By studying the interaction between X rays and matter, Charles Glover Barkla succeeded in determining important physical characteristics of X rays and the atomic structure of matter.

For several decades in the nineteenth century, physicists studied cathode rays. Cathode rays For an even longer time, scientists had known of the existence of atoms. Nevertheless, during the last decade of the nineteenth century, scientists still had great difficulty in comprehending the physical nature of either cathode rays or atoms. Apparently, atoms of some chemical elements were heavier and others were lighter. It was not known why. The reason could have been that atoms consisted of different materials or that the heavier ones had more of the same materials. Chemical facts gave clues about the existence of atoms. Cathode rays appeared to be “tentacles” originating from the atom. X rays;physical characteristics
[kw]Barkla Discovers the Characteristic X Rays of the Elements (1906)
[kw]X Rays of the Elements, Barkla Discovers the Characteristic (1906)
[kw]Rays of the Elements, Barkla Discovers the Characteristic X (1906)
[kw]Elements, Barkla Discovers the Characteristic X Rays of the (1906)
X rays;physical characteristics
[g]England;1906: Barkla Discovers the Characteristic X Rays of the Elements[01460]
[c]Science and technology;1906: Barkla Discovers the Characteristic X Rays of the Elements[01460]
[c]Physics;1906: Barkla Discovers the Characteristic X Rays of the Elements[01460]
Barkla, Charles Glover
Röntgen, Wilhelm Conrad
Stokes, George Gabriel
Thomson, Joseph John

In December, 1895, a sequence of clues, and tentacles, began to emerge. First, Wilhelm Conrad Röntgen reported from the University of Würzburg that he had discovered X rays. In 1896, Antoine-Henri Becquerel Becquerel, Antoine-Henri announced to the French Academy of Sciences his finding that uranium spontaneously emitted invisible radiation that would blacken a photographic plate yet seemed different from X rays. In 1898, Pierre Curie Curie, Pierre and Marie Curie Curie, Marie detected two new elements that apparently also emitted the same types of radiation. At the same time, in England, Sir Joseph John Thomson made remarkable progress in the study of cathode rays. He startled scientists by announcing his experimental confirmation that these rays consisted of charged particles more than one thousand times lighter than the smallest atom. He declared that these particles were “matter in a new state” and that the chemical elements were made up of matter.

By the beginning of the twentieth century, therefore, scientists were confronted with a number of intriguing questions about the nature of X rays, how one could account for radioactivity, and how one could reconcile the apparent endlessness of radioactive emanations with the conservation of energy. It was in this atmosphere of challenging scientific inquiry that Charles Glover Barkla began his scientific career. From 1899 to 1902, he conducted research with Thomson. In 1902, he attended University College in Liverpool and began his lifelong study of X rays.

The veteran mathematician and physicist George Gabriel Stokes proposed the “ether pulse” theory Ether pulse theory of X rays about the nature of X rays. He hypothesized that X rays are irregular electromagnetic pulses created by the irregular accelerations of cathode rays when they are stopped by the atoms in the target of the X-ray tube. Using this theory, Thomson derived a mathematical formula expressing the scattering of X rays by electrons. Barkla’s first research project was to test Stokes’s and Thomson’s theories experimentally.

Charles Glover Barkla.

(Library of Congress)

Five years previously, Georges Sagnac Sagnac, Georges had experimented in France on the absorption of X rays by solids—a phenomenon that is directly related to scattering. Sagnac found that the secondary scattered radiation was of distinctly greater absorbability. Barkla showed that the secondary radiation from light gaseous elements was of the same absorbability as that of the primary beam. He worked on air first, then extended the investigation to hydrogen, carbon dioxide, sulfur dioxide, and hydrogen sulfide. He used an electroscope to test for the presence of the secondary radiation, based on the assumption that the amount of ionization should be proportional to the intensity of the radiation passing through the instrument. To check the absorbability of the rays—primary and secondary—Barkla used a thin aluminum plate. He published the results in 1903. At the time, the fact that scattering did not modify the absorbability of the radiation appeared to be strong support for the ether pulse theory. From the same set of experiments, Barkla demonstrated that “this scattering is proportional to the mass of the atom.” This was a highly satisfying result because it supported the theory that the atoms of different substances are different systems of similar corpuscles, where, in the atom, the number is proportional to its atomic weight. These similar corpuscles, according to most physicists of the time, constituted Thomson’s “matter in a new state.”

