Peierls found out that a very interesting experiment was being done at Cambridge by Chadwick and Goldhaber. Goldhaber later became the Director of the Brookhaven National Laboratory. He was a very smart graduate student at that time, and they had discovered that the deuteron, that is the atom of hydrogen of atomic weight 2, could be disintegrated by gamma rays from a natural radioactive substance. You have to remember that at that time there was not yet available any accelerated proton which had high energy, so one had to rely on the high energy particles which nature provided, and fortunately the gamma ray from one of the thorium daughters has an energy just big enough to disintegrate a deuteron, in fact an energy of 2.6 million electron volts, and the binding energy of the deuteron is 2.2. So they managed to disintegrate the deuteron and get the track of the proton which comes out of it to measure the length of that track, and then using the range-energy relation they could determine that the proton which came out had an energy of about 0.2 million electron volts. Neutron obviously had the same energy, so they could calculate the deuteron binding energy.