Who proved the existence of neutron in 1934?
You mean who found evidence for the existence of massive nuclear particles, which are NEUTRALLY charged, in 1934.
See the link. James Chadwick was the pupil of Rutherford, and later served on the Manhattan project. The discovery and demonstration of the neutron was a much greater experimental tour de force than the discovery of the electron or the nuclear proton. Why should this be so?