X-rays were discovered by Wilhelm Konrad Roentgen (1845-1923) in 1895 at the University of Wursburg where he was professor of physics. During the Christmas week of that year he announced his discovery which aroused a great interest among laymen and scientists alike and stimulated a flurry of research activity. For the important discovery of these rays, which are also known as "Roentgen rays," he received the first Nobel prize in physics in 1901. Roentgen made his important discovery accidentally while studying the electrical discharges in low pressure gas discharge tube, called a cathode-ray tube. He observed that a paper screen coated with the fluorescent substance barium platino-cyanide would glow brightly with a greenish light in a darkened room at a distance of as great as two meter from the cathode-ray tube. The fluorescence could be observed even when the tube was covered with tightly fitting thin black cardboard. When objects were placed between the tube and the fluorescent screen, the intensity of the glow could be diminished and Roentgen concluded that rays emanated from the tube. And when their paths could not be affected by magnet, he concluded that the rays were not cathode-rays or electrons. Further experiments showed that the rays could penetrate substances of low density but opaque to ordinary light such as wood, paper, hard rubber, and thin sheets of aluminum. On the other hand, dense substances such as lead were opaque to the rays and hence it could be used as a shield to block off the rays. Roentgen also found that photographic paper was sensitive to the rays and would blacken when exposed to them. He used sheets of lead to protect the photographic paper until he wished to expose them. If the human hand were held between the discharge tube and the screen, Roentgen found that the darker shadow of the bones could be seen within the less intense shadow of the hand. Photographs of this effect produced a public sensation, and within a few weeks of his first announcement, physicians were using such photography for diagnostic purposes to set broken boned and to locate bullets.
Roentgen carried out a carefully planned sequence of experiments to determine the nature of the rays. Since the rays cast shadows of dense objects on a fluorescent screen, they must travel in straight lines. However, Roentgen was unable to produce reflection, refraction, diffraction or polarization which are common properties of light and other wave motion. On the other hand, he was unable to deflect them either by an electric or a magnetic field. This was taken to mean that they did not consist of charged particles. Being in doubt then about the nature of the emanation, Roentgen called them x-rays, or unknown rays. And the name stuck even after their nature became known.
In his earliest experiments Roentgen discovered that x-rays caused the discharge of an electroscope or other electrified bodies. The discharge was more rapid when the x-rays were more intense. Roentgen was led to suspect that the discharge occurred because the x-ray made the air around the charged body conductive. Further experiments showed this hypothesis to be true. This hypothesis could be explained on the further hypothesis that the x-rays produced ions (electrically charge atoms or molecules) within the gas and that migration of these ions accounts for the conductivity of the gas. This ionization of gases by x-rays was the first direct evidence that an atom is a complex structure and that electrical charges are constituents in that structure.
In his earliest experiments Roentgen found that the x-rays came from the spot on the wall of the glass discharge tube which fluoresces the strongest, that is, where the cathode rays strike the glass wall. If the cathode rays within the discharge tube were deflected by means of a magnet, the x-rays were observed to come from the new spot where the cathode rays struck the glass wall. He also observed that the x-rays could be produced in a metal plate enclosed in the discharge tube. Modern x-ray tubes use such a metal target as the anode to produce the x-rays. Early x-ray tubes used an induction coil (spark coil) to produce a high potential between the cathode and anode. The cathode-ray electrons were pulled from the cathode (cold cathode) and accelerated toward the anode by this high potential. The x-ray were produced by the cathode-ray electrons as they struck the target anode. The modern high vacuum Coolidge-type tube, which was introduced about 1913, uses a heated filament (hot cathode) as the source of the cathode-ray electrons (thermionic emission). The target is made of a metal of high melting point because of the large quantities of heat generated by the electrons striking the target anode; it is usually made hollow to allow a coolant such as water or oil to be circulated through it. The source of high potential is a high voltage power supply. The potential used depends upon the application of the x-rays, but usually ranges between ten thousand and a million volts.
