- 1895 (Nov 8) Roentgen discovers X-rays.
- 1895-1900 Photographic emulsions and electroscopes are primary instruments used when radiation is discovered.
- 1896 (Feb 3) First diagnostic X-ray in US (E. Frost).
- 1896 (Feb) First x-ray picture of a fetus in utero.
- 1896 (Mar) First application of X-rays in dentistry (C. Kells and W. Rollins).
- 1896 (Mar) Thomas Edison reports eye injuries from X-rays.
- 1896 (June) N. Tesla cautions experimenters not to get too close to X-ray tubes.
- 1896 First therapeutic applications of X-rays (Grubbe, Voigt, Despeignes)
- 1896 Dr. D. W. Gage (McCook, NB.) writing in New York's "Medical Record," notes cases of hair loss, reddened skin, skin sloughing off, and lesions. "I wish to suggest that more be understood regarding the action of the x rays before the general practitioner adopts them in his daily work."
- 1897 J.J. Thomson demonstrates corpuscular nature of cathode rays. He theorizes that these electrons might be a constituent part of all matter.
- 1898 (July) Marie & Pierre Curie coin word "radioactivity."
- 1898 Discovery of gamma rays by P. Villard.
- 1898 Becquerel receives skin burn from radium given to him by the Curies that he keeps in his vest pocket. He declares, "I love this radium but I have a grudge against it!"
- 1901 Max Planck proposes that atoms could gain and lose energy only in discrete quantities (quantum).
- 1902 Rollins experimentally shows X-rays can kill higher life forms.
- 1903 (Nov 12) Marie and Pierre Curie awarded the Nobel Prize for Physics.
- 1903 Sir William Crookes and, independently, Elster and Geitel discover that crystals of zinc sulfide emit tiny flashes of visible light (scintillations) when struck with alpha particles. Rutherford quickly adopts this detector for his work.
- 1904 (Oct) Clarence Madison Dally, a glass blower at Thomas Edison's Menlo Park lab, is first person known to have been killed by x-ray exposure. Severely burned in 1896, he still works with x-rays until 1898. His death causes Edison to discontinue radiation work in his lab.
- 1904 Rutherford shows that alpha particles are helium atoms and works out the natural decay series.
- 1904 Colormetric dosimetry system devised by Saboroud and Noire.
- 1904 Marie Curie publishes an observation that diamonds when exposed to radiation and later heated glow proportional to exposure. This is published in Research on Radioactive Substances . This is the basis for thermoluminescent dosimetry which waits until 1950 to be further developed.
- 1904 H. Nagaoka (Japan) publishes planetary hypothesis of atomic structure.
- 1904 Rutherford coins the term "half-life."
- 1905 Einstein publishes Theory of Relativity which explains the phenomenon called the Photoelectric Effect.
- 1905 Ionization unit proposed by M. Franklin.
- 1906 Ernest Rutherford conducts experiments where he bombards gold foil with alpha particles. Most of the alphas pass through. He theorizes that atoms are mostly space.
- 1910 Jesuit Father Theodor Wulf measures radiation at ground level and at top of Eiffel Tower. Radiation increases at higher elevation. Suspects extraterrestrial origins of this radiation. Suggests balloonists measure dose rates.
- 1911 (Aug) Rutherford and Geiger discover that atoms are mostly space using alpha particles to bounce off thin gold foil.
- 1911 Robert Andrews Millikan using oil droplets measures the charge of an electron.
- 1911 Soddy suggests that "the expulsion of the alpha particle causes the radioelement to change its position on the periodic table..."
- 1911 Charles Glover Barkla (England) shows certain x-rays predominate; these are termed characteristic x-rays.
- 1911 Microscope is used to count grain densities in photographic film.
- 1911-1912 Victor Hess (Austrian) takes balloon rides to measure radiation at heights up to 5000 meters. Discovers cosmic radiation which he names "Hoehenstrahlung" (high altitude rays.)
- 1912 T. Christen puts forth concept of half value layer for shielding x or gamma radiation, i.e., only half the incident radiation will be stopped by each successive shielding layer.
- 1912 Max von Laue (Germany) uses the crystals of zinc sulfide to diffract x-rays and measure their wavelength. He thereby proves the wavelike nature of x-rays.
- WW I Exposure of hundreds of girls to luminous paint compound for instrument dials in New York and Illinois.
- WW I Henry Gwyn-Jeffries Mosley killed at Gallipoli. Mosley, a student of Rutherford, had bombarded each of the known elements with a beam of electrons to show the number of electric charges in each nucleus was increased in regular steps between each element in the periodic table.
- 1913 Hans Geiger unveils his prototype gas-filled radiation detector.
- 1913 Niels Bohr (Denmark) applies the newly invented quantum theory to atomic electron orbitals. These stationary orbitals would allow an electron to orbit a nucleus without emitting energy.
- 1913 Soddy proposes the term "isotope" for atoms with the same number of protons and differing only in number of neutrons.
- 1915 (June) British Roentgen Society proposes standards for radiation protection workers; includes shielding, restricted work hours, medical exams; no limits because of lack of units for dose or dosimeters; voluntary controls.
- 1916 A. Sommerfeld (Germany) modifies Bohr's model of electron orbitals to allow elliptical orbits.
- 1920 Luminous dial painting expanded to clock factories.
- 1920 Rutherford suggests additional neutral nuclear particle (later called a neutron). "Such an atom would have very novel properties. Its external field would be practically zero, except close to the nucleus, and, in consequence, it should be able to move freely through matter."
- 1921 British X-ray and Radium Protection Committee present its first radiation protection standards.
- 1922 American Roentgen Ray Society adopts radiation protection rules.
- 1922 American Registry of X-ray Technicians founded.
- 1922 G. Pfahler recommends personnel monitoring with film.
- 1923 A.H. Compton reports wavelengths lengthened for bounced x-rays and gammas. Leads to Nobel prize for the "Compton Effect".
- 1923 A. Mutscheller puts forth first "tolerance dose" (0.2R/day).
- 1924 Description of jaw necrosis by dentist, Blum; attributed to radiation from deposited luminous paint.
- 1924 DeBroglie states that an electron has wave properties and assigns a wavelength to an electron much the same way Einstein assigns a mass to an electromagnetic wave in 1905. This standing wave allows an electron to exist a some distance from the nucleus without gaining or losing energy.
- 2007 Wikiradiographer created
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