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Discovery of Radioactivity

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Discovery of Radioactivity No. 1 in a series of essays on radioactivity produced by the Royal Society of Chemistry Radiochemistry Group Discovery of Radioactivity Being a mineralogist, Becquerel had a large collection of minerals, many of which exhibited phosphorescence. He experimented by wrapping a photographic plate in black paper (to protect it from direct light), placing a phosphorescent uranium mineral on it and exposing it to bright sunlight. When developed the plate bore Earth has been radioactive a clear image of the uranium since it was formed 4500 mineral. Initially, he thought of million years ago. In fact the this as confirmation of his age of the earth can be theory. Radioactivity blackens a photographic calculated from a detailed plate even through a layer of black examination of its paper radioactivity, particularly the decay of uranium to lead. Imagine his surprise to find the image as intense as in the Henri Becquerel original sunlight experiment. He now drew the correct Following the discovery of conclusion that this had nothing x-rays in 1895 by Roentgen, to do with light but the exposure Becquerel had the idea, came from uranium itself, even mistakenly, that minerals that in the dark. Radioactivity had were made phosphorescent by been discovered. He went on to visible light might emit x-rays. show that uranium minerals (X-rays are still used to look for were the only phosphorescent faults such as breaks and minerals to show this effect. cavities. X-rays, not absorbed by materials or the specimen, Henri Becquerel The Curies can be detected). In February 1896 there was Becquerel recommended that Phosphorescence is the ability very little sunlight in Paris for Marie Sklodowska Curie of a crystal to absorb light and several days (a common choose the subject for her re-emit light sometime after the occurrence at that time of year). doctoral thesis. During her exciting light has been Due to this, Becquerel stopped studies she quickly established removed. The Romans used his experiments and placed the that the amount of radioactivity phosphorescent minerals in the uranium mineral and wrapped emitted by a uranium mineral centre of some of their roads to photographic plate together in a was proportional to the amount illuminate them at night. The drawer. After several days he of uranium present in it. minerals were stimulated by developed the plate expecting a Further, that thorium minerals daylight then continued to re- weak image from the small behaved similarly. emit light during the hours of amount of phosphorescence darkness. that had not yet decayed away. Registered charity number 207890 1 No. 1 in a series of essays on radioactivity produced by the Royal Society of Chemistry Radiochemistry Group chemical separations is gas into a thin-walled glass monitored by their radioactivity. tube. Becquerel and the Curies received the 1903 Nobel Prize in Physics for these experiments. During this period the Curies did ALL the heavy work themselves on Pierre’s income of £20/month. For this he gave 120 lectures/year. By 1902 they had produced about 0.1 g of pure radium chloride (about 70mCi, millicuries, or 2600 MBq, megabecquerel). Marie Curie Rutherford, Royds and When she examined the Soddy uranium ore pitchblende she Ernest Rutherford found that it was four times Ionization is the removal of one, more radioactive than would be or more, electrons from an The wall of this tube was thin expected from the uranium atom, leaving it positively enough to allow most of the - mineral alone. The only charged. In studying the particles through. An evacuated conclusion was that it contained ionization of gases by uranium, collecting tube surrounded the another unknown radioactive Rutherford identified two types thin-walled tube containing the component. of radiation: -radiation, which radon. After two days the produces by far the greater part yellow helium line was clearly With the help of her husband of the ionization which can be visible in the outer tube by Pierre they boiled 40 pound (20 stopped by a single sheet of visible light spectroscopy. Then kilo) batches of pitchblende in paper, and -radiation, which after six days the whole cast iron cauldrons and in 1898 produces much less ionization spectrum of helium was visible. they succeeded in isolating two and is capable of penetrating The -particles thus gave rise new radioactive elements. several millimetres of Al, Cu, to the observed helium. etc. In 1900 -radiation was Identifying helium in this way is discovered by Villard who found like recognizing a neon light that radium emitted an from its colour. extremely penetrating radiation capable of detection through 20 Uranium Decay Series cm of iron or several cm of lead. -radiation is When uranium (U) and thorium electromagnetic radiation like (Th) emitted and particles light but the photons have they decayed to different Pierre (centre) and Marie Curie in their much shorter wavelengths and