X - rays

THE DISCOVERY AND RESEARCH OF X RAYS

    In last years of the 19th century numerous major discoveries were made concerning the structure of the microcosm and laws which govern it. One of the most important of them was the discovery of X rays.
    In 1895 Wilhelm Conrad Roentgen carried out research on the phenomenon of cathode rays. By accident he put a piece of cardboard covered with fluorescent mineral near the experimental set. The scientist noticed that it started to glow in the dark when the source of cathode rays was turned on. Roentgen immediately started an experiment aimed at investigation of the phenomenon.
    He discovered that if vacuum tube, used for experiments with cathode rays, was covered tightly with thin, black cardboard and placed in a darkened room, bright glow was observed during each discharge on a screen covered with fluorescent barium platinum cyanide (placed near the device). It did not matter which side (the one covered with the mineral or the other one) of the screen faced the device. Fluorescence occurred even when the screen was placed two meters from the vacuum tube.
    He assumed that the fluorescence must have been caused by an agent which could infiltrate from within the vacuum tube through dark cardboard (impermeable to visible or ultraviolet radiation) to the outside of the set. In subsequent experiments the scientist showed that this agent (which he called X rays) could penetrate through various different bodies to a different extend (the level of transparency of a given body was described by Roentgen in terms of the ratio of the brightness of the fluorescing screen placed behind the body to the brightness of the uncovered screen). For example paper and tin foil have a high transparency factor. It is slightly less for wood. Even lesser for aluminium. Thin plates of copper, silver, gold or platinum are transparent (but not thick plates). Compounds containing additives of lead (for example flint glass) are much less transparent. Layers of lead are virtually non-transparent.
    The scientist discovered that human body was also transparent to X rays - Roentgen put his hand between the vacuum tube and the screen and observed dark shadows of bones against a slightly shaded outline of his hand.

    Roentgen showed that under the influence of X rays not only barium platinum cyanide but also other substances fluoresce: fluorescent compounds of calcium, uranium glass, plain glass, calcite, rock-salt. Also photographic plates are sensitive to X rays. On the other hand retina of an eye is not sensitive to them.
    In another experiment Roentgen showed that X rays are not deflected, even in a very strong magnetic field.
    The scientist also showed that the main centre emitting X rays in all directions was the fluorescent spot on the wall of the discharge tube (the spot emerged where the beam of cathode rays falls). If the beam of cathode rays is deflected in a magnetic field and causes fluorescence in a different place, X rays appear in a different spot. Hence it would seem that they are simply cathode rays filtered through a glass casing. However, as opposed to cathode rays, X rays do not get deflected in a magnetic field.
Photograph of shades of human hand

    The scientist made numerous observations and photographs using X rays he had discovered. For example, he took a photograph of the shadow of the profile of the door separating the room containing the experimental devices and the room where he exposed the photographic plate. He took a photo of the shadow of the bone of an arm, shadow of a covered wooden spool with wire wound on, a galvanometer with a magnetic needle covered with metal. He also took a photo of a piece of metal inside which he managed to notice certain heterogeneity which was invisible form the outside.
    Roentgen also discovered that X rays are capable of ionising gases and are not deflected in the electric field (they do not convey any charge).
    The scientist believed that rays were a form of electromagnetic waves (he thought that those waves were of longitudinal character - but he was wrong).
    Roentgen published the results of his experiments in his works in December 1895 and March 1896. As early as on 20th January 1896 in Dartmouth in New Hempshire X rays were used for the first time to help set a broken arm of Eddie McCarthy. Scientists world-wide could take advantage of the new, powerful tool to be used in virtually all branches of science, including atomic science.
    In the following years X rays became the focal point for many researchers.
    Henri Becquerel discovered deflection, reflection and polarisation of X rays (the phenomena are difficult to discover and hence Roentgen did not observe them). After this discovery it became evident that X rays were plain transversal electromagnetic waves. The length of those waves is very small (therefore they can penetrate, virtually without any dispersion, many materials).
    In 1899 two Dutch researchers Haga and Wind passed a beam of X rays through a very narrow V-shaped slit. Rays passing through the opening created an image on a photographic plate. This image differed from the one that would emerge in case of rectilinear dispersion of a ray beam. Scientists knew the phenomenon of the diffraction of light rays and thought that similar phenomena would occur in the case of X rays. The Dutch scientists discovered that the observed phenomenon was actually a diffraction one. Basing on the widening of the image of rays, the researchers came to a conclusion that the length of their wave was about 10^-8 centimetre (1 angstrom).
    In 1906 researcher Marx measured the speed of X rays dispersion. The speed equalled the speed of light.

In years 1907-1908 Stark and Wien took advantage of the relationship between the length and energy of a wave as presented by Albert Einstein and Marx Karl Ernst Planck (1858-1947) to calculate that the wavelength of X rays equalled 0.5*10^-8 centimetre.

    I 1912 Arnold Somerfeld and Koch used a more technically advanced equipment to confirm empirically the results obtained by Haga and Wind - and theoretically by Wien and Stark - concerning the wavelength of X rays.
    However, not all scientists agreed with the idea that X rays were waves. It was discovered that during their absorption certain molecules - similar to beta molecules - were created. William Henry Bragg (1862-1942) and J.P.V. Marsden discovered that those molecules moved in the same direction as originally was the beam of X rays. Moreover their energy is approximately equal to the energy of cathode rays which brought about X rays. They argued that the energy of those molecules was not related to the distance covered by X rays from the source to the place where the molecules emerge. Therefore there is no noticeable energy loss. On the other hand, as Bragg argued, as the distance from the source increases a wave becomes more and more dispersed. According to the experiments, X rays maintained their properties (energy) many meters from the source and therefore, those two researchers claimed, they were molecules.
    So on the one hand X rays behaved like regular waves, but on the other hand they behaved like molecules. Those two views as different models survived till 1920ties when they became reconciled by quantum mechanics.
    At the turn of the 19th and 20th century numerous researches were carried out on the internal structure of crystals. A beam of light passed through a crystal gets refracted on its different layers at different angles, which helped investigate their structure. In early 1920ties scientists came to a conclusion that light rays could be substituted by much shorter X rays. Length of those rays, as was pointed out by Max von Laue (1879-1960), is approximately the same as the distance between crystal layers. Hence a crystal can constitute a diffraction grid for X rays.
    Soon afterwards, in 1912, Friedrich and Knipping passed beams of X rays through crystals. They discovered a pattern of dark spots on a photographic plate placed behind a small crystal.
    The pattern of spots discovered by Friedrich and Knipping was theoretically explained by William Henry Bragg. He analysed reflections of many waves of different lengths from each of numerous atomic surfaces in the crystal. On the basis of those theoretical calculations it was then possible to investigate precisely the internal structure of crystals.
    The discovery of X rays must be listed among major discoveries of the late 19th century. Those rays found immediate use in various branches of science. Precise description of their properties allowed scientists to take advantage of them in further inquiries into the microcosm.