ScienceWeek

X-RAYS AND X-RAY DIFFRACTION

There are several popular distortions concerning the history of the discovery of x-rays, one of which concerns the origin of the "Crookes tube", and the other of which concerns the scientific talents of the discoverer of x-rays, Wilhelm Roentgen (1845-1923).

Concerning the Crookes tube, the term "cathode rays" refers to streams of electrons emitted at the cathode in an evacuated tube containing a negative electrode (cathode) and a positive electrode (anode) that are part of a closed circuit containing a source of electromotive force. Cathode-ray investigations began in 1854 when Heinrich Geissler, a glassblower in the laboratory of the physicist Julius Pluecker (1801-1868) improved the vacuum tube. Pluecker discovered cathode rays in 1858 by sealing two electrodes inside a vacuum tube, evacuating the air, and forcing electric current between the electrodes. He observed a green glow on the wall of the glass tube and he attributed the glow to rays emanating from the cathode.

In 1869, with improved vacuums, Pluecker's student Johann W. Hittorf (1824-1914) (of later fame as an electrochemist) saw a shadow cast by an object placed in front of the cathode. The shadow proved that the cathode rays originated from the cathode. The physicist and chemist William Crookes (1832-1919) investigated cathode rays in 1879 and discovered the rays were bent by a magnetic field, with the direction of deflection suggesting the rays were negatively-charged particles. As the luminescence of the rays did not depend on what gas had been in the vacuum or on what metal formed the electrodes, Crookes suggested the rays were a property of the electric current itself. As a result of Crookes's work, investigations of cathode rays intensified, and the evacuated tubes came to be called "Crookes tubes". But if any name should be associated with evacuated glass tubes containing two electrodes, perhaps the deserved name would be "Pluecker tubes".

Concerning the scientific talents of Wilhelm Roentgen, it is a popular myth that Roentgen was a second-rate scientist lifted from obscurity by a completely accidental discovery. Details of the discovery are provided below, but concerning Roentgen's talents, he was apparently one of the best experimental physicists of his generation. Apart from his discovery of x-rays (for which he received the first Nobel Prize in Physics in 1901), Roentgen is perhaps best known for his experiments verifying the prediction of Oliver Heaviside (1850-1925) that magnetic effects would be produced by a dielectric rotated rapidly between the plates of a charged condenser. Roentgen also made a classic determination of the ratio of specific heats of gases, and investigated pyro- and piezo-electrical phenomena.

It is instructive to note that Crookes had actually observed x-rays before Roentgen, but Crookes was not astute enough to realize what he was looking at. Roentgen was an extremely sharp experimentalist, and he immediately realized the significance of his discovery. It is a tribute to Roentgen's mastery of experimental physics that most of the currently accepted basic properties of x-rays were described in the series of papers (1895-1897) in which his discovery was first announced. Roentgen's discovery provoked intense interest, and in 1896 over 400 papers on x-rays were published by other physicists. Roentgen won the Rumford Medal in 1896 and the Nobel Prize five years later. But he refused ennoblement by the king of Bavaria, and he made no attempt to patent any aspect of x-ray production or to make any financial gain from his work. He maintaining that his discovery should be used for the benefit of mankind. When he died in 1923 at the age of 78, Roentgen was completely impoverished and suffering from the effects of radiation disease due to years of x-ray exposure.

In Roentgen's first paper (Sitzungsber. der Wuerzburg. Ges. 1895; Ann. Physik. 64:1 1898), he reported the following observations concerning x-rays:

1) All substances are more or less transparent to x-rays. For example, wood 2 to 3 centimeters thick is very transparent. Aluminum 15 millimeters thick "weakens the effect considerably, although it does not entirely destroy the fluorescence." Lead glass is quite opaque, but other glass of the same thickness is much more transparent. "If the hand is held between the discharge tube and the screen, the dark shadow of the bones is visible within the slightly dark shadow of the hand."

2) Many other substances besides barium-platino-cyanide fluoresce: calcium compounds, uranium glass, rock salt, etc.

3) Photographic plates and films "show themselves susceptible to x-rays." Hence, photography provides a valuable method of studying the effects of x-rays.

4) X-rays are neither reflected nor refracted (so far as Roentgen could discover). Hence, "x-rays cannot be concentrated by lenses."

5) Unlike cathode rays, x-rays are not deflected by a magnetic field. They travel in straight lines (as Roentgen demonstrated by pinhole photographs).

6) X-rays discharge electrified bodies, whether the electrification is positive or negative.

7) X-rays are generated when the cathode rays of the discharge tube strike any solid body. A heavier element, such as platinum, however, is much more efficient as a generator of x-rays than is a lighter element such as aluminum.

In terms of modern physics, x-rays comprise electromagnetic radiation of wavelengths shorter than the ultraviolet, with x-rays produced by bombardment of atoms by high-quantum-energy particles. The range of x-ray wavelengths is 10^(-11) meters to 10^(-9) meters. Atoms of all the elements emit a characteristic x-ray spectrum when they are bombarded by electrons, the x-ray photons being emitted when the incident electrons knock an inner orbital electron out of an atom. When this occurs, an outer electron falls into the inner shell to replace the lost electron, the outer electron losing potential energy. The wavelength of the emitted photon will be given by l = ch/(deltaE), where (l) is the wavelength of the emitted photon, (c) is the velocity of light, (h) is Planck's constant, and (deltaE) is the potential energy loss.

The term "x-ray diffraction" refers to the diffraction of x-rays by a crystal. The wavelengths of x-rays are comparable in size to the distances between atoms in most crystals, and the repeated pattern of the crystal lattice acts as a diffraction grating for x-rays.