elements, this sequence was laboratory higher energies. called the natural decay series. The first they called Polonium By studying the chemistry of (Po), after Marie’s native Many experiments were carried the natural decay series of Poland, and the second out with radiation, usually uranium and thorium 30 new Radium (Ra) after the Latin for from radium prepared by the radioactive elements were ray (radius). Po was 60 and Ra Curies, which suggested that discovered, of which radium 400 times more radioactive the particle was a helium (Ra) was the best known. This than uranium (U). This was the nucleus. The definitive identity was a truly international effort invention of radiochemistry. In of -radiation was established but much of the work was done radiochemistry the destination by Rutherford and Royds in an by Rutherford and Soddy who of any species in physical or elegant experiment. They put a both obtained Nobel prizes in large quantity of purified radon Chemistry for this work. Registered charity number 207890 2 No. 1 in a series of essays on radioactivity produced by the Royal Society of Chemistry Radiochemistry Group From the detailed chemistry of the spintharoscope. The latter the decay series Soddy and being a method of quantitatively others noticed that the detecting radioactivity by eye chemistry of the daughter from the light flashes produced element was connected with on a zinc sulphide (ZnS) the emitted radiation. It screen. The light flash caused eventually dawned on the by radioactivity is a scintillation researchers that particle event. The spintharoscope was emission moved the element widely used by the early two places down the Periodic researchers, including Table, whereas emission Rutherford’s group when moved it one place up the discovering the nucleus. In Table. addition, spintharoscopes were sold to the general public for Measurement “after dinner” entertainment in well-to-do houses. Having identified the various types of radioactivity it was necessary to be able to Unravelling this chemistry was measure them. like the work of a detective. Rutherford’s doctoral thesis at Many of these “elements” could the Cavendish Laboratory not be separated by chemical under J.J. Thompson in 1895 methods, e.g. radiothorium involved studying the effects of (228Th) from Th, Ra from x-rays on gases. This later mesothorium and radium D extended to radioactivity. (Ra-D = 210Pb) from the final product in the decay chain, lead (Pb). Soddy concluded that such inseparable “elements” must occupy the same position in the Periodic Table and called them “isotopes”. Isotopes are atoms of the same nuclear charge but different mass; that is they contain the same number of protons (and are the Curie Electrometer same element) but different numbers of neutrons. Geiger-Muller Tube for the detection of The electrometer was used to ionizing radiation determine the presence of This concept was confirmed in radioactivity in solutions and the early 1920s when Aston, The work led to a clear precipitates. It was used by the using the first mass understanding of the transport Curies to measure the spectrometer (a device which of electricity through gases by radioactivity of their solutions uses a magnetic field to charged ions being attracted to when separating polonium and separate atoms of different the electrodes. This production radium from pitchblende. mass), was able to separate of charged ions was used for Electroscopes consist of two and measure isotopes for a detecting radioactivity in light gold leaves that, when number of elements, in ionization chambers and charged, spring apart. When particular 20Ne and 22Ne. The Geiger-Muller counters. radioactivity passes through the placement of these new electroscope the ionization elements in the periodic table in Sir William Crookes (1832- causes the charge to leak away 1913 was remarkably accurate 1919) had a private laboratory and the gold leaves to collapse when compared to the present at his home in Kensington together at a rate proportional allocation. Gardens, London. Here he to the number of ions produced, discovered thallium (Tl), and which, in turn, can be related to Registered charity number 207890 3 No. 1 in a series of essays on radioactivity produced by the Royal Society of Chemistry Radiochemistry Group the amount of radioactivity. compared to the approximate Modern versions are often a size of an atom (10-10m). metal-coated quartz fibre and are used as personal dosimeters. The cloud chamber was invented by C.T.R. Wilson after studying the meteorological effects of clouds on Ben Nevis. Rutherford’s scattering experiment. 1) In effect, the track of an ionizing alpha source, 2) lead shielding, 3) alpha particle could be photographed beam, 4) fluorescent detector, 5) gold as it made a “cloud trail” in a foil, 6) target, 7) some particles pass through, some are deflected chamber saturated with vapour. The principle is similar to the Assuming a nucleus to be the formation of aircraft trails at size of a balloon, then the high altitudes.
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