Roentgen was unable to determine the nature of x-rays which he indicated by naming them x-rays. He did eliminate some possibilities. Since x-rays are not deflected by electric or magnetic fields, they could not be charged particles. On the other hand, since they could not be perceptibly refracted or diffracted, they were not ordinary light. Roentgen proposed that x-rays might be the "longitudinal component" of light. This possibility was eliminated in 1905 when an English physicist, Charles Barkla (1877-1944), found that the rays could be polarized. Thus by 1912 it was believed that x-rays were electromagnetic waves with wavelength very much shorted than light. Wien suggested in 1907, applying Einstein's interpretation of the photoelectric effect to electrons ejected from metals by x-rays, that the wavelength of x-rays is in the order of 0.3 Angstrom. Seizing upon this suggestion that x-ray wavelengths seem to be in the order of tenths of an Angstrom, the German physicist Max von Laue (1879-1960) proposed to use the highly regular arrangement of atoms of a single crystal as a three dimensional diffraction grating. It had long been assumed that characteristic shapes and sharp cleavage planes of crystalline materials were the result of the highly regular arrangement of atoms or molecules in a lattice structure with fixed planes of atoms. Laue thought that since the distance between the layers or planes of atoms were of the order of a few Angstroms, they might act to give an effect similar to that produced by two optical gratings placed over one another so that their lines are at right angles and thus form an array of square openings. When a pencil beam of light falls upon them, it would produce a symmetrical pattern of spots. At Laue's suggestion the experiment was performed by two of his students, Friedrich and Knipping, and the results were announced in July of 1912. A narrow pencil beam of x-rays were passed through a crystal of zincblende (ZnS) for several hours exposing a photographic plate to the emerging rays. Friedrich and Knipping found on the photographic plate numerous faint spots lying a symmetrical pattern about the central spot due to the beam. Calculation of the wavelength of the x-ray was complicated by the fact that a crystal would not act as a two-dimensional grating but would constitute a kind of three-dimensional grating and furthermore that atomic layers could be thought of as existing in many planes. But using a series of photographs taken with the crystal oriented at various angles and using estimates of the atomic spacing in the various atomic layers in the crystal, they calculated that there were present in the x-ray beam wavelengths varying from 0.13 to 0.48 Angstroms.
The importance of these experiments was immediately recognized and Laue received the Nobel prize in physics in 1914. These x-ray diffraction experiments were the conclusive evidence of the wave nature of x-rays. Also it reinforced the hypothesis that the geometrical properties of crystals are the result of the regular arrangement of the atoms in a three-dimensional lattice structure. In addition, it opened up two new fields of study: the study of x-ray spectra of various target materials and the study of the crystal structure of solids.
Later in 1912 a simpler method of x-ray diffraction was devised by English physicists (father and son) W.H. Bragg (1862-1942) and W.L. Bragg (1890-1971). Instead of observing the effect of a pencil beam of x-rays as they passed through the crystal, the Braggs considered the diffuse scattering of a narrow flat beam of x-rays off the cleavage face of a crystal. When a x-ray wave front of this flat beam falls on such a face each atom absorbs the radiation and then becomes a source to radiate the x-rays in all directions (diffuse scatterings). In general, these waves from the individual atoms combine destructively. However, if two conditions are fulfilled simultaneously they will combine constructively. The first Bragg condition is that these waves combine destructively in all directions except in the direction of the rays whose scattering angle is equal to the glancing angle of the incident waves. It is customary to indicate the direction the incident beam with respect to the cleavage face of the crystal by giving the glancing angle θ, the angle between the direction of the incident beam and the cleavage face of the crystal upon which it is incident. (Note that this is not the angle of incidence which is between the direction of the incident beam and the normal to the surface upon which it is incident.) Only those scattered rays whose angle (scattering angle) with respect to the cleavage face of the crystal is equal to the glancing angle of the incident beam will emerge from the surface of the crystal. All others will be canceled by the destructive combination. This first condition is similar to the law of reflection in optics. This has led some to incorrectly refer to this phenomena as Bragg "reflection." This first condition is not enough; a second Bragg condition must be fulfilled in order that the scattered waves combine constructively. The Braggs considered the atoms of the crystal as arranged into different sets of parallel planes now called Bragg planes with different interplanar spacing d. The layer of atoms in the surface of cleavage face of a crystal and successive parallel layers of atoms below it constitute one of these sets of Braggs planes.
Two rays of the narrow flat beam of x-rays enter from the upper
left, making a glancing angle θ with respect to each of the Bragg
planes. Note that the rays scattered from the second layer must
travel a greater distance than those from the first layer. In
order for the rays scattered from the second plane to combine
constructively or reinforce the ray from the first plane, it is
necessary that this additional distance shall be some integral
multiple of the x-ray wavelength or mλ where m
is an integer. If we construct the lines AE and EC perpendicular
to the direction of the incident and scattered rays respectively,
then we find that each of these lines makes an angle θ with
respect to the line EB whose length is the interplanar spacing d.