The term "x-ray powder diffraction" refers in general to x-ray diffraction of a collimated monochromatic beam by a sample (a powder) containing a large number of tiny crystals having random orientations.

The term "x-ray crystallography" refers to the use of x-ray diffraction to determine the structure of crystals or molecules. In general, the technique involves directing a beam of x-rays at a crystalline sample and recording the diffracted x-rays on a photographic plate. The x-ray diffraction pattern consists of a pattern of spots on the plate, and the crystal structure can be determined from the positions and intensities of the diffraction spots. Since x-rays are diffracted by the electrons in a molecule, if molecular crystals of a compound are used, the electron density distribution in the molecule can be determined.

X-ray diffraction is currently one of the most important tools of solid-state physics and chemistry, the technique used for differentiation between crystalline and amorphous materials, determination of the structure of crystalline materials, determination of electron distributions within atoms and throughout unit cells of a crystal, determination of the orientation of single crystals, determination of the texture of polygrained materials, etc. In the biological sciences, x-ray diffraction has contributed greatly to our understanding of processes occurring in living cells, with the technique revealing the structures of proteins and nucleic acids and their constituent monomers, and also of drugs, hormones, and vitamins. Knowledge of 3-dimensional structures of various biomolecules has had a profound impact on the whole of biology.

The following points are made by Ron Jenkins (J. Chem. Educ. 2001 78:601):

1) In 1888 Roentgen became head of the physics department in the University of Wurzburg, and like many other scientists of his day, he studied the visible light emitted in a closed glass tube (Crookes tube) under reduced pressure when high voltage is applied. On November 8, 1895, Roentgen wrapped his glass tube in black paper to prevent escape of the light, and darkened the room. He used fluorescent screens in his experiments, and to his great surprise, when voltage was applied across the glass tube, the fluorescent screen showed flashes of light. Roentgen attempted to block the fluorescent screen with his hand, and he then saw the shadow of his hand upon the screen. He soon discovered that the radiation had great penetrating power, was absorbed by lead, and could blacken a photographic plate. On the 28th of December 1895, Roentgen published his first paper, in which he included a radiograph of the hand of his wife. The impact of his paper was enormous.

2) The main application of x-rays at the end of the 19th century was in radiography, and several German organizations, notably Siemens, began the commercial development of x-ray tubes and high-voltage generators. In 1912, there was much discussion at the University of Munich about whether x-radiation could best be described as wave or particle radiation. Max von Laue (1879-1960) wagered a bet that x-rays were electromagnetic waves that could be refracted by crystals. Two of von Laue's students then obtained a diffraction pattern from a single crystal. Soon after that, von Laue proposed a theory to describe the conditions for diffraction, and in England, the Braggs, William Henry Bragg (1862-1942) and William Lawrence Bragg (1890-1971), father and son, began their work on crystal structure analysis.

3) Using a much improved x-ray tube that produced a significant amount of characteristic radiation, and an ionization chamber as a detector, the Braggs demonstrated that x-rays are diffracted only in specific directions. The elder Bragg derived a simple equation relating the diffraction angle to the diffracted wavelength and the interplanar spacing of the crystal. The development of various x-ray methods followed rapidly between 1914 and 1937, including the beginning of the use of powder diffraction in 1916.

--------------------------------

MAX VON LAUE AND THE INTERACTION OF LIGHT AND MATTER

The principle governing the interaction of light and matter may be stated in simple terms: matter interferes with radiation when the atomic or sub-atomic dimensions (such as the distance between the atoms in a crystal, or the wavelength characterizing the motion of an extra-nuclear electron) are commensurate with the wavelength of the incident radiation. We may compare the surface of a crystalline solid to a series of turnstiles, spaced at equal distances, and leading, let us say, from one enclosure to another. The incident light may be compared to an orderly army advancing towards them. If the soldiers were Lilliputians, they would pass through the turnstiles without turning them, or affecting them in any other way; if they were giants, they would pass over and ignore them. But if the advancing host consisted of human beings, for the hindrance of whom the gates were erected, there would clearly be interference with their march at the boundary; some would be let through, some would not, while others would come out having turned round once inside the revolving gates.

The gamma-rays emitted by radioactive elements will pass through several centimeters of lead; and cosmic rays, which have even shorter wavelengths (possibly of a different kind), will penetrate the Earth to depths of hundreds of feet. Wireless waves, which are of great length, ignore material interference, unless provoked. We owe to Max von Laue (1879-1960) the suggestion that x-rays have a wavelength of the same order of magnitude as the distance between atoms in crystals, and the first demonstration that this distance can be calculated from the interference produced.

Adapted from: E.A. Moelwyn-Hughes: Physical Chemistry. 2nd Edition. Pergamon Press 1961, p.18.

Editor's note: Max von Laue (1879-1960), who essentially founded x-ray crystallography, first studied the interaction of x-rays and crystals in order to establish the wavelengths of x-rays. There were no techniques available to manufacture the fine grating required for such a measurement, and Laue hit on the idea of using a crystal of zinc sulfide as a grating. He was awarded the Nobel Prize in Physics in 1914 for his work on x-ray diffraction in crystals. Subsequently, by using x-rays of known wavelength, it became possible to study the atomic structure of various crystals, including polymers, where such structure was unknown. The metaphor used by Moelwyn-Hughes in his classic text, the image of an advancing army, is apt: Max von Laue was the son of an army official and spent his youth moving from one army post to another. Laue obtained his PhD in theoretical physics in 1903. Between 1905 and 1909, Laue worked as assistant to Max Planck (1858-1947), and they evidently established a close friendship.