X-rays are a type of high-energy, ionizing radiation. They are similar to visible light in that they are both forms of electromagnetic radiation, but X-rays have much higher energy levels and shorter wavelengths. X-rays have wavelengths ranging from 10 nanometers to 10 picometers (10×10-9 Hz to 10×10-12 Hz), which corresponds to frequencies in the range of 30 petahertz to 30 exahertz (3×1016 Hz to 3×1019 Hz). The energy of X-rays falls within the range of approximately 100 electron volts (eV) to several megaelectron volts (MeV).
Wilhelm Conrad Röntgen, a German scientist, discovered X-rays in 1895 [1]. In recognition of his groundbreaking work, X-radiation is often referred to as Röntgen radiation in many languages. Röntgen initially named it "X" to symbolize its unknown nature, and the term "X-ray" has since become a widely recognized term in scientific and medical communities.
History
It is generally believed that the earliest unintentional producer of X-rays was the renowned British scientist William Morgan (1750-1833)[2, 3]. In 1785, he submitted a paper to the Royal Society of London describing the phenomenon of generating visible light by passing an electric current through a partially evacuated glass tube. Unbeknownst to him, this light was actually produced by X-rays, a discovery not yet made at that time. This work was later expanded upon by British scientists Sir Humphry Davy (1778-1829) and Michael Faraday (1791-1867).
In 1893, American physicist Fernando Sanford (1854-1948) invented electron photography, inadvertently generating and detecting X-rays. In 1894, Nikola Tesla (1856-1943) noticed damage to film in his laboratory and began researching this invisible radiation.
On November 8, 1895, Wilhelm Röntgen accidentally discovered X-rays during experiments and started investigating them. He submitted a preliminary report titled "On a New Kind of Ray" on December 28, 1895, which was the first paper on X-rays. Röntgen referred to this radiation as "X," signifying it as an unknown type of radiation. While some called them "Röntgen rays," Röntgen himself opposed naming them after himself. He was awarded the first Nobel Prize in Physics for the discovery of X-rays. Röntgen's X-ray image of his wife's hand was the first image of a human body part using X-rays.
Realizing the potential medical applications of X-rays, Röntgen wrote to physicians he knew in Europe when he submitted his preliminary report. Scottish electrical engineer Alan Archibald Campbell-Swinton (1863-1930) was the second person to produce X-ray images of hands.
X-rays' remarkable physical properties and ability to see through objects generated immense interest among scientists and the public. Many papers and articles were published about X-rays in 1896. X-rays' mysterious qualities led to associations with extraordinary abilities such as X-ray vision and telepathy. Due to ethical concerns and the harm X-rays could cause to the human body, research into these aspects largely ceased.
Soon after Röntgen's discovery, many applications for X-rays emerged. The first clinical use of X-rays was by British physician John Hall-Edwards (1858-1926) in January 1896 when he took an X-ray image of a colleague's hand with a needle stuck in it. He also performed the first surgical procedure using X-rays on February 14, 1896. Georgian physiologist Ivane Javakhishvili (1846-1908) exposed frogs and insects to X-rays just weeks after Röntgen's discovery, concluding that X-rays not only could capture images but also affect living organisms.
Marie Curie (1867-1934) developed radiology vehicles in 1914 for diagnosing wounded soldiers during World War I. These vehicles enabled rapid and accurate surgery by providing X-ray images. Scientists also discovered the dangers of X-rays. In 1904, John Hall-Edwards developed cancer, referred to as X-ray dermatitis, due to X-ray exposure, leading him to publish warnings about X-ray hazards. He died in 1926 from X-ray-induced cancer.
X-rays found applications beyond medicine. In approximately 1906, British physicist Charles Glover Barkla (1877-1944) discovered that gases could scatter X-rays and that each element had characteristic X-ray spectra. He received the Nobel Prize in Physics in 1917 for this work. In 1912, German physicist Max von Laue (1879-1960) observed X-ray diffraction by crystals, earning him the Nobel Prize in 1914. British physicists Sir William Henry Bragg (1862-1942) and his son Sir William Lawrence Bragg (1890-1971) used X-rays to study crystal structures, founding the field of X-ray crystallography and winning the Nobel Prize in 1915. Henry Gwyn Jeffreys Moseley (1887-1915) used X-rays emitted by various metals in crystallography experiments and formulated Moseley's law, relating X-ray frequency to atomic number[4].
In the 1950s, X-ray microscopes were developed, and the Chandra X-ray Observatory was launched in 1999 to observe the universe in X-ray wavelengths, revealing celestial phenomena such as stars torn apart by black holes, galaxy collisions, stellar disintegration, and neutron star accretion. These cosmic events are often challenging to detect in visible light.
Physical Properties
X-ray photons possess sufficient energy to ionize atoms and break molecular bonds, classifying them as ionizing radiation, which can be harmful to biological tissues. High doses of radiation can lead to radiation sickness in a short period, while lower doses increase the risk of cancer. However, the ionizing capability of X-rays can also be harnessed in cancer treatment using radiation therapy to kill cancer cells.
Hard X-rays can penetrate relatively thick objects with minimal absorption or scattering. Therefore, X-rays are widely used for imaging the internal structures of opaque objects. The most common applications include medical radiology and airport security scanners, among others.
