Re: Who discovered the Nuclear Mass Defect?

It took a bit of digging, but the answer appears to be that the first references made to a measurement of the "mass defect" were in a paper by F.W. Aston in Royal Society Proceedings A, Vol. 115, pg 487 (1927):

The Packing Fraction
Ever since the discovery of the whole number rule it has been assumed that in the structure of atoms only two entities are ultimately concerned, the proton and the electron. If the additive law of mass mentioned above was as true when an atomic nucleus is built of protons plus electrons as when a neutral atom is built of nucleus plus electrons, or a molecule of atoms plus atoms the divergences from the whole number rule would be too small to be significant, and, since a neutral hydrogen atom is one proton plus one electron, the masses of all atoms would be whole numbers on the scale H -1. The measurements made with the first mass- spectrograph were sufficiently accurate to show that this was not true.

The theoretical reason adduced for this failure of the additive law is that, inside the nucleus, the protons and electrons are packed so closely together that their electromagnetic fields interfere and a certain fraction of the combined mass is destroyed, whereas outside the nucleus the distances between the charges are too great for this to happen. The mass destroyed corresponds to energy released, analogous to the heat of formation of a chemical compound, the greater this is the more tightly are the component charges bound together and the more stable is the nucleus formed. It is for this reason that measurements of this loss of mass are of such fundamental importance, for by them we may learn something of the actual structure of the nucleus, the atomic number and the mass number being only concertied with the numbers of protons and electrons employed in its formation. The most convenient and informative expression for the divergences of an atom from the whole number rule is the actual divergence divided by its mass number. This is the mean gain or loss of mass per proton when the nuclear packing is changed from that of oxygen to that of the atom in question. It will be called the " packing fraction " of the atom and expressed in parts per 10,000. Put in another way, if we suppose the whole numbers and the masses of the atoms to be plotted on a uniform logarithmic scale such that every decimetre equals a change of one per cent., then the packing fractions are the distances, expressed in millimetres, between the masses and the whole numbers.

A better explanation of the reasoning - or a first "theoretical" attempt to explain the "mass defect" appears in George Gamow's paper in the Proceedings of the Royal Society A, Vol. 126, 632 (1930). But both of the earlier papers appeared before the discovery of the neutron by Chadwick in 1932, so one could make the argument that the mass defect problem wasn't solved until the full structure of the nucleus was understood - that there existed a second nuclear particle which was not just an electron and a proton packed tightly but was a truly neutral species, with its own separate and distinct mass from that of the proton plus the electron.

"Radiation from Radioactive Substances" by E. Rutherford, J. Chadwick, and C.D. Ellis has an account of the earlier understanding and development of the atomic model and an explanation of the mass defect issue.

Hope this helps.