X-ray

This article is about the type of radiation. For the medical specialty, see Radiography. For other uses, see X-ray (disambiguation).

An X-ray is a form of high-energy electromagnetic radiation. Its wavelength is shorter than that of ultraviolet light but longer than that of gamma rays. X-rays have wavelengths from about 10 nanometers to 10 picometers. Their frequencies range from 30 petahertz to 30 exahertz (3×10¹⁶ Hz to 3×10¹⁹ Hz). Their photon energies are from 100 eV to 100 keV.

The German scientist Wilhelm Conrad Röntgen discovered X-rays in 1895. He called them X-radiation because they were a type of radiation nobody knew about yet.

X-rays can pass through many solid things, including building materials and living tissue. Because of this, X-ray radiography is widely used in medical diagnostics. Doctors use it to check for broken bones. It is also used in materials science. Scientists use it to identify some chemical elements and to find weak spots in building materials.

X-rays are ionizing radiation. Too much exposure can be harmful to health. It can damage DNA and may cause cancer. At higher levels, it can cause burns and radiation sickness. Because of this, health authorities carefully control how X-rays are made and used.

History

Scientists first noticed X-rays as an unknown kind of radiation coming from discharge tubes. Researchers were studying cathode rays, which are fast-moving electron beams first seen in 1869. Many early Crookes tubes made them. Some earlier experimenters, including William Morgan, Philipp Lenard, and Nikola Tesla, may have made X-rays without knowing what they were.

On 8 November 1895, German physics professor Wilhelm Röntgen discovered X-rays while experimenting with Crookes tubes. He called them "X" because they were an unknown type of radiation. He found that the rays could pass through cardboard, books, and paper. He also made a picture of his wife's hand. It was the first photograph of a part of the human body taken with X-rays. Doctors soon began using the rays. Thomas Edison developed a fluoroscope, which became the standard tool for medical X-ray checks. Röntgen received the first Nobel Prize in Physics for his discovery. Much later, the Chandra X-ray Observatory, launched on July 23, 1999, began studying the powerful processes in the universe that produce X-rays.

Energy ranges

X-rays with high photon energies, above 5–10 keV, are called hard X-rays. Those with lower energy are called soft X-rays, and the in-between range is often called tender X-rays. Hard X-rays can pass through objects, so they are used to see inside things, such as in medical radiography and airport security. The word X-ray can also mean a radiographic picture made this way. Hard X-rays have wavelengths about the size of atoms, so they help scientists find the shapes of crystals with X-ray crystallography. Soft X-rays are easily absorbed by air, and they cannot travel far through water.

Scientists do not agree on one clear rule for telling X-rays apart from gamma rays. One common way is by source: X-rays are made by electron deexcitation, while gamma rays come from the decay of atomic nuclei. Another way is by wavelength. Radiation with a wavelength shorter than 10⁻¹¹ m (0.1 Å) counts as gamma radiation. These two ways often agree, since X-ray tubes usually give off longer wavelengths than radioactive nuclei. Sometimes the names are chosen based on history, how the rays are detected, or how they are used. For example, gamma rays of 6–20 MeV used in medicine and industry, such as in radiotherapy, can also be called X-rays.

Properties

X-ray photons have enough energy to ionize atoms and break apart molecular bonds. This makes X-rays a type of ionizing radiation, which can harm living tissue. A very high radiation dose over a short time can cause burns and radiation sickness. Lower doses can raise the risk of radiation-induced cancer. In medical imaging, this risk is usually much smaller than the benefits of the test. The ability to ionize is also useful. It is used in cancer treatment to destroy malignant cells through radiation therapy, and in studying materials through X-ray spectroscopy.

Hard X-rays can pass through fairly thick objects without being much absorbed or scattered. This is why X-rays are widely used to image the inside of objects that cannot be seen directly. They are most often used in medical X-ray pictures and airport security scanners. They are also important in industry, such as industrial radiography and industrial CT scanning, and in research, such as small animal CT. The penetration depth changes by orders of magnitude across the X-ray range. Because of this, the photon energy can be adjusted to allow enough transmission through an object while still giving good contrast in the image. X-rays also have much shorter wavelengths than visible light. This means they can show structures far smaller than a normal microscope can see. This is used in X-ray microscopy for high-resolution images. It is also used in X-ray crystallography to find the positions of atoms in crystals.

