Some radioactive sources emit x-rays at accurately known wavelengths.
These are used in a special mode of the XMM detectors to calibrate their wavelength response. The protons in a nucleus strongly repel each other due to the electrostatic force but are held together by the attractive strong nuclear force which acts between nucleons in general and not just the protons. This means that by adding the uncharged neutrons we can increase the force that holds the nucleus together but do not add to the electrostatic repulsion. This explains why there are typically more neutrons than protons in stable nuclei. However, if this were the only factor in determining the stability it is clear that the most stable nuclei would contain enormous numbers of neutrons in comparison to protons.