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Discovery of Polonium and Radium

Marie was already young wife and mother, now it was a time to find a subject for doctorial thesis. She would become the first woman in Europe with PhD in Physics! Read the previous part: Maria and Pierre Curie: First Meeting, Love and Marriage .

electrometer-CuriesThe discovery of strange X-ray radiation by Rentgen, the radiation that showed bare bone in human hand became the most novel scientific curiosity. Henri Bequerel found that salts of uranium were also a source of this strange radiation. They made a mark on photographic paper without any access to the external light. So Marie naturally decided to follow up and investigate it. Pierre and Jacques Curie invented the electrometer (see the photo on the right) based on piezoelectric quartz that could detect very small amount of electricity. It was known that this strange radiation caused changes in the electric field, so Curies electrometer was a very suitable device to experiment with these strange and penetrating rays.

Marie collected as many samples of metals, rocks and minerals with uranium and other suspected sources of radiation as she could and started her experiments in a tiny room of School of Physics and Chemistry where Pierre also worked. She measured the strength of radiation by detecting the amount of electricity they caused in the air. Within days she had initial results that showed that the strength of the rays depends on the amount of uranium in the sample, the more uranium, the more detected electricity, thus radiation was seen. She also tested thorium, which was known for emitting strange radiation, she found out that the thorium radiation strength is the same like uranium. For now it was obvious that this radiation is not only specific to uranium, instead of calling it “uranium rays” so she started using the term “radioactivity”, which is still used today and sounds almost the same in French like in English.

Marie started investigating all sorts of materials to find out whether any other elements emit radioactivity. Among them she measured mineral called pitchblende, a black rock which contains mainly uranium oxide, but also some lead, thorium and other rare earth elements. It contained much more radioactivity than it should have if only the mass of uranium and thorium was counted. Marie repeated the experiment about twenty times and the result was the same. There could be only one explanation – there is something else in this rock which gives off so much radiation. In that time 83 elements of periodic table were known, today we know about 115. Marie concluded that this element has to be present in minute amounts and its “specific radioactivity” (activity per mass) is very high. Marie and Pierre were hypothesizing that this element may not be known yet.

Pierre decided to join Marie in her tedious work. By July 1898 they were able to confirm the presence of the first element. They named it Polonium after Poland, Marie’s country of origin which was not existed on political maps in that time. Polonium is chemically similar to bismuth. By the end of the same year they discovered one more element which they called Radium. Polonium, although highly radioactive, never gained such scientific significance as radium due to its properties and occurrence – it is more rare. Now we know that both elements are parts of the radioactive decay chain. Polonium’s best known radioisotope, Po-210, with the halflife (time of decay of a half of existing atoms) equal to 138 days, emits mainly alpha rays, which do not penetrate far therefore they are difficult to detect. It may cause a death if injected (like it happened in the famous case of a death of Alexander Litvinenko, a Russian dissident, in 2006). Polonium is almost impossible to detect it through industrial methods since it does not emit any gamma (far range) radiation, thus its application is also limited. Radium was chemically similar to barium. Its radioactivity was much more powerful than polonium. Marie and Pierre were determined to isolate it in the measurable amounts, but it order to do so they needed much more amounts of pitchblende – several tones at least. Pitchblende was mined in St. Joachimstal on the boarder of Germany and what is today Czech Republic. They needed several tones of the ore, not an easy task. In the next article, entitled:

Working through Pitchblende to Separate Radium & Polonium

I will describe how they were able to obtain sufficient quantities of polonium and radium they discovered.

Baba Jaga Corner: Visit Jaga Polish Culture Website at: www.polishsite.us

References:

Beverley Birch “Marie Curie – courageous Pioneer in the Study of Radioactivity

Eve Curie “Madame Curie

Barbara Goldsmith:“ Obsessive Genius

 

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