She used a radioactive source of 𝛂-particles to irradiate paraffin wax, which has a high content of hydrogen. An 𝛂-particle may collide with a hydrogen nucleus (proton), knocking that proton out of the wax and into the photographic emulsion, where it produces a visible track of silver halide grains. After many trials, using different plates and careful shielding of the emulsion from unwanted radiation, she succeeded in making the first ever observation of proton tracks in a nuclear emulsion.