma ray (1115.5 keY). Second, some of the components have relatively short half-lives. Several remedies to these deficiencies have been used but nis article discusses some of the coincidence summing problems encountered when measuring aqueous samples in Marinelli beakers using germanium gamma-ray spectroscopy. Determination of the coincident gammaray radionuclides 134CS and 60Co is discussed. Calibrations with the coincident gamma-ray radionuclides 88y, 60Co, 133Ba,152Eu, and 154Eu are compared to essentially coincidence-free calibrations. For these measurements, standards were prepared by adding 500:t 2 mL ofa liquid of known radionuclide concentration to 500-mL, plastic Marinelli beakers of the type described in ANSI/IEEE Std 680-1978. Gamma-ray measurements were performed using a 19.4%, 84-cm3, p-type, closed coaxial, intrinsic germanium detector. Essentially coincidence-free standards for this particular detector/ geometry combination were prepared using 241 Am