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X-ray is the letter X, in the NATO phonetic alphabet.
X-rays (German: Ršntgenstrahlen) are a form of electromagnetic radiation
with a wavelength approximately in the range of 5 pm - 10 nanometers
(corresponding to frequencies in the range 30 PHz - 60 EHz).
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.
Exposure to X-ray radiation is dangerous and causes cancer.
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 Barkla, and
Wilhelm Conrad Ršntgen.
Physicist Johann Hittorf observed tubes with energy rays extending from a
negative electrode. These rays produced a fluoresce 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. He 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 places
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, futher researched this effect. He developed a version of the
cathode tube and studied the penetration of X-rays through 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 his own
devices as well as Crookes tubes. Tesla did this by experimenting with high
voltages and vacuum tubes. From Nikola Tesla's technical publications, it is
indicated that he invented and developed a special single-electrode X-ray
tube. Tesla's tubes differ from other X-ray tubes in that they have no
target electrode. He stated these facts in his 1897 X-ray lecture before the
New York Academy of Sciences. The modern term for this process is 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 experiments with them. Tesla 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 first 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).
On November 8 1895, Wilhelm Ršntgen, a German scientist, began observing and
futher documenting X-rays while experimenting with vacuum tubes. Ršntgen, on
December 28, 1895, wrote a preliminary report "On a new kind of ray: A
preliminary communication". He submitted it to the Wźrzburg's
Physical-Medical Society journal. This was the first formal and public
recognition of X-rays. Ršntgen referred to the radiation as "X", to indicate
that it was an unknown type of radiation. The name stuck, although (over
Ršntgen's objections), many of his colleagues suggested calling them Ršntgen
rays. They are still referred to as Ršntgen 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. He found that calcium tungstate was the most effective
substance. Around March 1896, the fluoroscope he developed became the
standard for medical X-ray examinations. 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 for this discovery.