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You are here: Home page > Tools, instruments, and measurement > X rays


X RAYS

   
   
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by Chris Woodford. Last updated: October 31, 2022.

If our eyes could detect super-energetic forms of radiation such as X rays,
looking at our friends would be an altogether more surreal experience: we'd be
able to see straight through their skin and watch their bones jiggling about
underneath! Perhaps it's fortunate that we don't have that kind of ability—but
we can still enjoy the benefits of using X rays all the same: they're hugely
important in medicine, scientific research, astronomy, and industry. Let's take
a closer look at what X rays are, how they work, and how we make them!

Photo: Once X rays had to be treated like old-fashioned photographs. Now,
they're as easy to study and store as digital photographs on computer screens.
Photo by Kasey Zickmund courtesy of U.S. Air Force.

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CONTENTS

 1. What are X rays?
 2. What are X rays used for?
 3. How are X rays produced?
 4. How were X rays discovered?
 5. Find out more


WHAT ARE X RAYS?

Imagine you had the job of redesigning light to make it a bit more powerful—so
you could see through bodies, buildings, and anything else you fancied. You
might come up with something a bit like X rays.

X rays are a kind of super-powerful version of ordinary light: a higher-energy
form of electromagnetic radiation that travel at the speed of light in straight
lines (just like light waves do). If you could pin X rays down on a piece of
paper and measure them, you'd find their wavelength (the distance between one
wave crest and the next) was thousands of times shorter than that of ordinary
light. That means their frequency (how often they wiggle about) is
correspondingly greater. And, because the energy of electromagnetic waves is
directly related to their frequency, X rays are much more energetic and
penetrating than light waves as well. So here's the most important thing you
need to remember: X rays can travel through things that ordinary light waves
can't because they're much more energetic.



Artwork: The electromagnetic spectrum, with the X-ray band highlighted in yellow
over toward the right. You can see that X rays have shorter wavelengths, higher
frequencies, and higher energy than most other types of electromagnetic
radiation, and don't penetrate Earth's atmosphere. Their wavelengths are around
the same scale as atomic sizes. Artwork courtesy of NASA (please follow this
link for a bigger and clearer version of this image).

We all know that some materials (such as glass and plastic) let light pass
through them very easily while other materials (such as wood and metal) don't.
In much the same way, there are materials that allow X rays to pass straight
through them—and materials that stop X rays dead in their tracks. Why is this?
When X rays enter a material, they have to fight their way through a huge scrum
of atoms if they're going to emerge from the other side. What really gets in
their way is the electrons whizzing round those atoms. The more electrons there
are, the more chance they have of absorbing the X rays and the less likely the X
rays are to emerge from the material. X rays will tend to pass through materials
made from lighter atoms with relatively few electrons (such as skin, built from
carbon-based molecules), but they're stopped in their tracks by heavier atoms
with lots of electrons. Lead, a heavy metal with 82 electrons spinning round
each of its atoms, is particularly good at stopping X rays. (That's why X-ray
technicians in hospitals wear lead aprons and stand behind lead screens.) The
fact that some materials let X rays travel through them better than others turns
out to be very useful indeed.



Artwork: Lead is a heavy element that you'll find toward the bottom of the
periodic table: its atoms contain lots of protons and neutrons, so they're very
dense and heavy. Lead is very good at stopping X rays.

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WHAT ARE X RAYS USED FOR?

From studying tooth decay in your mouth to detecting events in distant galaxies,
X rays are useful in many different ways.


MEDICINE

One of the first uses people found for X rays was in medicine—and they're still
best known as a medical tool, used in both diagnosis and treatment. Hard
materials such as bones and teeth are very good at absorbing X rays, whereas
soft tissues like skin and muscle allow the rays to pass straight through. That
makes X-ray photographs (which look like shadows of the things inside your body)
extremely useful for all kinds of medical diagnosis: they can show up broken
bones, tumors, and also help to diagnose lung conditions such as tuberculosis
and pneumonia. Dental X rays help your dentist understand what's happening in
parts of your mouth—inside your teeth and gums—that they could not otherwise
see.