Interaction with matter

X-rays interact with matter in three main ways: photoabsorption, Compton scattering, and Rayleigh scattering. How strongly they interact depends on the X-ray's energy and on the elements in the material. Chemical properties matter much less, because an X-ray's energy is far higher than the energy that holds chemical bonds together. Photoabsorption is the main interaction for soft X-rays and for lower-energy hard X-rays. Compton scattering is the main one at higher energies. Most materials absorb low-energy X-rays strongly, especially those with high atomic number, so the X-rays travel only short distances through them. This pattern changes near absorption edges, where the chance of interaction jumps suddenly. Soft tissue absorbs X-rays mainly through photoabsorption up to about 26 keV. Bones show up clearly on medical X-ray pictures because they contain calcium and are dense.

In photoabsorption, the X-ray gives all of its energy to an electron. The electron is knocked out of its atom, and it can knock out more electrons along its path. An outer electron then fills the empty spot and gives off either a characteristic X-ray or an Auger electron. Scientists use these effects to find out which elements are present, using X-ray spectroscopy or Auger electron spectroscopy. In Compton scattering, an X-ray hits an outer electron and gives it part of its energy. The X-ray then becomes longer in wavelength. This is an inelastic scattering process. The X-ray can leave in almost any direction, though a direction close to its original one is more likely, especially for high-energy X-rays. The Klein–Nishina formula describes how likely each angle is. The energy passed on can be worked out from that angle, using the conservation of energy and momentum. Rayleigh scattering is the main elastic scattering process in the X-ray range.

Production

X-rays are made when fast charged particles, such as electrons or ions, hit a material. In an X-ray tube, a vacuum tube uses high voltage to speed up electrons from a hot cathode. These electrons crash into a metal target called the anode, which makes X-rays. Medical X-ray tubes usually use tungsten as the target. Molybdenum is used for special jobs like mammography-style imaging. Crystal studies often use a copper target, and cobalt is used when iron in the sample could cause problems. For lower energies, as in X-ray photoelectron spectroscopy, aluminium or magnesium targets are common. The energy of each X-ray photon can be no higher than the energy of the electron that produced it.

There are two ways X-rays are made in the target. In the first, called characteristic X-ray emission, an electron knocks out a tightly held electron from an inner electron shell. Electrons from higher levels then fill the gap. This gives X-rays at specific frequencies, shown in an emission spectrum. In the second, called bremsstrahlung, electrons give off X-rays when the strong electric field near atomic nuclei pulls them off course. This creates a continuous spectrum. Both processes are inefficient. Most of the electric power used by the tube becomes waste heat, and only about one percent turns into X-rays. A research source that is becoming widely used is synchrotron radiation from particle accelerators. It gives much stronger X-rays with excellent collimation and linear polarization. X-rays also appear in lightning, in terrestrial gamma-ray flashes, and in some laboratory sparks. In those sparks, fast electrons from streamers may form run-away electrons.

Detectors

X-ray detectors come in different shapes because they do different jobs. Early detectors for radiography used photographic plates and later photographic film. Today, most of these have been replaced by digital types, such as image plates and flat panel detectors.

To keep people safe from radiation, the amount of direct exposure is often checked with ionization chambers. Dosimeters measure the radiation dose a person has received. X-ray spectra can be measured with spectrometers. For X-ray diffraction work, such as X-ray crystallography, hybrid photon counting detectors are widely used.

Medical uses

Main article: Projectional radiography

Main article: CT scan

Main article: Fluoroscopy

Since Röntgen found that X-rays can show bones, they have been used for medical imaging. The first medical use came less than a month after his paper on the subject. By 2010, billions of medical images had been taken around the world. Doctors use projectional radiography to make flat pictures. Bones show up clearly because they contain calcium and have a high atomic number, which helps them block X-rays. Chest X-rays can help find pneumonia, problems related to lung cancer, or pulmonary edema. Dental radiography helps find cavities. Doctors also use angiography. In this test, a contrast agent is put into blood vessels so a radiologist can check for blockages.

Computed tomography (CT scanning) joins many X-ray images taken from different directions. This makes tomographic images and a three-dimensional picture of the inside of the body. Fluoroscopy shows moving images of the body as they happen, and radiation therapists use it too. Using X-rays to treat illness is called radiation therapy. It is mainly used to manage cancer, including palliation.

Adverse effects

X-rays are a form of ionizing radiation. The World Health Organization's International Agency for Research on Cancer and the U.S. government list X-rays as a carcinogen, which means they can cause cancer. Most of the X-rays that can add risk come from computed tomography (CT) scans, because these scans use a large dose. These X-rays can increase the risk of developmental problems and cancer. Scientists do not yet know of a threshold dose of radiation below which there is no added cancer risk, and many now doubt that such a level exists. Doctors weigh the benefits of an X-ray against its risks. They avoid X-rays that are not needed, especially CT scans. For pregnant patients, the risk to the unborn baby is greater, so doctors often use ultrasounds instead, because ultrasounds do not use radiation.

