In an experiment using a gamma spectrometer, which is a cylindrical crystal NaI(Tl) detector, the Cs-137 isotope with a current activity of 8.6 Ci was placed at a distance of 3 cm from the detector and a 500s count was taken. The net photopic count was found to be 2504535 at 662 keV energy in the received spectrum. Accordingly, calculate the unique detector efficiency of the detector at 662 keV energy. (The radius of the NaI(Tl) crystal is 2.55 cm; the probability of emitting a gamma ray with 662 keV energy is 85.21%).