An X-ray (German: Röntgenstrahlen) is a form of electromagnetic radiation with a wavelength approximately in the range of 5 pm to 10 nanometers (corresponding to frequencies in the range 30 PHz to 60 EHz). X-rays are primarily used for diagnostic medical imaging and crystallography. X-rays are a form of ionizing radiation and as such can be dangerous.
X-rays with a wavelength longer than 0.1 nm are called soft X-rays. At wavelengths shorter than this, they are called hard X-rays. Hard X-rays overlap the range of long-wavelength (low energy) gamma rays, however the distinction between the two terms depends on the source of the radiation, not its wavelength: X-ray photons are generated by energetic electron processes, gamma rays by transitions within atomic nuclei.
The detection of X-rays is based on various methods. Most commonly known is the photographic plate, well known from its use in hospitals. The X-rays blacken the photographic plate (negative), it is black where the X-rays go through ("soft" parts of the body like organs and skin) and white where the X-rays are stopped ("hard" parts like bones, or contrast product containing iodine injected in blood). Another method is to use a fluorescent plate, e.g. sodium iodide NaI. These methods give no information about the energy of the X-ray photons, just their spatial density.
Initially, most common detection methods were based on the ionisation of gases, as in the Geiger-Müller counter: a sealed cylinder with a polymer window contains a gas, and a wire, and a high voltage is applied between the cylinder (cathode) and the wire (anode). When a X-ray photon enters the cylinder, it ionises the gas which becomes conducting, creating a current flow (a kind of flash); this peak of current is detected and is called a "count".
When the high voltage between anode and cathode is decreased, the detector is no longer saturated, and the height of the current peak is proportionnal to the energy of the photon; it is thus called a "proportional counter". Most of times, the cylinder is not sealed but is constantly fed with "fresh gas", is thus called a "flow counter". This proportionality property allows filtering the "interesting" peaks from the noise and other photons, but the resolution in energy is not enough to determine the energy spectrum; such a feature requires a diffracting crystal to first separate the different photons, the method is called wavelength dispersive X-ray spectroscopy (WDX or WDS).
Some materials such as NaI can "convert" a X photon to a visible photon; an electronic detector can be build by additing a photomultiplier. These detectors are called "scintillators" or "scintillation counters".
Since the 1990s, new detectors based on semiconductors were developed (silicon or germanium doped with lithium, Si(Li) or Ge(Li)). X-ray photons are converted to electron-hole pairs in the semiconductor, and are collected to detect the X-rays. When the temperature is low enough (the detector is cooled by Peltier effect or best by liquid nitrogen), it is possible to directly determine the X-ray energy spectrum; this method is called energy dispersive X-ray spectroscopy (EDX or EDS); it is often used in small X-ray fluorescence spectrometers. These detectors are often called "solid detectors".
Since the discovery by Röntgen (see below) that X-rays can identify bony structures, X-rays have been developed for their use in medical imaging. Indeed, this is probably the most common use of X-ray technology.
The use of X-rays is limited to the detection of lesions in bone, and they are generally unsuitable for detecting pathology in soft tissue, such as brain and muscle. Notable exceptions are the very common chest X-ray, which can identify lung disease (pneumonia, lung cancer, pulmonary oedema), and the plain abdominal film, which can detect ileus (blockage of the intestine), free air (in visceral perforations) and flee fluid (in ascites). In some cases, the use of X-rays is debatable, such as gallstones (which are rarely radioopaque) or kidney stones (which are often visible, but not always).
Imaging alternatives for soft tissue problems are computed axial tomography (CAT or CT scanning) and magnetic resonance imaging. The former relies on computed X-ray measurements in reconstruction three-dimensional images, while the latter uses other physical principles altogether. Ultrasound, another medical imaging modaltiy, relies on sound waves rather than electromagnetic radiation for diagnosic use.
X-rays are also used "real-time" in angiography and contrast studies of the hollow organs (e.g. barium enema of the small or large intestine). Angioplasty, medical interventions of the arterial system, rely heavily on X-ray-sensitive contrast to identify potentially treatable lesions.
