I got the following graph in the multichannel experiment for Cs137: http://img830.imageshack.us/i/cs137.png/ where the y-axis is the energy and the x-axis the number of channels. My attempt: The peaks A, B and C are likely caused by backscattering. The peak D then again is likely caused by annihilation due to the pair formation with the surrounding materials. The peak E is caused either by Compton scattering, photoelectric effect or by pair formation. I guess that it is caused by photoelectric scattering. This fact would imply that the peak is caused by photons. (1) The peak E is high so a rather energetic particle must have caused the peak. (2) The fact (1) suggests me that the particle is photon but (2) suggests me that it is likely an alpha particle because alpha particle carries more energy than photon. 1. Which type of scattering does cause the peak E? 2. Which type of particle is responsible of the peak E?