 In their famous paper, Cockcroft and Walton noted that the sum of the rest-masses of the proton and the Lithium nucleus (i.e., the reactants) was \(1.0072 + 7.0104 = 8.0176\) amu. However, the sum of the rest-masses of the two \(\alpha\)-particles (i.e., the products) was 8.0022 amu. Thus, it seems as if an amount of mass of 0.0154 amu has “disappeared” from the reactants. Cockcroft and Walton also observed that the total energy (in the reference frame in which the \({^7}\Li\) nucleus is at rest) for the reactants was 125 KeV. However, the total kinetic energy of the \(\alpha\)-particles was observed to be 17.2 MeV. Thus, it seems as if an amount of energy of approximately 17 MeV has “appeared” in the reaction.