For comparison, a plain chest X-ray exposes a person to about as much radiation as they get from background radiation in about 10 days. A dental X-ray equals about 1 day. In the United States, medical X-rays were a large source of radiation made by people. By 2006, medical procedures made up nearly half of all radiation exposure from all sources. This increase came from more imaging, especially CT scans, and more use of nuclear medicine.

Other uses

X-rays have many other uses, including:

• X-ray crystallography studies the pattern made when X-rays pass through the tightly packed atoms in a crystal. Scientists study that pattern to learn how the atoms are arranged. A related method, fiber diffraction, was used by Rosalind Franklin to help discover the double helical shape of DNA.
• X-ray astronomy is a branch of astronomy. It studies X-rays that come from objects in space.
• X-ray microscopic analysis uses electromagnetic radiation in the soft X-ray range to make images of very small objects.
• X-ray fluorescence happens when strong X-rays hit a sample. The sample fluoresces, giving off a pattern of light that matches its atoms. This lets people test many things without harming them.
• X-ray photoelectron spectroscopy is a way to study the chemistry of a material. It is based on the photoelectric effect and is usually used in surface science.
• Industrial radiography uses X-rays to check industrial parts, especially welds.
• Radiography of cultural objects is most often used on paintings. X-rays can show underdrawing, pentimenti (changes made while painting or by later restorers), and sometimes older paintings hidden beneath the surface. Many pigments, such as lead white, show up clearly in X-ray images.

• Automated X-ray inspection uses X-rays to check packaged items for authenticity and quality. It can also find foreign material in foods and medicines.
• Industrial CT (computed tomography) uses X-ray equipment to make three-dimensional pictures of parts, both outside and inside. A computer combines many images taken from different directions.
• Airport security luggage scanners use X-rays to check inside bags for safety threats before they are loaded onto aircraft.
• Border control truck scanners and domestic police departments use X-rays to look inside trucks.

• X-ray art and fine art photography is an artistic use of X-rays. Examples include works by artists such as Stane Jagodič or Peter Dazeley.
• X-ray hair removal was a popular method in the 1920s, but it is now banned by the FDA.
• Shoe-fitting fluoroscopes became popular in the 1920s. They were later banned in the US in the 1960s, in the UK in the 1970s, and later in other parts of Europe.
• Roentgen stereophotogrammetry is used to track how bones move by placing markers in them.
• In thermonuclear weapon design, radiation implosion is a process in which X-rays from a first stage (the primary) squeeze nuclear fuel until a second stage (the secondary) begins to fuse.

Visibility

X-rays are usually invisible, but in special cases people can see them with their eyes. Soon after Röntgen's 1895 paper, Brandes reported that after his eyes adjusted to the dark and he put his eye close to an X-ray tube, he saw a faint "blue-gray" glow that seemed to come from inside the eye. When Röntgen looked back at his notes, he found that he had seen the same effect. He had also noticed a blue glow when he placed an X-ray tube on the other side of a wooden door. He first thought his observations were wrong because he only saw it with one type of tube. Later he realized that this tube was the only one strong enough to make the glow clearly visible, so the experiment could be easily repeated. Today this knowledge is mostly forgotten. People no longer want to repeat an experiment with ionizing radiation that is now seen as risky and possibly harmful. Scientists do not know exactly how the eye creates this effect. Cherenkov radiation caused by X-rays passing through the clear fluid inside the eye is a likely explanation. Another possible explanation is that X-rays directly activate cells in the retina, similar to some light flashes seen in experiments on cosmic ray visual phenomena.

Although X-rays are otherwise invisible, it is possible to see the ionization of air molecules if the X-ray beam is intense enough. The beam from a wiggler at the European Synchrotron Radiation Facility is one example of such a high-intensity beam.

Units of measure and exposure

The ability of X-rays to create electric charge in matter is called the exposure. The coulomb per kilogram (C/kg) is the SI unit of exposure to ionizing radiation. The older roentgen (R) unit measured the charge made by an electrostatic unit of charge in one cubic centimeter of dry air. One roentgen equals 2.58×10⁻⁴ C/kg.

Radiation affects matter, especially living tissue, more by the energy it leaves behind than by the charge it creates. This energy is measured as the absorbed dose. The gray (Gy) is the SI unit of absorbed dose. One gray equals one joule of energy deposited in one kilogram of matter. The equivalent dose measures the effect on human tissue. For X-rays, it equals the absorbed dose. The older unit for equivalent dose is the Roentgen equivalent man (rem), where 100 rem equals 1 Sv. The sievert (Sv) is the SI unit of equivalent dose and of effective dose. For X-rays, the equivalent dose in sieverts has the same number as in grays (1 Sv = 1 Gy). However, the effective dose is usually not the same number as the gray.