The discovery of spontaneous radioactivity overturned the prevailing belief that atoms were indivisible. While investigating phosphorescence, Becquerel noticed that uranium compounds darkened photographic plates even without light exposure. He hypothesized that the blackening was caused by electrically charged particles or high-energy rays and designed systematic tests. By using several salts such as uranyl acetate and uranyl sulfate, he proved the effect came from the uranium element itself. He concluded that uranium nuclei must decay spontaneously, releasing energy in the process—an unprecedented idea at the time. The finding demonstrated that natural elements store enormous energy inside, laying the groundwork for later nuclear power applications. It also initiated technologies such as radiation therapy in medicine and radiometric dating in geology. Becquerel’s work provided a cornerstone for the nuclear physics later advanced by the Curies and Ernest Rutherford.

Becquerel’s 1896 experiment was performed in the wake of the fascination with phosphors that followed the discovery of X-rays. He intended to examine whether UV-excited fluorescence of uranium salts produced X-rays, using photographic plates shielded by black paper. Chance intervened when, after being stored in a drawer without UV exposure, the plates nonetheless became fogged. Further tests showed that the darkening survived passage through aluminium foil or thin glass but was blocked by thicker metal, allowing Becquerel to quantify the penetrating power of the radiation. He coined the term “Becquerel rays” and, from ionization observed inside an electrified cloud chamber, inferred that part of the emission consisted of charged particles. His papers also noted that calorimetric measurements revealed negligible heat release compared with the radiative energy detected, implying a high-energy atomic process. Because the effect occurred without optical stimulation, it was later interpreted as nuclear decay that requires no external energy input. The discovery triggered a re-examination of atomic structure, leading to Thomson’s electron discovery and Rutherford’s nuclear model. In the modern SI system, the unit becquerel (Bq), meaning one nuclear decay per second, commemorates his contribution. Understanding this historical and scientific context remains essential for nuclear physics, radiation science, and debates on energy policy.

Becquerel placed crystals of K2UO2(SO4)2·2H2O, a self-luminescent phosphor, on photographic plates and detected mixed β and γ components without external excitation. Between March and June 1896 he compared doses with a gold-leaf electroscope, demonstrating that the radiation intensity was independent of chemical form or crystalline state and proportional to the number of U atoms. He showed that thermal treatment or dissolution did not alter the activity, implying that the phenomenon was intrinsic to the nuclide rather than to electronic configuration. Laboratory notes reveal he used semi-log plots to obtain transmission curves, effectively recognizing exponential attenuation early on. Ultraviolet components seen in the self-emission spectra of some samples were later re-interpreted as scintillation, becoming a precursor to radiation detection methods. Becquerel also measured recombination currents in ionized gas, anticipating the concept of linear energy transfer (LET). His raw data were re-analysed later that year by Carveth and Tarbell, hinting at the presence of uranium-series daughter nuclides. Although the mechanism of spontaneous decay was unknown, the observations provided statistical evidence that nuclear binding energies lie in the mega-electronvolt range. These primary measurements fed directly into the Rutherford–Soddy transmutation law and the formulation of α-decay theory, forming a foundation for nuclear physics. Recent historiographical studies have found qualitative discussions of half-lives in Becquerel’s manuscripts, further justifying the modern adoption of the becquerel (Bq) unit.