Atomic nuclei are tightly bound collections of protons and neutrons, normally hard to break apart. Cockcroft and Walton invented a voltage-multiplying circuit that produced about 800 kV and used it to accelerate protons. They fired these protons at lithium-7 and observed the reaction 7Li + p → 2α + 17.2 MeV, the first man-made splitting of a nucleus. This process, called artificial nuclear transmutation, greatly advanced measurements of nuclear reaction energies and our understanding of nuclear structure. Their high-voltage technique became a cornerstone for later linear accelerators and synchrotrons, which are now common in medicine and materials science.

By the late 1920s Rutherford’s laboratory needed an accelerator delivering energies beyond natural α-particles. The Cockcroft–Walton circuit, a cascade of rectifier valves and capacitors, produced steady DC voltages up to ~800 kV with currents of several hundred microamps—remarkable for the time. In 1932 they used ~400 keV protons on a thin LiF target, detected backward-emitted α-particles, and verified Einstein’s E = mc² from the 17 MeV energy release. The experiment highlighted the energy potential of nuclear reactions and inaugurated systematic cross-section measurements. Bridging accelerator physics, electronic engineering, and nuclear physics, the work paved the way for β-decay studies and, ultimately, uranium fission research.

The Cockcroft–Walton generator, an enhancement of the Greinacher multiplier, incorporated symmetric ripple suppression and inter-stage capacitor balancing, operating reliably at ~800 kV. The Li(p,α)α cross-section displayed sharp resonances between 150 and 400 keV, matching Gamow’s tunneling calculations. Using a cloud chamber supplemented by ZnS scintillation screens, they counted α tracks, performed calorimetric energy measurements, and determined the 17.2 MeV mass defect within ±5 %. High-voltage DC sources derived from their design later served as injectors for cyclotrons and linear accelerators, becoming a standard front-end for ion sources. The data also verified Bethe’s stopping-power formula with screening corrections and seeded the low-energy nuclear reaction databases used today.