Cockcroft
and Walton had used an accelerator to split atoms of
lithium. This was the first nuclear disintegration that
was entirely under human control. The reaction they
had observed was:
Li
+ H = He + He + 17 MeV
The
combined mass of the resulting helium nuclei is actually
slightly less than the combined mass of the original
lithium and hydrogen nuclei. The very small change in
mass becomes energy, its amount calculated using Einstein's
famous equation E = mc². Cockcroft and Walton measured
the speed of the two helium nuclei and found that the loss
could be accounted for by this mass difference, so their
experiment was the first verification of Einstein's
law, E = mc².
