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Gamez_girl

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Joined
Jan 6, 2002
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175
X ray



Source Database: UXL Science

Table of Contents
Source Citation | View Multimedia File(s)

X rays are a form of electromagnetic radiation with wavelengths that range from about
10-7 to about 10-15 meter. No sharp boundary exists between X rays and ultraviolet
radiation on the longer wavelength side of this range. Similarly, on the shorter wavelength
side, X rays blend into that portion of the electromagnetic spectrum called gamma rays,
which have even shorter wavelengths.

X rays have wavelengths much shorter than visible light. (Wavelengths of visible light
range from about 3.5 x 10-9 meter to 7.5 x 10-9 meter.) They also behave quite
differently. They are invisible, are able to penetrate substantial thicknesses of matter,
and can ionize matter (meaning that electrons that normally occur in an atom are stripped
away from that atom). Since their discovery in 1895, X rays have become an extremely
important tool in the physical and biological sciences and the fields of medicine and
engineering.

History

X rays were discovered in 1895 by German physicist William Roentgen (1845-1923) quite
by accident. Roentgen was studying the conduction of electricity through gases at low
pressure when he observed that a fluorescent screen a few meters from his experiment
suddenly started to glow. Roentgen concluded that the glow was caused by certain
unknown rays that were given off in his experiment. Because of its unknown character,
he called this radiation X rays.

Roentgen discovered that these rays were quite penetrating. They passed easily through
paper, wood, and human flesh. He was actually able to insert his hand between the
source and the screen and see on the screen the faint shadow of the bones in his hand.
He concluded that more dense materials such as bone absorbed more X rays than less
dense material such as human flesh. He soon found that photographic plates were
sensitive to X rays and was able to make the first crude X-ray photographs.

Production of X rays

The method by which X rays were produced in Roentgen's first experiments is basically
the one still used today. As shown in the accompanying X-ray tube drawing, an X-ray
tube consists of a glass tube from which air has been removed. The tube contains two
electrodes, a negatively charged electrode called the cathode and a positively charged
target called the anode. The two electrodes are attached to a source of direct (DC)
current. When the current is turned on, electrons are ejected from the cathode. They
travel through the glass tube and strike a target. The energy released when the electrons
hit the target is emitted in the form of X rays. The wavelength of the X rays produced is
determined by the metal used for the target and the energy of the electrons released
from the cathode. X rays with higher frequencies and, therefore, higher penetrating
power are known as hard X rays. Those with lower frequencies and lower penetrating
power are known as soft X rays.

Applications of X rays

Medical

The earliest uses of X rays were based on the discoveries made by Roentgen, namely
their ability to distinguish bone and teeth from flesh in X-ray photographs. When an X-ray
beam is focused on a person's hand or jaw, for example, the beam passes through flesh
rather easily but is absorbed by bones or teeth. The picture produced in this case
consists of light areas that represent bone and teeth and dark areas that represent flesh.
Some applications of this principle in medicine are the diagnosis of broken bones and torn
ligaments, the detection of breast cancer in women, or the discovery of cavities and
impacted wisdom teeth.

X rays can be produced with energies sufficient to ionize the atoms that make up human
tissue. Thus, X rays can be used to kill cells. This is just what is done in some types of
cancer therapy. X-radiation is directed against cancer cells in the hope of destroying
them while doing minimal damage to nearby normal cells. Unfortunately, too much
exposure of normal cells to X rays can cause the development of cancer. For this reason,
great care is taken by physicians and dentists when taking X rays of any type to be sure
that the exposure to the rest of the patient's body is kept at an absolute minimum.

A relatively new technique for using X rays in the field of medicine is called computerized
axial tomography, producing what are called CAT scans. A CAT scan produces a
cross-sectional picture of a part of the body that is much sharper than a normal X ray.
Normal X rays are taken through the body, producing a picture that may show organs and
body parts superimposed on one another. In contrast, in making a CAT scan, a narrow
beam of X rays is sent through the region of interest from many different angles. A
computer is then used to reconstruct the cross-sectional picture of that region.

Nondestructive testing

The term nondestructive testing refers to methods that can be used to study the
structure of a material without destroying the material itself. For example, one could find
out what elements are present in a piece of metal alloy by dissolving the alloy in acid and
conducting chemical tests. But this process of testing obviously destroys the alloy being
tested.

X rays can be used to study the structure of a material without actually destroying it.
One approach is based on the usual method of producing X rays. A sample of unknown
material is used as the target in an X-ray machine and bombarded with high energy
electrons. The X-ray pattern produced by the sample can be compared with the X-ray
patterns for all known elements. Based on this comparison, the elements present in the
unknown sample can be identified. A typical application of this technique is the analysis of
hair or blood samples or some other material being used as evidence in a criminal
investigation.

