Let me take you back to 8th November 1895, to the University of Wurzburg, Germany and in particular, Professor Wilhelm Roentgen’s laboratory. . Late into the evening, whist experimenting with a cathode ray tube, Roentgen observed barium platino-cyanide crystals fluorescing. And although he did not yet know it, Roentgen had begun the story of one of the most significant inventions to impact on man-kind – X rays.
So what was Roentgen observing that night? The cathode ray tube is quite simply an electron beam. If we look at a simple model of an atom, and an incoming electron, it is possible to see that when an electron collides with the atomic electron, it can cause the electron from inside the atom to move from an inner shell to an outer shell. Energy is needed for this process because the atomic electron moves away from the nucleus of the atom. This process is known as excitation. The electronic configuration in an excited atom is unstable because an electron that moves to an outer shell leaves a vacancy in the energy level that it moves from. When this vacancy is filled by an electron from an outer shell moving into it, the electron emits a photon. This process is known as de-excitation. The energy of the photon is equal to the energy lost by the electron and therefore the atom. However, an atom / electron in its excited state can de-excite directly or indirectly to the ground state, regardless of how the excitation took place. If we now look at an energy level diagram, for example of a mercury atom (pg 37), it is possible to see that whilst the electron has been excited to the 5.7eV (electron Volt) energy level, it can de-excite in two ways and therefore produce three different photons. The electron can de-excite from 5.7eV to the ground state directly by emitting a photon of 5.7eV or it can de-excite indirectly, it can de-excite from 5.7eV to 4.9eV by emitting a 0.8eV photon and then further de-excite to the ground state by emitting a second photon this time with energy of 4.9eV. When the de-excitation occurs, the material seems to glow with visible light, this process is known as fluorescence. And this is what Roetengen observed that evening. So how did barium platino-cyanide crystals fluorescing lead to the discovery of X-rays? Roentgen knew that the gas particles within the Cathode Ray tube would fluoresce with visible light. However, he covered the tube with black paper, which had been known to absorb the light, and so when the barium platino-cyanide crystals started fluorescing (these were on the other side of the room), he knew that a different form of light (which was invisible) had been emitted causing the crystals to excite using photons (this is a similar to principle to excitation by electrons, however, the photon must have energy exactly equal to the energy needed to move the electron to a different energy level), and then de-excite emitting visible light. This invisible light, had not been discovered before and was capable of passing through heavy black paper. As Roentgen continued his experiments, he discovered that these rays (now known as X rays), passed through most solid substances and would cast a shadow on photographic film. This application of the production of pictures of X rays was quickly interwoven within medicine. Another potential application of X rays came in 1912 from Max von Laue, whose worked related to X ray diffraction and crystallography. Research was already being carried out into the interference between radiation with large wavelengths (practically visible light) on a crystalline model based on resonaters (during this period of research the discussion of wave-particle duality was happening). Laue realised that using much shorter electromagnetic rays (which X rays had been theorised to be due to Barkla’s discovery that X rays could be polarised but not made to diffract, and calculated by Laue’s colleague), some kind of diffraction or interference would take place if X rays were sent through a medium (which crystals could be used as). Laue demonstrated this by allowing X rays into a lead box containing a crystal (he initially tested the principle with copper sulfate crystals) with sensitive photographic film behind and to the sides. When the film was developed there was a large central point from the incident X rays, but there was also many smaller points in a regular pattern. These could only be due to the diffraction of the incident beam by the atoms within the crystals and the interference of many beams. By using a crystal as a diffraction grating, laue had proved that X rays were not particles but waves of light with very small wavelengths. However, there was one major issue with Laue’s work. Laue had recorded his work photographically, with bright spots showing points where many X-rays were in phase together. However, there were a large number of points where these spots appeared to be “missing” i.e. diffracted beams of X-rays were expected in these direction, but didn’t occur. Laue suggested that this was because X-rays only contained certain wavelengths which would therefore account for the missing diffracted beams. However, William Lawrence Bragg was not convinced by this explanation. Bragg thought that x-rays must be made up of a continuous spectrum of all possible wavelengths similar to visible light. If this was true, then the missing directions of diffraction would not be because of the wavelength of X-rays, but due to some property of the crystal being examined. Bragg therefore examined each plane of atoms in a crystal as a reflecting surface. The X-rays hit each plane of atoms in turn, reflecting first off the surface layer, then the one below it and so on. If the X-rays reflected off al the surfaces were in phase, with their peaks and troughs all aligned, then a very strong signal could be measured from the reflection. Over the following years, Bragg developed his equation which allowed him to explain the crystal structure of Zincblende crystals (the photograph that Laue had initially taken). Since this point, X ray diffraction has developed rapidly, to the extent that Rosalind Franklin could photograph the structure of DNA in 1952, which then led to the discovery of the double helix structure of DNA. This has paved the way for scientific research, and has allowed genetic modification to develop. However, it is not only X ray diffraction that has developed over the last 100 years. X-rays are now classified into two types; hard and soft. Soft X-rays are found in the electromagnetic spectrum between Gamma Rays and UV light, with a wavelength of about 10 nanometres to about 100picometres and with very high frequencies (around 3x1016Hz to 1018Hz). Hard X rays, on the other hand, have a higher frequency (around 1018Hz up to 1020 Hz) and therefore occupy the same region of the Electromagnetic spectrum as gamma-rays. So, what is the difference? There is only one, and it links to the way that they are produced. Hard X-rays are produced by accelerating electrons, whereas gamma-rays are produced by the decay of atomic nuclei. Hard X-rays are now most often produced in syncotrons, for example at the Diamond light source. The uses of X-rays have also increased, from purely being used as an opportunity to image bones within the human body when they were first discovered, theey are now used to identify flaws or cracks in structural components, airport security and fishery, to name just a few, clearly showing the wide ranging significance of Roentgen’s discovery on mankind.
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