Doctoral Thesis: Recherches Sur Les Substances Radioactives

Overview

Marie Curie's 1903 doctoral thesis "Recherches sur les substances radioactives" presents a coherent and methodical account of the experimental program that established radioactivity as a fundamental atomic property and led to the isolation of two new elements. The thesis synthesizes precise measurements of emission from uranium-bearing minerals, careful chemical separations, and rigorous quantitative reasoning to show that certain substances exhibit radioactivity far in excess of their uranium content. That synthesis transformed scattered observations into a reproducible scientific discipline.

Background and aims

Building on Henri Becquerel's discovery of spontaneous emission from uranium salts and her own earlier work with Pierre Curie, Marie Curie set out to determine whether radioactivity depended solely on the presence of uranium or whether other elements could exhibit similar emissions. The thesis aims to measure the activity of various minerals, to compare activity with chemical composition, and to isolate and characterize the sources of anomalously high radioactivity found in pitchblende and related ores. A central goal is to show whether the observed excess activity can be explained by unknown radioactive constituents.

Experimental methods and measurements

The experimental approach rests on painstaking chemical fractionation combined with electrical measurements of ionization produced by the materials. Curie refined the use of the sensitive electrometer developed by her husband and colleagues to quantify ionization currents, enabling comparisons between samples of differing mass and purity. Samples were subjected to repeated precipitations, separations, and recrystallizations; activity was tracked through each step to locate the most active fractions. Great attention is paid to controlling contamination and verifying that measured activity correlates with specific chemical species rather than impurities or artifacts of handling.

Main results: isolation of polonium and radium

Systematic fractionation of pitchblende residues led Curie to identify two distinct highly radioactive substances. One, later named polonium, was noted for its strong activity despite its minute quantity and its association with bismuth-bearing fractions. The other, radium, emerged from barium-like fractions and displayed exceptionally high radioactivity relative to known elements. Curie provides experimental evidence that these substances are new elements by showing consistent chemical behavior across separations while their activity persists and concentrates. Quantitative comparisons demonstrate that the radioactivity of these elements far exceeds that of uranium on a per-mass basis, implying fundamentally different atomic properties.

Conclusions and legacy

The thesis concludes that radioactivity is an intrinsic property of certain atoms and not simply a characteristic of mineral matrices, and it identifies polonium and radium as new members of the chemical inventory. These conclusions supplied the experimental foundation for a new branch of physics and chemistry, enabling subsequent work on atomic structure, nuclear decay, and applications in medicine and technology. The importance of Curie's findings was recognized with the 1903 Nobel Prize in Physics, making her the first woman laureate. The methods and standards of quantitative separation and measurement established in the thesis continued to influence experimental practice and opened pathways that reshaped understanding of matter at its most fundamental level.