Photo: Taking a dental X ray with modern, digital technology. This equipment
uses low-power (and therefore safer) X rays and instead of the dentist having to
develop an old-fashioned photo, the results show up almost instantly on their
computer screen. Photo by Matthew Lotz courtesy of US Air Force.

There's a limit to what a physician can understand from a two-dimensional
photograph of your three-dimensional body, especially with so much packed inside
such a small space, but 3D-scanning technology helps to overcome that. CT or CAT
(computerized axial tomography) scanners draw what are effectively 3D, X-ray
pictures on screens by firing pencil-thin beams of X rays through a patient's
body and using computer technology to turn lots of 2D pictures into a single 3D
image.



Photo: A typical CT scanner. The patient lies on the bed, which slides through
the hole in the donut-shaped scanner behind. The scanner unit contains one or
more rotating X-ray sources and detectors. Photo by Francisco V. Govea II
courtesy of US Air Force and Wikimedia Commons.

Since X rays are highly energetic, they can damage living tissue when they pass
through it. On one hand, this means X rays have to be used cautiously and quite
selectively—and X-ray technicians (known as radiographers) have to take
precautions about absorbing too much of the radiation during their work. But on
the other hand, X rays can also be used to sterilize medical equipment (because
they destroy germs) and kill tumors in the treatment of cancer. This is known as
X-ray therapy (also called radiation therapy and radiotherapy).

Find out more about medical uses of X-rays from the US government's National
Institute of Biomedical Imaging and Bioengineering (NIBIB).


SECURITY

X ray scans that show up the organs lurking inside your body are just as useful
for checking bags at airport check-ins: X rays pass straight through soft
materials such as leather and plastic but are blocked by the metal in guns,
knives, and weapons. Typically suitcases and bags travel up through large
scanners on conveyor belts, with X ray images of their contents appearing
instantly on computer screens studied by security guards. CT scans are
increasingly being used in airport scanners to measure the density of liquids
being carried in luggage; this has proved to be a quick and effective way of
detecting some kinds of explosives. Scanners such as this are called CTX
machines and are made by companies such as GE InVision.



Photo: Using digital X ray equipment (left) to check the contents of a
suspicious package (on the floor, right). Photo by Jonathan Pomeroy courtesy of
US Air Force.


INDUSTRIAL APPLICATIONS

If you can use X rays to study lung problems or scan airport baggage, why not
use it in a similar way to detect faults lurking inside machines? That's the
theory behind nondestructive testing, where engineers X ray all kinds of
industrial equipment to help them track down things like cracks and fatigue in
metal components that might otherwise go undetected. Turbine blades in airplane
jet engines are tested in this way to make sure they're not harboring any
problems that would cause them to fail suddenly during flight. All kinds of
other products are also routinely studied with X rays. Oil paintings, for
example, are often X rayed to prove their authenticity (occasionally showing up
earlier versions of a picture or entirely different images by the same artist on
the same canvas).



Photo: Nondestructive X ray testing is one way to inspect planes without taking
them apart. Here, a plane has just been tested in a lead-lined hangar at
Randolph US Air Force Base, Texas. The warning signs you can see on the door
indicate the potential dangers from the X rays. Photo by Steve Thurow courtesy
of US Air Force.

Tiny, precise X-ray beams can also be used as microscopic machine tools. The
miniature circuit patterns of integrated circuits (silicon chips) can now be
drawn using immensely precise beams of X rays using a technique called X-ray
lithography. Light beams were once used for this purpose; using X rays, which
are thousands of times finer, allows components to be made smaller, which in
turn makes for smaller and more powerful chips.


SCIENTIFIC RESEARCH



Photo: Studying semiconductor materials with X-ray spectroscopy. Photo by Jim
Yost courtesy of US DOE/NREL.

Apart from medicine, the other original use for X rays was in studying the inner
structure of materials. If you fire a beam of X rays at a crystal, the atoms
scatter the beam in a very precise way, casting a kind of shadow of the
crystal's interior pattern from which you can measure the distance between one
atom and its neighbors. This is called X-ray diffraction or X-ray
crystallography, and, thanks to British scientist Rosalind Franklin, it played a
hugely important part in the discovery of DNA's structure in the 1950s.


ASTRONOMY



Photo: X-ray image of the Sun produced by the Soft X-ray Telescope (SXT). Photo
courtesy of NASA Goddard Space Flight Center (NASA-GSFC).

