Purpose: Annual efficiencies of our PET hot lab well counters using a new ⁶⁸Ge/⁶⁸Ga calibration rod source in 2020, were substantially lower than historical averages. An independent verification as to whether or not the stated activity of the new source was correct, was performed.
Methods: A positron detection lab setup was employed for coincidence counting of the source. Two Harshaw 3" diam. x 3" thick NaI(Tl) detectors faced each other, separated by 18.6 cm and connected to Ortec pre-amplifiers (PAs). PA signals were input into Ortec amplifier and single-channel analyzers (SCAs) windowed around 511 keV. The two SCA logic pulses were input to an Ortec universal coincidence (UC) unit, and UC output logic pulses were then fed into an Ortec timer and counter. ²²Na coincidence efficiency (cpm/dpm) was determined using two 3-mm calibrated button sources centered between the detectors, and converted to that for ⁶⁸Ga based on positron yield. The 2020 and a 2015 source were then coincidence counted (cpm) and the results were converted to dpm.
Results: The estimated dpm of the 2015 source was 7.6% lower than the decayed stated activity, but that for the 2020 source was 64.4% lower. A second new source sent by the manufacturer yielded an identical result (64.4% lower). All three sources were returned for investigation, and the manufacturer discovered an error in their source activity calibration method. A new source calibrated with a corrected method was sent, and the efficiencies of two well counters using that source (0.60 cpm/dpm) were within the range of those for the prior four years using the 2015 source (0.58 - 0.64).
Conclusion: A coincidence counting method of measuring ⁶⁸Ge/⁶⁸Ga source activity, in addition to the well counter efficiencies based on that source, resulted in discovery and correction of an error in the manufacturer's activity calibration procedure.
Coincidence Counting, Calibration, Source Strength