My question relates to calculating the decay lifetime of an unstable isotope. The information given is the average energy of the emitted gamma ray from the decay has an average energy of say 100kev and a line-width of 5 x 10^-6ev. From this information I need to work out the average lifetime for the isotope. From what I can gather the problem seems to be similar to the Doppler shift effect. From the equation E = hf the energy of the gamma ray is related to its frequency and therefore the change in frequency can be worked out. Does the line width mean the upper and lower boundry of the gamma ray energy with the 100keV in the middle of that range? The other thing is that the shift of the energy either means the source is moving or the energy of the gamma rays emitted is changing for some other reason. The relation between this and the lifetime is not obvious to me. Any help would be much appreciated. I am probably missing something obvious.