Radiotherapy, a curative medical intervention now used almost exclusively for cancer, employs somewhat stronger forms of radiation.
Among the important early researchers in X-rays were Sir William Crookes, Johann Wilhelm Hittorf, Eugene Goldstein, Heinrich Hertz, Philipp Lenard, Hermann von Helmholtz, Thomas Edison, Nikola Tesla, Charles Grover Barkla, and Wilhelm Conrad Röntgen.
Physicist Johann Hittorf (1824 - 1914) observed tubes with energy rays extending from a negative electrode. These rays produced a fluorescence when they hit the glass walls of the tubes. In 1876 the effect was named "cathode rays" by Eugene Goldstein. Later, English physicist William Crookes investigated the effects of energy discharges on rare gases, and constructed what is called the Crookes tube. It is a glass vacuum cylinder, containing electrodes for discharges of a high voltage electric current. He found, when he placed unexposed photographic plates near the tube, that some of them were flawed by shadows, though he did not investigate this effect. In 1892, Heinrich Hertz began experimenting and demonstrated that cathode rays could penetrate very thin metal foil (such as aluminum). Philip Lenard, a student of Heinrich Hertz, further researched this effect. He developed a version of the cathode tube and studied the penetration by X-rays of various materials. Philip Lenard, though, did not realize that he was producing X-rays.
In April 1887, Nikola Tesla began to investigate X-rays using high voltages and vacuum tubes of his own design, as well as Crookes tubes. From his technical publications, it is indicated that he invented and developed a special single-electrode X-ray tube, which differed from other X-ray tubes in having no target electrode. He stated these facts in his 1897 X-ray lecture before the New York Academy of Sciences. The principle behind these devices is nowadays called the bremsstrahlung process, in which a high-energy secondary X-ray emission is produced when charged particles (such as electrons) pass through matter. By 1892, Tesla performed several such experiments, but he did not categorize the emissions as what were later called X-rays, instead generalizing the phenomenon as radiant energy. He did not publicly declare his findings nor did he make them widely known. His subsequent X-ray experimentation by vacuum high field emissions led him to alert the scientific community to the biological hazards associated with X-ray exposure.
Hermann von Helmholtz formulated mathematical equations for X-rays. He postulated a dispersion theory before Roentgen made his discovery and announcement. It was formed on the basis of the electromagnetic theory of light (Wiedmann's Annalen, Vol. XLVIII). However, he did not work with actual X-rays.
On November 8, 1895, Wilhelm Röntgen, a German scientist, began observing and further documenting X-rays while experimenting with vacuum tubes. Röntgen, on December 28, 1895, wrote a preliminary report "On a new kind of ray: A preliminary communication". He submitted it to the Würzburg's Physical-Medical Society journal. This was the first formal and public recognition of the categorization of X-rays. Röntgen referred to the radiation as "X", to indicate that it was an unknown type of radiation. The name stuck, although (over Röntgen's great objections), many of his colleagues suggested calling them Röntgen rays. They are still referred to as Röntgen rays in some countries. Roentgen received the first Nobel Prize in Physics for his discovery.
In 1895, Thomas Edison investigated materials' ability to fluoresce when exposed to X-rays, and found that calcium tungstate was the most effective substance. Around March 1896, the fluoroscope he developed became the standard for medical X-ray examinations. Nevertheless, Edison dropped X-ray research around 1903 after the death of Clarence Madison Dally, one of his glassblowers. Dally had a habit of testing X-ray tubes on his hands, and acquired a cancer in them so tenacious that both arms were amputated in a futile attempt to save his life
In 1906, physicist Charles Barkla discovered that X-rays could be scattered by gases, and that each element had a characteristic X-ray. He won the 1917 Nobel Prize in Physics for this discovery.
The use of X-rays for medical purposes (to develop into the field of radiation therapy) was pioneered by Major John Hall-Edwards in Birmingham, England. In 1908, he had to have his left arm amputated owing to the spread of X-ray dermatitis