In 1904, Barkla began a new series of experiments that would disclose additional physical characteristics of X rays. Because ordinary light, as the propagation of electromagnetic oscillations, is a transversal wave, it can be polarized relatively easily: When it is scattered in the direction at right angles to the incident (primary) beam, the transverse vibrations constituting the light are confined to a plane perpendicular to the primary beam. Barkla was researching the question of whether X rays could be polarized in the same way so that they could be confirmed as electromagnetic waves. This proved to be a serious challenge. It took Barkla two years to perform the difficult experiment and arrive at a clear conclusion that the scattered beam was highly polarized; thus X rays were most probably transversal waves, like ordinary light.

While investigating the intensity in different directions of the secondary radiation, Barkla found that light elements, such as carbon, aluminum, and sulfur, showed marked variation in intensity with direction; calcium showed much less. With iron and even heavier elements, there was practically no difference in intensity in different directions. This salient phenomenon led Barkla to investigate more closely the relation between atomic weight and absorbability. His experiments showed that for light elements, the scattered radiation closely resembled the primary radiation, but for elements heavier than calcium, the scattered radiation was quite different from the primary. When Barkla examined the scattered (secondary) radiation more closely, he found that the secondary radiation from metals contained not only scattered radiation of the same character as the primary but also homogeneous radiation that was characteristic of the metallic element itself.

Meanwhile, Barkla also discovered an X-ray phenomenon that was analogous to a discovery made by Stokes: Fluorescent substances fluoresced only when exposed to light of shorter wavelength than that of the fluorescent light emitted by the substances. This is known as Stokes’s law. Stokes’s law[Stokess law] Barkla found that the emission of the homogeneous (secondary) radiation occurred only when the incident X-ray beam was harder than the characteristic radiation itself. Moreover, he found some revealing facts about the homogeneous characteristic radiations. Beginning with calcium and moving toward the heavier elements, the characteristic X-ray radiations form one of two series. From calcium (atomic weight 40) to rhodium (atomic weight 103), there appeared a K series; from silver (atomic weight 108) to cerium (atomic weight 140), there appeared a K series and an L series; from tungsten (atomic weight 184) to bismuth (atomic weight 208), there appeared an L series only. K radiations were softer; L radiations, harder. The heavier the atom, the harder its characteristic radiations. Such phenomena, closely correlating atomic weight to characteristic X rays, showed that the latter must have originated from the atom. In fact, Barkla’s discoveries anticipated the assignment to each chemical element an atomic number, which, in general, was recognized as about one-half the element’s atomic weight.

Following these discoveries in 1906, Barkla and other physicists researched interactions between X rays and matter and achieved historic results. These achievements, accomplished from 1909 to 1923, may be categorized in three stages: First, X rays were found to interact with crystal lattices. In 1909, Max von Laue Laue, Max von attended the University of Munich and was influenced by Röntgen and mineralogists who informed him of theories on the structure of crystal solids. Von Laue proceeded to combine the study of X rays with the study of solid structures. He developed a mathematical theory based on the assumption that crystal lattices could serve as “diffraction gratings” (instruments used in optical experiments that demonstrate the wave character of light) for X rays. This idea was experimentally confirmed, and that confirmation has been highly praised, as it opened vast potentials for studying the nature of X rays and the structure of crystal solids. Shortly after von Laue’s publication of his work in 1912, William Henry Bragg Bragg, William Henry and his eldest son, Lawrence Bragg, Bragg, Lawrence founded the science of crystallography. Crystallography In particular, William Henry Bragg created the ionization spectrometer for measuring the exact wavelengths of X rays; Lawrence Bragg derived the influential equation now named after him. The Bragg equation tells at what angles X rays will be most efficiently diffracted by a crystal layer.

