X-rays appear when charged particles have very high accelerations. Collisions in a plasma at 2 million degrees produce black body x-rays. In a typical x-ray tube the x-rays are created as 20-200 KeV electrons collide with a metal target. The maximum possible x-ray photon energy corresponds to the total KE of a single electron. A typical tube is about 1% efficient with 99% of the electron energy converted to heat in the target. In high-current, high-voltage tubes, the target is water cooled, oil cooled or spun to stop it from melting.
The early tube at right has a concave cathode C, an anode A, and an anticathode (target) AC. The target is platinum because it is a durable metal with a high melting point, and the intensity of x-ray emission is dependent on atomic number. A boron nitride target would be less effective.
The design of a modern x-ray tube is essentially unchanged from the tubes of 100 years ago. Electrons emitted from a cathode are accelerated in an electric field to impact a target. In the tube pictured the target rotates to distribute the heat over a large area so the tube can be operated with a higher anode current than would otherwise be possible.
The continuous spectrum given off by decelerating electrons, as they impact the target of an x-ray, tube is called Bremsstrahlung.
Emission lines superimposed on the Bremsstrahlung, are due to transitions within the electronic shells of the target material. High energy electrons excite inner electrons. An outer electron falling to the vacant inner level emits an x-ray photon.
Note: many transitions take place between electron shells disturbed by the incident electrons. Only transitions to the innermost levels emit in the x-ray region and are superimposed on the bremsstrahlung. Transitions to the innermost level of the hydrogen atom are in the UV because the 'potential energy well' for hydrogen is less deep than it is for the heavy elements.
The separation of atoms in crystals is about 10-10 meters (one Angstrom). The wavelength of x-rays is in the same region, ~ one Angstrom. A crystal acts like a diffraction grating for x-rays. The effect was (and is) used to determine the crystal structure of complex crystalline proteins etc. When the crystal is salt with a simple cubic structure the diffraction patterns (dark spots on photographic negatives) are also in a cubic structure and relatively easy to interpret. When the crystal is something like hemoglobin the patterns of spots are very complex and the task is daunting to say the least!
In 1914 Moseley found that the energy of the highest energy x-ray photons, emitted by electronic transitions to the lowest electronic level of atoms in a target material, was proportional to the square of the atomic number of the target nuclei.
(Moseley is shown here as a relatively young man.)
| Note: the concept of photon energy was not well established in 1914. What he actually plotted was atomic number against the square root of the frequency. The square root of frequency is proportional to the square root of the photon energy. |
Moseley's law provided direct evidence of the importance of atomic number, when considering the x-ray radiation emitted from atoms. In modern terms, he had established the link between the energy of the lowest level of the electronic shells, (the ground state), and the atomic number.
X-rays caused a sensation when first introduced for medical purposes. It is difficult now - only 100 years later - to imagine a hospital without an x-ray machine. Lead shielding is used and x-ray technicians wear a strip of photographic film that is developed every week or so to make sure they are not being exposed to harmful ionizing radiation.
It was not always this way. The editor remembers as a child of five, in 1950, being taken to a shoe-shop to look at his toes inside new shoes with an x-ray fluoroscope. The 80 cm high box had a slot at the bottom. The image was pale green. The machines were removed a year later for public health reasons. Pity - lots of things that are fun are 'bad' for you.
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