In 1932 John Douglas Cockcroft and Ernest Thomas Sinton Walton accelerated hydrogen ions from a discharge source through an evacuated tube onto lithium at . A side-facing zinc sulfide screen, viewed by microscope, showed particle scintillations. Mica stopped scattered protons; switching off or blocking the beam removed the flashes. An external screen allowed absorption-range measurements.[ref]
Experiments with High Velocity Positive Ions. II. The Disintegration of Elements by High Velocity Protons by John Douglas Cockcroft and Ernest Thomas Sinton Walton. Experiments with High Velocity Positive Ions. II. The Disintegration of Elements by High Velocity Protons, 1932, pp. 230–232, sections 2–3, Fig. 1; primary text inspected in the Scribd transcription
Magnetic deflection checked voltage calibration. Opposite screens and independent observers found excess coincidences for a thin lithium target, supporting paired alpha particle emission; geometry and screen efficiency limited detection.[ref]
Experiments with High Velocity Positive Ions. II. The Disintegration of Elements by High Velocity Protons by John Douglas Cockcroft and Ernest Thomas Sinton Walton. pp. 231, 237–238, section 4, Figs. 5–6: corrected voltage calibration, paired screens and random-coincidence comparison
John Douglas Cockcroft's 1951 lecture recalls detection at and an absorption range about , then refined alpha particle energies about each. He places the work under Ernest Rutherford's direction. The inferred nuclear reaction was lithium-7 plus a proton producing two alpha particles.[ref]
Experiments on the interaction of high-speed nucleons with atomic nuclei by John Douglas Cockcroft. Experiments on the interaction of high-speed nucleons with atomic nuclei, lecture delivered 11 December 1951, pp. 167–170, Figs. 2–4; primary text inspected in the Scribd transcription
The logic has several independent links. Beam blocking tests whether the flashes depend on bombardment. An absorber tests whether scattered beam particles could reach the screen. A particle range tests product identity and kinetic energy, while coincidences test whether two products belong to one event. These tests support a reaction interpretation jointly; counting flashes alone is not an isotope identification, and a range-to-energy conversion needs calibration. The lithium proton reaction energy below is a modern calculation, separate from those historical measurements.