Discovery of Radiation and Radioactivity

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The History of the Discovery of Radiation and Radioactivity
Introduction: Radiation can be defined as the propagation of energy through matter or space. It can be in the form of electromagnetic waves or energetic particles.

Ionizing radiation has the ability to knock an electron from an atom, i.e. to ionize. Examples of ionizing radiation include: • alpha particles • beta particles • neutrons • gamma rays • x-rays Non-ionizing radiation does not have enough energy to ionize atoms in the material it interacts with. Examples of non-ionizing radiation include: • microwaves • visible light • radio waves • TV waves • Ultraviolet radiation (except for the very shortest wavelengths) The earth has been radioactive ever since its formation into a solid mass over 4½ billion years ago. However, we have only known about radiation and radioactivity for just over one hundred years. [One hundred and four years and 10 months to be exact.] You are probably familiar with the radiation warning sign, the “trefoil”. See http://www.orau.com/ptp/articlesstories/radwarnsymbstory.htm for a history of the symbol.

A Brief History of the Discovery of Radiation and Radioactivity The History of the Discovery of Radiation and Radioactivity

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1895: The Discovery of the X-ray Radiation was discovered by Wilhelm Conrad Roentgen on November 8, 1895. http://www.orcbs.msu.edu/radiation/radhistory/wilhelmrontgen.html Roentgen, like many other physicists at that time, had been experimenting in his laboratory with the discharge of electricity in “vacuum tubes”. The glass tubes were evacuated (and made airtight using Bank of England sealing wax) and had metal plates sealed at the ends. The metal plates could be connected to a battery or an induction coil. This flow of electricity was necessary in order for the tube to glow. The glow emerged from the negative plate (the cathode) and disappeared into the positive plate (the anode). If a circular anode was sealed into the middle of the tube, the glow (the cathode rays) could be projected through the circle and into the other end of the tube. If the beam of cathode rays were energetic enough to hit the glass, the glass would glow (fluoresce). The glass tubes were given various names, e.g. Crookes tubes (named after William Crookes) or Hittorf tubes (Johann Hittorf) based on the individual able to design a tube better able to generate or hold vacuum. Roentgen’s work was carried out using a Hittorf tube. [See www.chem.uiuc.edu/demos/cathode.html for a demo of an early cathode ray tube in operation.]

It had already been demonstrated that the emission coming from the cathode, the “cathode rays”, were not very penetrating. On the evening of November 8, 1895, Roentgen had covered the tube completely with black cardboard. The laboratory was completely dark. Several feet away from the tube was a piece of paper, used as a screen, covered with barium-platinum cyanide. In the darkened room, Roentgen noticed the screen fluorescing, emitting light. Something must have hit the screen if it reacted by

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emitting light. However, since the Hittorf tube was covered in cardboard, no cathode rays or light could have come from Hittorf tube. Very surprised, Roentgen began investigating this strange occurrence. He turned the paper screen so that the side without barium platinum-cyanide faced the tube – still the screen fluoresced. He moved the screen farther from the tube and still the screen fluoresced. Then he placed several objects between the tube and the screen but all appeared to be transparent. When he put his hand in front of the tube he saw his bones on the screen.

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Roentgen experimented with these “new rays”, alone in his laboratory, for many weeks. He found that objects were transparent to these rays in different degrees. Photographic plates were sensitive to x-rays. He could not detect any appreciable reflection or refraction of the rays, nor could he deflect them with a magnetic field. The rays...
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