The additional length of the ray scattered from the second layer is
AB plus BC, each of which is equal to d sin θ. Hence,
the second Bragg condition is
mλ = 2d sin θ, where m = 1,2,3...
and is the order of the spectrum.
This relation is known as Bragg's Law.
The device employing Bragg's method of x-ray diffraction is called
an x-ray spectrometer. X-rays from the tube are formed or collimated
into a flat narrow beam by the two sheets of lead which absorb
all rays except those which pass through the narrow rectangular
slits in them. This beam falls on a crystal which can be rotated
about an axis parallel to the slits and perpendicular to the plane
of the figure. The glancing angle between the cleavage face of
the crystal and the direction of the x-ray beam can be measured.
Since Bragg scattering can take plane constructively only in
a direction 2θ from the direction of the incident beam, a detector
(a fluorescent screen, photographic film or ionization chamber)
can detect the scattered rays in only one direction. Whether
or not the scattered rays constructively reinforce one another
in this direction depends upon whether the second Bragg condition
is satisfied. If the interplanar spacing d is known, it
is possible to calculate by Bragg's law the wavelength of the
x-rays scattered at any particular angle θ. For a cubic crystal
the interplanar spacing may be computed by the following formula:
d = 3√[M / (2N0ρ)],
where M is the gram molecular weight of the substance,
N0 is Avogradro's number and ρ is the mass
density of the substance.
The Braggs, using a rock-salt crystal and an ionization chamber detector in order to measure the intensity at different wavelengths, studied the spectrum of x-rays coming from a platinum target. Laue and his associates had thought that the spectrum was discrete or bright line. The Braggs, however, found that it was continuous with several very intense bright lines superimposed on the continuous background. Almost at the same time H.G.J. Moseley (1887-1915) at Manchester University, England, observed the same features of the x-ray spectrum. A few years earlier (1908) Barkla and his coworkers made detailed measurements of the absorption coefficients (a measure of the ability of a material to absorb x-rays) of the characteristic x-radiation from various elements used as targets in the x-ray tube. They found that the elements have, in general, two types of characteristic radiation which differ greatly in their absorption coefficients. These he called the K- and L-radiation; the K-radiation are more penetrating (hard rays) and the L-radiation much less penetrating (soft rays). Moseley identified the bright lines of the x-ray spectra with Barkla's K- and L-radiation. Later more sensitive detectors found additional series of bright lines (denoted as M, N, etc.) at longer wavelengths. The wavelengths of the K, L, M, ... lines are characteristic of the target element in the x-ray tube. In contrast to the complicated optical (visible) spectra of the elements, the x-ray spectra of elements are quite simple and are very similar for different elements.
The continuous spectrum has a minimum wavelength m below which
there is no x-rays. This wavelength is independent of the target
element. Duane and Hunt in 1915 found that this minimum wavelength
was universally proportional to the accelerating potential between
the cathode and anode of the x-ray tube. This is called Duane
and Hunt's Law and is given by the following formula:
λm = 1.2396 × 10-6 / V,
where λm is measured in meters and V is
measured in volts.
Moseley in 1913 and 1914 made a systematic investigation of the
bright line spectra and found a simple relation, known as Moseley's
Law, between the frequency of a given line for successive
elements and the atomic number of the elements. The atomic number
Z of an element is an integer which gives the ordinal position
of that element in the periodic table; thus for hydrogen, Z = 1;
for oxygen, Z = 8; for copper, Z = 29; and for uranium
Z = 92. Moseley found that x-ray lines shifted progressively
to shorter wavelengths as the atomic number of target element increased.
However the sequence was not perfect; he found gaps and wavelengths
out of order. Moseley considered the progression of wavelength
more important than the atomic weights and therefore inverted
the order of the elements in the periodic table. Compare, for
example, the elements nickel and cobalt; the atomic weight of
cobalt is less than that of nickel, but the progression of wavelengths
places Co before Ni in the periodic table. Other inversions of
order in the periodic table which were made in order to place
the elements in the right family were justified on the basis of
the progression of x-ray spectra wavelength (for example, Te and
I; and Ar and K). Moseley attributed the gaps in the sequence
to undiscovered elements. Moseley's law is given by the following
equation:
f = a2(Z - b)2.
For the K series b = 1 and a2 =
2.48 × 1015 cycles per second for K line,
the first line of the K series. Thus the frequency f
of a given line, say the K line, is proportional to
(Z - b)2 in the regular progression
through the periodic table.