Dense tissue scatters x-rays, producing an image, while the radiation passes through soft tissue. |
Radiation And
X-Rays
X Ray Definition and Properties (X
Radiation)
What You Need
to Know About X-Rays
By Anne Marie Helmenstine, Ph.D.
X-rays or x-radiation are
part of the electromagnetic spectrum with shorter wavelengths (higher
frequency) than visible light.
X-radiation wavelength ranges from 0.01 to 10
nanometers, or frequencies from 3×1016 Hz to 3×1019 Hz.
This puts the x-ray wavelength between ultraviolet light and gamma rays.
The
distinction between x-ray and gamma rays may be based on wavelength or on
radiation source.
Sometimes x-radiation is considered to be radiation emitted
by electrons, while gamma radiation is emitted by the atomic nucleus.
German scientist Wilhelm
Röntgen was the first to study x-rays (1895), although he was not the first
person to observe them.
X-rays had been observed emanating from Crookes tubes,
which were invented circa 1875. Röntgen called the light "X-radiation"
to indicate it was a previously unknown type.
Sometimes the radiation is
called Röntgen or Roentgen radiation, after the scientist.
Accepted
spellings include x rays, x-rays, xrays, and X rays (and radiation).
The term x-ray is also used
to refer to a radiographic image formed using x-radiation and to the method
used to produce the image.
HARD AND
SOFT X-RAYS
X-rays range in energy from
100 eV to 100 keV (below 0.2–0.1 nm wavelength). Hard x-rays are those
with photon energies greater than 5-10 keV. Soft x-rays are those with
lower energy. The wavelength of hard x-rays is comparable to the diameter of an
atom. Hard x-rays have sufficient energy to penetrate matter, while soft x-rays
are absorbed in air or penetrate water do a depth of about 1 micrometer.
SOURCES
OF X-RAYS
X-rays may be emitted
whenever sufficiently energetic charged particles strike matter. Accelerated
electrons are used to produce x-radiation in an x-ray tube, which is a vacuum
tube with a hot cathode and a metal target. Protons or other positive ions may
also be used. For example, proton-induced x-ray emission is an analytical
technique.
Natural
sources of x-radiation include radon gas, other radioisotopes, lightning, and
cosmic rays.
HOW
X-RADIATION INTERACTS WITH MATTER
The three ways x-rays
interact with matter are Compton scattering, Rayleigh scattering, and
photoabsorption. Compton scattering is the primary interaction involving high
energy hard x-rays, while photoabsorption is the dominant interaction with soft
x-rays and lower energy hard x-rays. Any x-ray has sufficient energy to
overcome the binding energy between atoms in molecules, so the effect depends
on the elemental composition of matter and not its chemical properties.
USES OF
X-RAYS
Most people are familiar with
x-rays because of their use in medical imaging, but there are many other
applications of the radiation:
In diagnostic medicine,
x-rays are used to view bone structures. Hard x-radiation is used to minimize absorption
of low energy x-rays. A filter is placed over the x-ray tube to prevent
transmission of the lower energy radiation. The high atomic mass of
calcium atoms in teeth and bones absorbs x-radiation, allowing most of the
other radiation to pass through the body. Computer tomography (CT scans),
fluoroscopy, and radiotherapy are other x-radiation diagnostic techniques.
X-rays
may also be used for therapeutic techniques, such as cancer treatments.
X-rays are used for
crystallography, astronomy, microscopy, industrial radiography, airport
security, spectroscopy,
fluorescence, and to implode fission devices. X-rays may be used to create art
and also to analyze paintings. Banned uses include x-ray hair removal and
shoe-fitting fluoroscopes, which were both popular in the 1920s.
RISKS
ASSOCIATED WITH X-RADIATION
X-rays are a form of ionizing
radiation, able to break chemical bonds and ionize atoms. When x-rays were
first discovered, people suffered radiation burns and hair loss. There were
even reports of deaths. While radiation sickness is largely a thing of the
past, medical x-rays are significant source of man-made radiation exposure,
accounting for about half the total radiation exposure from all sources in the
U.S. in 2006.
There
is disagreement about the dose that presents a hazard, partially because risk
depends on multiple factors. It is clear x-radiation is capable of causing
genetic damage that can lead to cancer and developmental problems. The highest
risk is to a fetus or child.
SEEING
X-RAYS
While x-rays are outside the
visible spectrum, it's possible to see the glow of ionized air molecules around
an intense x-ray beam. It's also possible to "see" x-rays if a strong
source is viewed by a dark-adapted eye. The mechanism for this phenomenon
remains unexplained (and the experiment is too dangerous to perform). Early
researchers reported seeing a blue-gray glow that seemed to come from within
the eye.
Anne Helmenstine, Ph.D., is an author and
consultant with a broad scientific and medical background.
Experience
Anne has taught chemistry, biology, and physics
at the high school, college, and graduate level. In her doctoral work, Anne
developed ultra-sensitive chemical detection and medical diagnostic tests. She
has worked abstracting/indexing diverse scientific literature for the
Department of Energy. She presently works as a freelance writer and scientific
consultant. She enjoys adapting lab-based science projects so that they can be
performed safely at home.
Education
Dr. Helmenstine has bachelor of arts degrees in
physics and mathematics with a minor in chemistry from Hastings College in
Nebraska and a doctorate of philosophy in biomedical sciences from the
University of Tennessee at Knoxville.
Anne Marie Helmenstine, Ph.D.
Chemistry is part of everyone's life, from
cooking and cleaning to the latest computer chip technology and vaccine
development. It doesn't have to be intimidating and it doesn't have to be hard
to understand.
You can read more about Anne's current and past
work on her Google Profile: Anne Helmenstine. Find Anne's printable
periodic tables and science projects at Science Notes.
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