We're used to the idea of looking through telescopes to see light from distant
objects—even ones far out into space. But not all telescopes work this way.
Radio telescopes, for example, are more like giant satellite-dish antennas that
capture radio waves being given off from those distant sources. X rays also
travel through space and we can study them in a similar way with telescopes
tuned to recognize their particular frequency. Unfortunately for astronomers,
but possibly fortunately for the benefit of our own health, Earth's atmosphere
absorbs X rays coming from space before they reach our planet's surface. That
means we have to study sources of X rays with telescopes located in space
instead of ones based here on Earth. Find out more on NASA's page about X Ray
Astronomy.


HOW ARE X RAYS PRODUCED?

If you've read our main article on light, you'll understand that you see things
when they reflect light rays. More specifically, reflection happens when the
electrons in atoms inside objects move position to absorb and then re-emit light
energy. If you want to make red light, you can shine a flashlight on a tomato so
the red part of the original white light in your flashlight beam is reflected
back. X rays are produced in a more energetic version of the same process. If
you want to make X rays, you simply fire a beam of really high-energy electrons
(accelerated using a high-voltage electricity supply) at a piece of metal
(typically tungsten). What gets reflected back, in this case, is neither light
nor electrons but a beam of X rays. Generally speaking, the higher the voltage
you use, the faster the electrons go, the more energetically they crash into the
tungsten, and the higher the energy (and frequency) of the X rays they produce.


HOW WERE X RAYS DISCOVERED?



Photo: Wilhelm Röntgen's X-ray photograph of his wife's hand. Note the rings!
Photo believed to be in the public domain, courtesy of the National Library of
Medicine's Images from the History of Medicine (NLM) collection and the National
Institutes of Health.

Here's a brief history of X rays from their discovery, at the end of the 19th
century, up to modern times:


19TH CENTURY

 * 1895: German physicist Wilhelm Röntgen (1845–1923) discovers X rays while
   experimenting with cathode rays (the name then given to electron beams) in a
   glass tube. The X rays leak through the glass and into a nearby cardboard
   box, where they make paper coated with a fluorescent material glow. Röntgen
   doesn't know what these rays are so he calls them "X rays" (X being the name
   typically given to unknown quantities in mathematical problems). This
   discovery earns him the very first Nobel Prize in Physics in 1901.
 * 1896: Inspired by this discovery, prolific American inventor Thomas Edison
   (1847–1931) develops an X-ray viewer called a fluoroscope.


20TH CENTURY

 * 1906: Charles Barkla (1877–1944), a British physicist, shows that X rays can
   be polarized in a similar way to beams of light. This provides important
   evidence that X rays are essentially like light waves only of different
   wavelength and frequency.
 * 1912: German physicist Max von Laue (1879–1960) discovers he can measure the
   wavelength of X rays by firing them through crystals, roughly confirming the
   wavelength of X rays and the regular atomic nature of crystals.
 * 1913-1914: British physicist William Henry Bragg (1862–1942) and his son
   (William) Lawrence Bragg (1890–1971) effectively reverse this experiment,
   showing how X rays of known wavelength can be used to measure the atomic
   spacing of crystals—and developing the field of X-ray crystallography. For
   this, they earn the 1915 Nobel Prize in Physics.
 * 1913: American physicist William David Coolidge (1873–1975) develops the
   practical X-ray-making machine. Known as a Coolidge tube, it's a long glass
   jar with an electron beam and a metal target inside. When the beam is fired
   at the target, X rays are produced. Increasing the voltage produces faster
   and more energetic X rays with higher frequencies and shorter wavelengths.
   Coolidge patents his invention in 1916. Most X-ray machines still work
   broadly this way today.
   
   
   
   Illustration: A typical Coolidge tube. Artwork courtesy of the Wellcome
   Collection published under a Creative Commons (CC BY 4.0) licence.

