the detector of problem 6 has intrinsic peak efficiency at 1 MeV of 12%. the point source emits a 1 MeV gamma ray in 80% of its decays and has an activity of 20 kBg. neglecting attenuation between the source and detector, calculate the number of counts that will appear under the 1 MeV full-energy peak in the pulse height spectrum from the detector over a 100 second count.