X-rays have much shorter wavelengths compared to visible light, allowing them to probe structures smaller than those observable with visible light microscopes. This characteristic is employed in X-ray microscopy to obtain high-resolution images and is also used in X-ray crystallography to determine the positions of atoms within crystals.
Interaction with Matter
X-rays interact with matter primarily through two typical quantum physics mechanisms: the photoelectric effect and Compton scattering. The strength of these interactions depends on the energy of the X-rays and the elemental composition of the material [5]. However, they are relatively independent of the chemical properties of the material since the energy of X-ray photons is much higher than chemical bond energies.
1. Photoelectric Effect: This is the dominant interaction mechanism for X-rays with lower energies. The probability of the photoelectric effect occurring is roughly proportional to the cube of the atomic number of the material and the cube of the energy of the incident X-ray photons. However, this relationship does not hold near the binding energy of inner-shell electrons, where the interaction probability sharply increases, known as the absorption edge. X-ray photons absorbed through the photoelectric effect transfer all their energy to the interacting electron, ionizing the atom to which the electron is bound and generating a photoelectron that can potentially ionize more atoms along its path. Outer-shell electrons then fill the vacant electron positions, emitting characteristic X-rays.
2. Compton Scattering: This is the dominant interaction mechanism for higher-energy X-rays. Compton scattering involves the non-elastic scattering of X-ray photons by outer-shell electrons. Some of the energy of the scattered photon is transferred to the electron, leading to the ionization of the atom. The scattered photon can propagate in any direction, but it is more likely to scatter in a direction close to its original trajectory. The probabilities of different scattering angles are described by the Klein-Nishina formula. Energy transfer can be directly obtained from the scattering angle, while energy and momentum conservation principles are upheld.
Production
X-rays are produced through the interaction of high-speed ions (positive or negative) with matter, leading to the emission of electromagnetic radiation in the X-ray part of the electromagnetic spectrum. The specific method of X-ray generation can vary depending on the application, with conventional X-ray tubes and synchrotrons being two common sources[5].
Production by interaction of electrons with matter
X-ray Tubes, Conventional X-ray
X-rays can be generated by an X-ray tube, a vacuum tube that uses a high voltage to accelerate the electrons released by a hot cathode to a high velocity. The high velocity electrons collide with a metal target, the anode, creating the X-rays. When the electrons hit the target, X-rays are created by two different atomic processes:
- Bremsstrahlung x-ray (Braking Radiation): When these electrons collide with the metal atoms in the target, they undergo deceleration or braking due to the electromagnetic forces between electrons and atomic nuclei. As the electrons slow down or change direction, they emit energy in the form of X-ray photons. This process results in a continuous spectrum of X-ray energies, which can be used for various diagnostic purposes.
- Characteristic X-rays: In addition to Bremsstrahlung radiation, when electrons collide with the metal target, they may also dislodge inner-shell electrons from metal atoms. Electrons from outer shells then fill these vacancies, emitting characteristic X-rays with specific energies corresponding to the energy level differences between electron shells. These characteristic X-rays are used in X-ray spectroscopy and can help identify the type of metal in the target.
The maximum energy of the produced X-ray photon is limited by the energy of the incident electron, which is equal to the voltage on the tube times the electron charge, so a 120 kV tube cannot create X-rays with an energy greater than 120 keV.
Production by interaction of positive ions with matter
(Synchrotron Radiation, Advanced X-ray Sources)
- Synchrotrons are advanced particle accelerators used to generate extremely bright and focused X-ray beams. In a synchrotron, electrons are accelerated to nearly the speed of light and are forced to travel in a circular path by magnetic fields. As they change direction, they emit highly collimated X-rays. Unlike conventional X-ray tubes, synchrotrons produce X-rays with specific wavelengths and high intensity, making them valuable tools for a wide range of scientific research, including structural biology, materials science, and more.
Production in lightning and laboratory discharges
The production of X-rays in lightning and laboratory discharges is a fascinating phenomenon that involves the acceleration of electrons in electric fields and the subsequent generation of X-ray photons.
In natural lightning events and terrestrial gamma-ray flashes (TGFs), X-rays are produced as a result of the acceleration of electrons in the electric fields associated with the lightning discharge. This acceleration leads to the emission of X-ray photons through a process known as Bremsstrahlung. X-rays produced in these events can have energies ranging from a few kiloelectron volts (keV) to several tens of megaelectron volts (MeV). This wide range of energies suggests that X-rays emitted in lightning and TGFs can vary in intensity and characteristics.
In laboratory experiments, researchers have observed the production of X-rays with a characteristic energy of 160 keV. These experiments typically involve a setup with a gap size of approximately 1 meter and a peak voltage of 1 megavolt (MV). The observed X-rays are a result of the electrical discharge in the laboratory setting.
The exact mechanism behind X-ray production in lightning and laboratory discharges is complex and not fully understood. One possible explanation involves the interaction of two streamers, which can lead to the production of high-energy runaway electrons. However, it has been noted that the duration of the electric field enhancement between two streamers may be too short to produce a significant number of runaway electrons. Recent research suggests that air perturbations in the vicinity of streamers (the branching, luminous channels that propagate during electrical discharges) may facilitate the production of runaway electrons and, consequently, the generation of X-rays in discharges. These perturbations could play a crucial role in enhancing the conditions necessary for X-ray emission.