PITFALLS IN THE USE OF CO-57 FLOOD SOURCES

Long-lived Co-57 flood sources, used for routine quantitative monitoring of scintillation camera hood-held uniformity, have recently become available with nominal activities of 370 MBq and higher. Non-uniformity problems have been observed. These are presumably related to high overall count rates from Co-57 and other radioactive cobalt isotopes present as impurities. This was investigated in five different cameras from four manufacturers using three cobalt sources of different ages and strengths. Count rates were measured within the energy range 30 keV to 1200 keV, and compared to the count rates in the 122 keV photopeak window. For a new flood source placed on the collimator, the overall count rate was about 400% of the photopeak window count rate. About 200% was due to high energy photons with energies above 134 keV. The count rate diminished when the source was moved away to a distance of 50 cm. The high energy contribution was less with the older sources, due to the faster decay of the cobalt impurities. Uniformity images were visually and quantitatively evaluated. Non-uniformity patterns were observed. These were dependent on the actual source area viewed by the camera, the age of the source, the distance of the source from the collimator, and the type of camera. Problems can be circumvented by purchasing lower activity sources (180 MBq), and increasing the source-collimator distance. Uniformity must first be carefully assessed before the source is put into use.