So I have a problem where I'm given the decay rate of a sample which contains Cs137, 10 mCi, and I'm basically given the percentage of radiation detected by the detector. But the wording of the question is: "What count rate would be observed in a perfectly efficient gamma detector" Sounds to me like they only care about the intensity of gamma rays and not other particles which are released. I found this diagram but it's what's causing my confusion: http://en.wikipedia.org/wiki/Caesium-137#mediaviewer/File:Cs-137-decay.svg So 95% of the nuclei decay into an excited Ba nucleus which then decays into a stable barium nucleus and a gamma ray. This seems to be the only way gamma rays are produced from the Cs137. Makes sense. So what is the 85%?? Only 85% of the excited barium nuclei release a gamma ray? Or 85% of the Cs137 release a gamma ray? What happens to the remaining excited barium nuclei, do they decay in a different way which doesn't release a gamma ray? If we let P be the percentage of Cs radiation detected and R the overall decay rate of the sample and we suppose 85% of Cs decays result in a gamma ray, then is the detection rate of gamma rays ##r=0.85RP##?