A radioactive nucelus (cobalt - 60, has 27 p+'s and 33 e-'s) emits a photon (gamma ray) of wavelength 2.7 x 10^-13 m. Assuming the nucleus was intially at rest, what si its recoil velocity upon emitting this photon? I have already figured out the momentum using this information. When I looked at the solution guide of how to find the mass, I am not sure why they use: m = 60(1.67 x 10^-27 kg)? to figure out the mass? I mean, I can understand the number 60, because that's the atomic mass, but why multiply it by the mass of a proton? Cobalt has two charges: 2+ and 3+. The most common one is 2+, so why multiply it by the mass of a proton, rather than an alpha particle (which has a 2+ charge)? If someone can explain this, that would be greatly appreciated, because I am so confused. Thank you.