The Cockcroft-Walton experiment -- we already point that out; no need to say it again.
Perhaps the most precise direct empirical verification of E=mc<sup>2</sup> was done in 2005 by Simon Rainville ''et. al.''<ref>[http://www.nature.com/nature/journal/v438/n7071/full/4381096a.html Nature 438, 1096-1097 (22 December 2005)] doi:10.1038/4381096a; Published online 21 December 2005</ref> The article states that "Einstein's relationship is separately confirmed in two tests, which yield a combined result of 1−Δmc²/E=(−1.4±4.4)×10<sup>−7</sup>, indicating that it holds to a level of at least 0.00004%. To our knowledge, this is the most precise direct test of the famous equation yet described."
==The Cockcroft-Walton experiment
, not to be taken as a verification or "proof"==
This experiment is not one of the ones usually cited as validating E=mc². That was not its goal. The generally accepted important tests of this equation are the measurements of alpha decay energies, described above.