X-ray

An X-ray is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. These powerful waves of energy can pass through many solid objects, including human bodies and construction materials, making them very useful in many fields.

X-rays were discovered in 1895 by the German scientist Wilhelm Conrad Röntgen, who named them X-radiation because their properties were unknown at the time. Since then, X-ray radiography has become an important tool in medical diagnostics, helping doctors see inside the body without surgery. For example, doctors use X-rays to find broken bones or to check for problems inside the chest.

While X-rays are very helpful, they are also a type of ionizing radiation, which means they can damage cells and increase the risk of health problems like cancer if a person is exposed to too much. Because of this, the use of X-rays is carefully controlled to make sure people are only exposed to safe amounts.

History

X-rays were first noticed as a type of unknown radiation coming from special glass tubes used in experiments. These tubes, called Crookes tubes, could create beams of electrons. Early scientists observed strange effects from these beams, which later turned out to be X-rays.

The discovery of X-rays is credited to Wilhelm Conrad Röntgen in 1895. While experimenting with Crookes tubes wrapped in black paper, Röntgen noticed a glow on a nearby screen. He discovered that invisible rays were passing through the paper and could affect photographic plates. Röntgen shared his findings in a paper, calling these rays “X” to show they were unknown. His work led to the rapid spread of X-ray technology for medical imaging and other uses.

Energy ranges

X-rays with higher energy, called hard X-rays, are used to see inside objects, like in hospitals and airports. They can also help scientists study the structure of crystals. X-rays with lower energy, called soft X-rays, are easily absorbed by air.

There is some confusion about the difference between X-rays and gamma rays. One way to tell them apart is by where they come from: X-rays come from electrons, while gamma rays come from the centers of atoms. But sometimes it can be hard to tell them apart because they can have similar energies.

Properties

X-rays are a type of high-energy radiation that can pass through solid objects and are used to see inside things that are normally hard to see, like in hospitals or at airports. They have enough energy to affect living tissue, but doctors use them carefully to make sure they help more than they harm.

Because X-rays have very short wavelengths, they can show very tiny details, much smaller than what normal microscopes can see. This helps scientists study the tiny building blocks of materials and even look at the arrangement of atoms in crystals.

Interaction with matter

X-rays interact with matter in three main ways: photoabsorption, Compton scattering, and Rayleigh scattering. These interactions depend mostly on the energy of the X-rays and the types of atoms in the material, not on how the atoms are arranged in chemicals.

Photoabsorption happens more with lower energy X-rays and atoms that have higher atomic numbers, like those in bones. When a photoabsorbed X-ray transfers its energy to an electron, it can create a photoelectron and sometimes produce new X-rays or electrons that help scientists identify elements. Compton scattering is more common with higher energy X-rays and soft tissue, where the X-ray photon loses some energy and changes direction. Rayleigh scattering is an elastic scattering that mostly keeps the X-ray’s energy the same.

Production

X-rays are made when charged particles like electrons hit a material. One common way to create X-rays is by using an X-ray tube. This tube uses a high voltage to speed up electrons from a hot part called a hot cathode. These fast electrons then crash into a metal target, called the anode, and X-rays are produced.

The X-rays can be created in two main ways during this crash. First, if an electron knocks out another electron from inside the atom, X-rays are released when other electrons fill in the empty spot. Second, X-rays are also made when electrons are bent by the strong pull of the atom’s nucleus. Both of these processes create X-rays that doctors and scientists use for different kinds of work.

Detectors

X-ray detectors come in different shapes and sizes depending on what they are used for. In the past, detectors for taking pictures of bones or other body parts used special photographic plates and film. Today, most of these detectors are digital, like image plates and flat panel detectors.

To keep people safe from too much X-ray exposure, special tools called ionization chambers and dosimeters are used. These help measure how much radiation a person has been exposed to. For studying the details of X-rays, scientists use devices called spectrometers, and for looking at the structure of tiny things like crystals, they often use hybrid photon counting detectors.

Medical uses

Since Wilhelm Conrad Röntgen discovered that X-rays could show bone structures, they have been widely used in medical imaging. By 2010, over five billion medical imaging exams had been done around the world. In 2006, about half of all exposure to ionizing radiation in the United States came from medical imaging.

Projectional radiography creates flat, two-dimensional pictures using X-rays. Because bones contain a lot of calcium, they block X-rays well and appear clearly on the image. This helps doctors see problems in the skeletal system and some issues in softer parts of the body, like the lungs. Common tests include chest X-rays to look for lung diseases and abdominal X-rays to find blockages or fluid in the belly.

Other medical uses of X-rays include computed tomography (CT scans), which create detailed slices of the body, and fluoroscopy, which shows moving images of internal structures. X-rays are also used in radiation therapy to treat cancers by targeting them with controlled radiation beams.

Adverse effects

X-rays are a type of ionizing radiation and can be harmful if not used properly. They are known to increase the risk of cancer, especially with repeated or high-dose exposures like CT scans. The risk depends on the type of X-ray and the part of the body being imaged. For example, a simple chest X-ray has a very low risk, similar to a short period of natural background radiation, while a CT scan exposes the body to much more radiation.

Pregnant women should avoid X-rays when possible, as radiation can affect the developing fetus. Doctors often use ultrasound instead for pregnant patients because it does not use radiation. It is important to use X-rays only when necessary to reduce any possible health risks.

Other uses

X-rays have many important uses beyond medical imaging. For example, X-ray crystallography helps scientists study the tiny structures inside crystals, and X-ray astronomy lets us observe high-energy light from stars and other objects in space.

Other uses include checking the quality of products in factories with industrial radiography and automated X-ray inspection, scanning luggage at airports for safety with airport security, and even creating art with fine art photography. These examples show how useful X-rays can be in many different fields.

Visibility

X-rays usually can't be seen because they are outside the range of colors our eyes can detect. But in special cases, after the eyes have adjusted to the dark, people have reported seeing a faint blue-gray glow when very close to a strong X-ray tube. Scientists think this might be because of a special kind of light called Cherenkov radiation, or possibly because the X-rays excited cells in the eye.

Even though X-rays are invisible, very strong beams can make the air glow. This happens when the X-rays hit air molecules hard enough to make them light up, like what can be seen at places with very powerful X-ray machines.

Units of measure and exposure

X-rays can affect materials by depositing energy, and scientists measure this using different units. The SI system uses the gray (Gy) to measure how much energy is absorbed, where 1 Gy means one joule of energy deposited in one kilogram of material. There is also an older unit called the rad, where 100 rad equals 1 gray.

Another important measure is the equivalent dose, which shows the biological effect of radiation on human tissue. For X-rays, this is the same as the absorbed dose. The sievert (Sv) is the SI unit for this, and for X-rays, 1 Sv equals 1 Gy.