The second stage of Barkla’s achievements was the discovery that X rays interact with atoms, especially heavy atoms. Henry Moseley Moseley, Henry used the Bragg spectrometer soon after its introduction to study the characteristic X rays from the atom. With the new, powerful instrument, Moseley turned to Barkla’s line of investigation. Moseley could now measure Barkla’s K series and L series with exactness. Significantly extending such measurements, he made wonderful discoveries that led to what has since been called Moseley’s law: Moseley’s law[Moseleys law] the mathematical formula that relates the X-ray spectrum of an element to its atomic number. Moseley also made a series of verifiable predictions about the periodic table of elements. Tragically, Moseley was killed in World War I. Later, Karl Manne Georg Siegbahn took up the study of X-ray spectroscopy and its interpretation, work for which he was awarded the Nobel Prize in Physics in 1924. Nobel Prize recipients;Karl Manne Georg Siegbahn[Siegbahn]

The third stage in Barkla’s achievements was the finding that X rays interact with light atoms—that is, free electrons. Barkla was awarded the Nobel Prize in Physics in 1917, Nobel Prize recipients;Charles Glover Barkla[Barkla] and when he delivered his Nobel lecture in 1920, he declared that in the phenomena of scattering, there is strong positive evidence against any quantum theory. Three years later, in 1923, Arthur Holly Compton Compton, Arthur Holly was to prove the folly of Barkla’s statement. He followed Barkla in experimenting on the comparison of secondary X rays with primary X rays, especially when the former were scattered from light atoms. Compton experimented with the spectrometer and theoretically with the concept of photons; he was awarded the Nobel Prize in Physics 1927 for his discovery of the change in wavelength of scattered X rays. Nobel Prize recipients;Arthur Holly Compton[Compton]
X rays;physical characteristics

• Allen, H. S. “Charles Glover Barkla, 1877-1944.” Obituary Notices of Fellows of the Royal Society of London 5 (1947): 341-366. A substantial biography written by a colleague Barkla. Includes a complete list of Barkla’s publications. Suitable for nonspecialists.
• 
  Beam Line: A Periodical of Particle Physics 25 (Summer, 1995). http://www.slac.stanford.edu/pubs/beamline/pdf/95ii.pdf. Special issue titled “One Hundred Years of X Rays” is devoted to discussion of the history and uses of X rays. Articles include “Early History of X Rays,” “Medical Applications of X Rays,” and “The X-Ray Universe.” Features photographs, diagrams, and reproduced newspaper articles from the late nineteenth and early twentieth centuries. (Beam Line is published by the Stanford Linear Accelerator Center.)
• Forman, Paul. “Charles Glover Barkla.” In Dictionary of Scientific Biography, edited by Charles Coulston Gillispie. New York: Charles Scribner’s Sons, 1970. This brief biography is forthright in making critical comments on Barkla’s scientific outlook and attitude. Valuable for nonspecialists because of its sharp and accurate comments.
• Heathcote, Niels Hugh de Vaudrey. Nobel Prize Winners in Physics, 1901-1950. New York: Henry Schuman, 1953. Although the “physics” included in this book covers fifty years, the subject matter bears close relevance to Barkla’s discoveries.
• Michette, Alan, and Sawka Pfauntsch, eds. X-Rays: The First Hundred Years. New York: John Wiley & Sons, 1996. Collection of essays published in commemoration of the one hundredth anniversary of Röntgen’s discovery of X rays. Reviews the history of scientific work related to X rays as well as modern applications. Includes an extensive glossary.

Röntgen Wins the Nobel Prize for the Discovery of X Rays

Becquerel Wins the Nobel Prize for Discovering Natural Radioactivity

X-Ray Crystallography Is Developed by the Braggs

Discovery of the Compton Effect