 In these reactions, when the nucleus of an atom (in this case either \({^{32}}\S\) or \({^{28}}\Si)\) captures the neutron, a new isotope is created in an excited state. In returning to its ground state, the isotope emits a \(\gamma\)-ray. According to Einstein’s equation, the difference in the rest-masses of the neutron plus nucleus, on the one hand, and the new isotope in its ground state on the other hand, should be equal to the energy of the emitted photon. Thus, Rainville et al. test \(\Delta E = \Delta mc^2\) by making very accurate measurements of the rest-mass difference and the frequency, and hence energy, of the emitted photon. Rainville et al. report that their measurements show that Einstein’s equation obtains to an accuracy of at least 0.00004%.