 * 1922: Arthur H. Compton (1892–1962), another American physicist, studies the
   reflection of X rays from highly polished glass and measures their wavelength
   very precisely. He discovers the phenomenon now called the Compton effect (or
   Compton scattering): the scattered X rays have less energy than the particles
   in the original beam, providing evidence for the particle-nature of
   electromagnetic radiation.
 * 1953: Francis Crick (1916–2004) and James D. Watson (1928–) work out the
   structure of DNA with help from X-ray diffraction images produced by Rosalind
   Franklin (1920–1958).
 * 1972: British electronics engineer Godfrey Hounsfield (1919–2004) invents the
   CT scanner, which makes 3D images of the inside of a person's body using thin
   X-ray beams.
 * 1980s: Powerful X-ray lasers are proposed that would produce X rays through a
   process of stimulated emission (where atoms are made to emit radiation in a
   precise way by persistently "pumping" them with energy in a space between two
   parallel mirrors).
 * 1999: The Space Shuttle launches the Chandra X-ray Observatory—the most
   sensitive X-ray telescope to date.
   
   
   
   Photo: The Chandra X-ray telescope just before it was released from the Space
   Shuttle Columbia on on July 23, 1999. Photo courtesy of NASA/JSC


21ST CENTURY

 * 2000s: CT X-ray scanners are used to improve baggage-screening security in
   airports.
 * 2009: Scientists at SLAC National Accelerator Laboratory, Menlo Park,
   California produce a powerful X-ray laser described as "the world's brightest
   X-ray source."
 * 2018: Researchers in New Zealand develop a medical scanner that can produce
   3D color X rays of the human body.
 * 2019: Singapore scientists demonstrate how perovskite crystals could make
   better X ray detectors.

   
   
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FIND OUT MORE


ON THIS WEBSITE

 * Atoms
 * Electromagnetic spectrum
 * Light
 * Magnetism
 * Space telescopes


ON OTHER SITES

 * X Rays: What does an X ray test involve? What will it feel like? Are there
   any risks? The U.S. National Library of Medicine's Medline tells you all you
   need to know.
 * X Rays: The UK government's NHS website sets out the procedure of having an X
   ray and describes the benefits and risks (compared to other natural risks we
   all experience every day).


BOOKS

 * Chandra's Cosmos: Dark Matter, Black Holes, and Other Wonders Revealed by
   NASA's Premier X-Ray Observatory Hardcover by Wallace H Tucker. Smithsonian,
   2017. Explores some of the great discoveries scientists have made using
   images and data from the Chandra X-Ray Observatory.
 * X-Rays and Extreme Ultraviolet Radiation: Principles and Applications by
   David Attwood and Anne Sakdinawat. Cambridge University Press, 2016. Covers
   the basic concepts of high-energy radiation and its applications in
   lithography, microscopy, astronomy, and lasers.
 * X ray by Nick Veasey. Goodman/Carlton Books, 2013. A collection of intriguing
   X rays of everyday things, including photos of plants, people, and gadgets.
 * X rays: The First Hundred Years by Alan G. Michette et al (eds). John Wiley &
   Sons, 1996. A collection of papers published to commemorate 100 years since
   Röntgen's discovery.


ARTICLES

 * That Lead Apron in the X-Ray Room? You May Not Need It by By Mary Chris
   Jaklevic, The New York Times, January 14, 2020. A look at the latest thinking
   on shielding.
 * X-ray Detection May Be Perovskites’ Killer App by Jean Kumagai. IEEE
   Spectrum, May 20, 2009. How perovskite crystals could lead to more sensitive
   X ray detectors.
 * 3-D Color X Rays Could Help Spot Deadly Disease Without Surgery by Emily
   Baumgaertner. The New York Times, July 17, 2018. New X ray medical scanners
   can produce more realistic color images.
 * X rays Map the 3D Interior of Integrated Circuits by Rachel Courtland. IEEE
   Spectrum, March 17, 2017. How scientists are using X rays to probe the inner
   structure of microchips.
 * You Probably Don't Need Dental X Rays Every Year by Austin Frakt. The New
   York Times, July 25, 2016. Do you really need to have dental X rays so often?
 * Less Is More With Next-Generation Medical X rays by Mark Anderson. IEEE
   Spectrum, February 27, 2014. A new technique called X-ray phase-contrast
   imaging (XPCI) promises more comprehensive images with smaller X ray doses.

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Text copyright © Chris Woodford 2009, 2022. All rights reserved. Full copyright
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