The Curies noticed that the radioactivity of pitchblende ore was far higher than could be accounted for by its uranium content. Through hundreds of cycles of fractional precipitation and sulfide formation, they devised methods to measure the radiation of trace constituents. In 1898 they isolated polonium (element 84) with very high activity and later that year separated radium (element 88) whose activity was thousands of times stronger. Their measurements employed a quartz leaf electrometer and an improved ionization chamber, achieving sensitivities about 100 times better than previous instruments. Radium salts emitted enough energy to heat themselves and became pioneers of cancer treatment via “radium needles.” The couple concluded that radioactivity is a nuclear property intrinsic to each element, cementing the concept of atomic transmutation. Their process highlighted the importance of reproducible quantitative measurement and large-scale sample processing, bridging analytical chemistry and atomic physics. Subsequent work by Rutherford and others on α- and β-ray separation and nuclear reaction theory directly built on the Curies’ achievements.

The Curies began their work by procuring several tons of Joachimsthal pitchblende, an industrial by-product, dissolving it in sulfuric and hydrochloric acids, and precipitating it as barium salts in liter-scale beakers. Radiation was measured with a quartz electrometer developed by Pierre, capable of detecting currents at the 10^-11 A level through differential measurements of natural discharge rates. By analyzing distribution coefficients and precipitation curves, they predicted a new element, radium, chemically similar to barium. Spectroscopic lines and atomic weight determinations placed radium’s atomic mass near 225 and fixed its location in the periodic table. The heat release of roughly 100 kcal per day per gram of radium salt dwarfed chemical reactions, hinting at enormous nuclear energy density. The couple also investigated the time dependence of activity, providing experimental support for the exponential decay model of radioactivity. Their data fed into Soddy’s later extension of the conservation of mass–energy and the establishment of the isotope concept. Medically, sealed radium sources were first tried as “radium needles,” demonstrating efficacy in reducing malignant tumors. The heavy radiation exposure the Curies endured led to health issues, prompting recognition of the need for radioprotection science. Training programs at the Curie laboratory produced a generation of specialists who formed the backbone of early 20th-century nuclear chemistry and radiation biology.

The Curies extended ionization-based activity measurements to sensitivities of µC s^-1, estimating the Ra/Ba ratio in enriched fractions by gravimetry long before ICP techniques existed. Their 1899 paper reported a specific activity of 3.7×10^10 Bq g^-1 for radium salts, which later defined the curie (Ci) unit. They examined the polonium decay series, suggesting MeV-scale energies through calorimetric observations. Leveraging his magnetism background, Pierre studied temperature dependence of activity and found a near-zero coefficient, reinforcing the nuclear origin of radioactivity. Using ZnS screens, they attempted scintillation observation of α-particles, qualitatively demonstrating the particulate nature of radiation by counting tiny flashes. Comparing radium γ-ray spectra to Röntgen rays, they concluded the presence of fixed-wavelength peaks independent of tube voltage, implying internal excitation. Anhydrous radium salts self-heated, and calorimetry yielded a decay power density of about 140 W kg^-1. These data were employed in early validations of the Rutherford–Wilson equation and nuclear reaction cross-section theory. Marie improved spectroscopes and first identified characteristic X-rays of radium daughter nuclides, establishing spectroscopic isotope identification. Such achievements expedited completion of decay series charts and are still referenced as foundational data in modern nuclear databases.