Henri Becquerel had already noticed that uranium bearing crystals in a locked drawer could darken wrapped photographic film and such ’emanations’ – as he called them – could turn air into an electrical conductor, it seemed. His crystals needed no energy source, they seemed to provide their own. The Curies, Marie and Pierre, who shared the Nobel Prize with Becquerel in 1903, brought fresh, original minds to the problem. They used an ore of uranium, pitchblende that produced a current 300 times larger than Becquerel’s original and reasoned that the ore must contain a new substance. They named it Polonium, after Marie’s home country of Poland and coined a new word – “radio-active”. The work was hard, gruelling and tedious, but they finally isolated another element, Radium, established the chemical properties of both and set the first standards by which the emission rate from different materials could be measured and compared. Additionally, they realised that the rate of emission decreased over time which could be calculated and predicted. Her greatest achievement however was to realise that radioactivity was a unique atomic property of matter. All this before Rutherford established for all time the essential structure of the atom.
Three types of ’emanation’ were eventually discovered.
Alpha particles are helium nuclei; large, heavy and lose energy quickly. A hand or thin piece of paper stops them. Beta particles are high speed electrons that travel close to the speed of light and can penetrate a hand but not concrete. Gamma rays are emitted with either of the other two because the nucleus is left in an excited state after emission of alpha or beta. A nucleus thus has a radioactive ‘fingerprint’ which is unique to a particular isotope in terms of decay pattern and half life. The activity or the rate of decrease is predictable and can be described with accuracy, vastly increasing our ability to date the biological events of our planet.