What Are X-Rays?
X-rays are a very energetic form of electromagnetic radiation that can be used to take images of the human body.
Credit: Fotokon | Dreamstime

X-rays are a form of electromagnetic radiation, as are radio waves, infrared radiation, visible light, ultraviolet radiation and microwaves. One of the most common and beneficial uses of X-rays is for medical imaging. X-rays are also used in treating cancer and in exploring the cosmos. 

Electromagnetic radiation is transmitted in waves or particles at different wavelengths and frequencies. This broad range of wavelengths is known as the electromagnetic spectrum. The EM spectrum is generally divided into seven regions in order of decreasing wavelength and increasing energy and frequency. The common designations are: radio waves, microwaves, infrared (IR), visible light, ultraviolet (UV), X-rays and gamma-rays. 

The electromagnetic spectrum is generally divided into seven regions, in order of decreasing wavelength and increasing energy and frequency: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays.
The electromagnetic spectrum is generally divided into seven regions, in order of decreasing wavelength and increasing energy and frequency: radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays.
Credit: Biro Emoke Shutterstock

X-rays are roughly classified into two types: soft X-rays and hard X-rays. Soft X-rays fall in the range of the EM spectrum between (UV) light and gamma-rays. Soft X-rays have comparatively high frequencies — about 3 × 1016 cycles per second, or hertz, to about 1018 Hz — and relatively short wavelengths — about 10 nanometers (nm), or 4 × 10−7 inches, to about 100 picometers (pm), or 4 × 10−8 inches. (A nanometer is one-billionth of a meter; a picometer is one-trillionth of a meter.) Hard X-rays have frequencies of about 1018 Hz to higher than 1020 Hz and wavelengths of about 100 pm (4 × 10−9 inches) to about 1 pm (4 × 10−11 inches). Hard X-rays occupy the same region of the EM spectrum as gamma-rays. The only difference between them is their source: X-rays are produced by accelerating electrons, while gamma-rays are produced by atomic nuclei. 

X-rays were discovered in 1895 by Wilhelm Conrad Röentgen, a professor at Würzburg University in Germany. According to the Nondestructive Resource Center's "History of Radiography" Web page, Röentgen noticed crystals near a high-voltage cathode-ray tube exhibiting a fluorescent glow, even when he shielded them with dark paper. Some form of energy was being produced by the tube, and it was penetrating the paper and causing the crystals to glow. Röentgen called the unknown energy "X-radiation." Experiments showed that this radiation could penetrate soft tissues but not bone, and would produce shadow images on photographic plates. 

For this discovery, Röentgen was awarded the very first Nobel Prize in Physics, in 1901. During World War I, X-rays were already being used for medical purposes.

According to NobelPrize.org, "X-rays are produced when electrons strike a metal target. The electrons are liberated from the heated filament and accelerated by a high voltage towards the metal target." When the electrons strike the target, their energy is converted to X-rays.  

X-rays can also be produced by a synchrotron, a type of particle accelerator that causes charged particles to move in a closed, circular path. When high-speed electrons are forced to move in a circular path by a magnetic field, the angular acceleration causes the particles to emit photons. If the energy is great enough, the electrons will emit X-rays. 

Synchrotron radiation was seen for the first time at General Electric in the United States in 1947, according to the European Synchrotron Radiation Facility. This radiation was considered a nuisance because it caused the particles to lose energy, but it was later recognized in the 1960s as light with exceptional properties that overcame the shortcomings of X-ray tubes. One interesting feature of synchrotron radiation is that it is polarized; that is, the electric and magnetic fields of the photons all oscillate in the same direction, which can be either linear or circular. 

Due to their ability to penetrate certain materials, X-rays are used for a number of nondestructive evaluation and testing (NDE/NDT) applications, particularly for identifying flaws or cracks in structural components. According to the NDT Resource Center, "Radiation is directed through a part and onto [a] film or other detector. The resulting shadowgraph shows the internal features" and whether the part is sound.

X-rays are also essential for transportation security inspections of cargo, luggage and passengers. Electronic imaging detectors allow for real-time visualization of the content of packages and items that passengers might carry on their persons. 

The original use of X-rays was for imaging bones, which were easily distinguishable from soft tissues on the film that was available at that time. However, more accurate focusing systems and more sensitive detection methods, such as improved photographic films and electronic imaging sensors, have made it possible to distinguish increasingly fine detail and subtle differences in tissue density, while using much lower exposure levels. Additionally, computed tomography (CT) combines multiple X-ray images into a 3D model of a region of interest. The U.S. Food and Drug Administration states that X-ray imaging exams are recognized as a valuable medical tool for a wide variety of examinations and procedures. They are used as a noninvasive and painless method for diagnosing disease and monitoring therapy, and supporting medical and surgical treatment planning. They are also used in guiding medical personnel as they insert catheters, stents or other devices into the body; treat tumors; or remove blood clots or other blockages.

Radiation therapy uses high-energy radiation to kill cancer cells by damaging their DNA. However, the treatment can damage normal cells as well as cancer cells. Therefore, the National Cancer Institute recommends that treatment must be carefully planned to minimize side effects. 

According to the U.S. Environmental Protection Agency, ionizing radiation from X-rays deposits a large amount of energy into a small area, enough energy to strip electrons completely way from atoms, thus altering their chemical properties and breaking molecular bonds. In sufficient doses, this can damage or destroy cells. While this cell damage can cause cancer, it can also be used to fight it. By directing X-rays at cancerous tumors, the abnormal cells can be killed. 

The problem, though, is that this also kills healthy cells along the path of the beam. To reduce this problem, the patient lies on a table and is treated with radiation from multiple directions, Texas Oncology states. The exposure to surrounding tissues is minimized, because healthy tissue receives only a single small dose from the moving beam, while the tumor receives doses from every angle. 

According to Robert Patterson, professor of astronomy at Missouri State University, celestial sources of X-rays include close binary systems containing black holes or neutron stars. In these systems, the more massive and compact stellar remnant can strip material from its companion star forming a disk of extremely hot X-ray-emitting gas as it spirals inward. Additionally, supermassive black holes at the centers of spiral galaxies can emit X-rays as they absorb stars and gas clouds that fall within their gravitational reach. 

X-ray telescopes use low-angle reflections to focus these high-energy photons that would otherwise pass through normal telescope mirrors. Because the Earth's atmosphere blocks most X-rays, observations are typically conducted using high-altitude balloons or orbiting telescopes. 

Additional resources