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Scientific Background

From Women in European History

When Marie Curie decided to study radioactivity, the field barely even existed. Many physicists at the time were preoccupied (and understandably so) by the phenomenon of X-rays, which had been discovered in 1895 by the German physicist Wilhelm Röntgen. Even nonscientists in the public had been captivated by ghostly images of bones underneath living flesh as revealed by the strange new form of radiation.
Henri Becquerel thought he was studying X-rays, in fact, when he accidentally discovered radioactivity. Becquerel was interested in how X-rays interact with matter to produce phosphorescence[1]. He irradiated various materials with X-rays and tested whether they could then darken a photographic plate. Uranium, he found, could do so. In fact, he discovered, uranium could even darken a photographic plate after being exposed to regular sunlight! As Becquerel later told the story[1], his experiments were then inconveniently frustrated by cloudy weather. For whatever reason, he chose to develop the photographic plate that had been lying for several days surrounded by black paper under a piece of uranium in a dark drawer. Much to his amazement, it had been exposed. Radiation seemed to be coming spontaneously from the uranium itself, apparently violating the much-cherished principle of conservation of energy. This observation at first didn’t attract the attention that it probably should have, perhaps because Becquerel rays, as they came to be called, seemed much less energetic than X-rays—certainly not energetic enough to take pictures of the human skeleton.

Marie Curie was one of the few physicists at first to take an active interest in Becquerel rays. Critically, she was also one of the first to recognize the importance of quantifying this radiation. The device that Marie and Pierre Curie built to measure radioactivity was remarkable for its time[2]. Ionizing radiation, as its name would imply, ionizes (or strips electrons off) molecules of air. This ionization causes air to become slightly conductive. This conductivity can be quantified by measuring the tiny current passing between two high-voltage metal plates. The Curies’ device, based on the piezoelectric effect that Pierre Curie had studied previously, was able to reproducibly measure currents down to picoamps (trillionths of an Ampère)[3]. Modern devices with this level of sensitivity still cost hundreds of dollars. Without such precise equipment, it is unlikely that Marie Curie would have been able to discover the greater-than-normal radioactivity of pitchblende residues that eventually led her and Pierre on the quest for new radioactive elements. Her discovery was not merely the result of inspiration but also of a technical tour de force on the part of her and her husband.

File:Curie_apparatus_fig1.gif

In the apparatus shown above[3], two metal plates (A and B) are charged with a high voltage source (P). The radioactive substance is placed on plate B and ionizes the air between the two plates. Plate B is initially connected to the high end of the voltage source and A is connected to ground through connection D-C. When A is disconnected from ground, charge builds up on A due to current between A and B through the ionized air. This causes the needle on an electrometer (E) to move. In principle, it would be possible to measure the current by measuring the rate of the electrometer needle's displacement. But a much more clever and sensitive way developed by the Curies exploits the piezoelectric effect of quartz previously studied by Pierre Curie. Force exerted on a piece of quartz (Q) by a mass (H) generates a small voltage between the two sides of the quartz. Marie Curie was able to measure the electric current between plates A and B very sensitively by determining the amount of force applied on the quartz (and hence, the voltage) required to exactly cancel out this current, bringing the electrometer reading to zero.

Notes

  1. 1.0 1.1 Quinn, S. (1995) Marie Curie: A Life. New York: Simon and Schuster.
  2. Molinie, P. and Boudia, S. (2009) Mastering picocoulombs in the 1890s: The Curies’ quartz–electrometer instrumentation, and how it shaped early radioactivity history. Journal of Electrostatics. 67, 524-530.
  3. 3.0 3.1 Curie, M.S. (1904) Radio-Active Substances. London: Chemical News Office.

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