PITFALLS IN THE USE OF CO-57 FLOOD SOURCES

Long-lived Co-57 flood sources, used for routine quantitative monitori ng of scintillation camera hood-held uniformity, have recently become available with nominal activities of 370 MBq and higher. Non-uniformit y problems have been observed. These are presumably related to high ov erall count rates from Co-57 and other radioactive cobalt isotopes pre sent as impurities. This was investigated in five different cameras fr om four manufacturers using three cobalt sources of different ages and strengths. Count rates were measured within the energy range 30 keV t o 1200 keV, and compared to the count rates in the 122 keV photopeak w indow. For a new flood source placed on the collimator, the overall co unt 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 vis ually and quantitatively evaluated. Non-uniformity patterns were obser ved. These were dependent on the actual source area viewed by the came ra, the age of the source, the distance of the source from the collima tor, and the type of camera. Problems can be circumvented by purchasin g lower activity sources (180 MBq), and increasing the source-collimat or distance. Uniformity must first be carefully assessed before the so urce is put into use.