The evaluation of true coincidence summing effect on CTBTO-type sample geometry

The CTBTO sample geometry is filter paper compressed to a 7 cm diameter disk counted directly on the endcap of a HPGe detector. This geometry has high efficiency, but the activity calculated from the spectrum is affected by true coincidence summing (TCS). TCS reduces the peak area for coincidence gamma rays, thus the calculated activity will be low unless TCS is accounted for. NIST-traceable standards were used to calibrate the detectors and measure the TCS effect for both the 5 mm and 15 mm thick disks. Results show the TCS impact on ¹³⁴Cs to make the reported activity about 25% low if the summing correction is not applied.