C. C. Hagan

“One such scientist Henri Becquerel also discovered another type of radiation emanating from uranium compounds or salts. He was testing responses to sunlight when during cloudy weather he noticed the uranium salts still gave off radiation which could pass through metal foil and make impressions on photographic plate bordered by black paper.¹⁶ The Becquerel rays forming this radiation had ionized the air allowing it to conduct electricity.¹⁷

Marie wisely chose the Becquerel rays to study for her PhD thesis. For this work she was permitted to use a cramped storeroom at the Paris Municipal School of Industrial Physics and Chemistry where Pierre worked as professor of physics.¹⁸ When she began her experiments towards her thesis Marie noted that the Becquerel rays ionized the air and saw that measuring the electricity in the air would be a different way of measuring the rays than merely observing them fogging up photographic plate. Pierre’s invention of the electrometer was ideal for this task as it could measure minute electrical current. Marie later explained: “Instead of making these bodies act upon photographic plates, I preferred to determine the intensity of their radiation by measuring the conductivity of the air exposed to the action of the rays.”¹⁹ Her experiments with different uranium compounds confirmed some of Becquerel’s work but she started to develop a ground breaking hypothesis, namely, that the rays emanated from the atoms of uranium rather than being some other cause such as stored sunlight. Marie’s insight that this was a property of the atom of the radioactive element itself and that radioactivity was the emission of particles led to the insight that there were sub atomic particles which would break the atomic paradigm of the day – in other words, the atom was not the smallest unit of matter.²⁰ She later discovered that a second element thorium emitted Becquerel rays and this prompted her to coin the word “radioactivity”.²¹ She did not realize she was becoming one of the founders of nuclear physics. (Note today that X rays are considered electromagnetic radiation that emanate from the electron cloud of the atom and are not termed radioactive. Radioactivity is reserved for particle radiation that originates from nuclear processes. For example gamma radiation is considered to be part of radioactivity because it originates from the nucleus.)

Pierre soon joined her project as he was fascinated with her progress (suspending work on his piezoelectric crystals). She had discovered something curious, namely, that two uranium ores, pitchblende and chalcolite displayed more radioactivity than elemental uranium. Marie believed there was something else in the uranium ores that was doing this.²² Marie described her next step: “I undertook next to discover if there were other elements possessing the same property, and with this aim I examined all the elements then known, either in their pure state or in compounds. I found that among these bodies, thorium has an intensity of the same order as uranium, and is, as in the case of uranium, an atomic property of the element… Certain ores proved radioactive; these were containing uranium and thorium; but their radioactivity seemed abnormal, for it was much greater than the amount I had found in uranium and thorium…This abnormality greatly surprised us…..I then made the hypothesis that the ores uranium and thorium contain in small quantity a substance much more strongly radioactive than either uranium or thorium. This substance could not be one of the known elements, because these had already been examined; it must, therefore, be a new chemical element… Pierre [joined me] in the search for this unknown substance. We chose, for our work, the ore pitchblende, a uranium ore…Since the composition of this ore was known through careful chemical analysis, we could expect to find, at a maximum, 1 per cent of the new substance. The result of our experiment proved that there were in reality new radioactive elements in pitchblende, but that their proportion did not even reach a millionth per cent!”²³ In other words it was a substance so tiny in quantity yet so mighty in power.

Marie found that the pitchblende gave off radiation four times greater than uranium which caused her to double check everything to eliminate the possibility of errors. But after checking she wrote in excitement to her sister Bronya: “The radiation that I couldn’t explain comes from a new chemical element. The element is there and I’ve got to find it. We are sure!”²⁴

Extract from Chapter 16 CURIE’S SCIENCE

¹⁶ J. Borzendowkski, Marie Curie- Mother of Modern Physics, Sterling Publishing Co.Inc, 2009, p45
¹⁷J. Borzendowkski (ibid), p45
¹⁸N. Pasachoff, exhibited in American Institute of Physics paper (Editor :S.R. Weart) based on her book Marie Curie and the Science of Radioactivity, Oxford University Press, 1996, p4 of paper., p33
¹⁹ N. Pasachoff, (ibid) , p34
²⁰ N. Pasachoff, (ibid) , p34
²¹ N. Pasachoff, (ibid) , p34
²²N. Pasachoff, (ibid) , p35
²³ N. Pasachoff, (ibid) , pp35-36
²⁴ J. Borzendowkski (ibid), p50