X rays are used for nondestructive testing in business and industry in many other ways.
For example, X-ray pictures of whole engines or engine parts can be taken to look for
defects without having to take an engine apart. Similarly, sections of oil and natural gas
pipelines can be examined for cracks or defective welds. Airlines also use X-ray detectors
to check the baggage of passengers for guns or other illegal objects.

Synchrotron radiation

In recent years an interesting new source of X rays has been developed called
synchrotron radiation. Synchrotron radiation is often produced by particle accelerators
(atom-smashers). A particle accelerator is a machine used to accelerate charged
particles, such as electrons and protons, to very high speeds. As these particles travel in
a circle around a particle accelerator, they may give off energy in the form of X rays.
These X rays are what make up synchrotron radiation.

One of the more important commercial applications of synchrotron radiation is in the field
of X-ray lithography. X-ray lithography is a technique used in the electronics industry for
the manufacture of high density integrated circuits. (A circuit is a complete path of
electric current, including the source of electric energy.) The size of the circuit elements
is limited by the wavelength of the light used in them. The shorter the wavelength the
smaller the circuit elements. If X rays are used instead of light, the circuits can be made
much smaller, thereby permitting the manufacture of smaller electronic devices such as
computers.
 

Gamez_girl

Thread Starter
Joined
Jan 6, 2002
Messages
175
X-ray machine



Source Database: UXL Science

Table of Contents
Source Citation

The very first X-ray device was discovered accidentally by the German scientist Wilhelm
Röntgen in 1895. He found that a cathode-ray tube emitted certain invisible rays that
could penetrate paper and wood, causing a screen of fluorescent material several yards
away to glow. Though he used his device to examine the bone structure of the human
hand, Röntgen's machine was really just a modified cathode-ray tube. True X-ray
machines were not invented for several years.

Upon their discovery in 1895, X rays were advertised as the new scientific wonder and
were seized upon by entertainers. Circus patrons could view their own skeletons and were
given pictures of their own bony hands wearing silhouetted jewelry. While many people
were fascinated by this discovery, others feared that it would allow strangers to look
through walls and doors and eliminate privacy.

What are X rays?

X rays are waves of electromagnetic energy. They behave in much the same way as light
rays, but at much shorter wavelengths (approximately 1,000 times shorter than light).
When directed at a target, X rays can often pass through the substance uninterrupted,
especially when it is of low density. Higher density targets (such as the human body) will
reflect or absorb the X rays, because there is less space between the atoms for the short
waves to pass through. Thus, an X-ray image shows dark areas where the rays traveled
completely through the target (such as with flesh) and light areas where the rays were
blocked by dense material (such as bone).

Medical use of X rays

The most important application of the X ray has been its use in medicine. This importance
was recognized almost immediately after Röntgen's findings were published in 1895. Within
weeks of its first demonstration, an X-ray machine was used in America to diagnose bone
fractures. Thomas Alva Edison invented an X-ray fluoroscope in 1896. American
physiologist Walter Bradford Cannon used Edison's device to observe the movement of
barium sulfate through the digestive system of animals and, eventually, humans. (Barium
sulfate is a fine white powder that is still used as a contrast medium in X-ray photography
of the digestive tract.) In 1913, the first X-ray tube designed specifically for medical
purposes was developed by American chemist William Coolidge. X rays have since become
the most reliable method for diagnosing internal problems.

Modern medical X-ray machines have been grouped into two categories: those that
generate hard X rays and those that generate soft X rays. Soft X rays are the kind used
to photograph bones and internal organs; they operate at a relatively low frequency and,
unless they are repeated too often, they cause little damage to tissues. Hard X rays are
very high frequency rays designed to destroy the molecules within specific cells, thus
destroying tissue. Hard X rays are used in radiotherapy, a treatment for cancer. Because
of the high voltage necessary to generate hard X rays, they are usually produced using
cyclotrons or synchrotrons, which are variations of particle accelerators (atom smashers).

One of the more familiar X-ray machines is the security scanner used to examine baggage
at airports. These machines use a very low-power scanner to illuminate the interior of
purses and suitcases without causing damage to the contents, such as undeveloped film.

X-ray crystallography

At the same time the medical applications of X rays were first explored, a new science
was being founded. It was based on the principles introduced by German physicist Max
von Laue (1879-1960). Laue theorized that crystals could be to X rays what diffraction
gratings were to visible light. He studied the interference pattern of X rays passing
through a crystal. These patterns revealed a great deal of information about the internal
structure of the crystal. British physicists William Henry Bragg and his son William
Lawrence Bragg took this field even further, developing a system of mathematics that
could be used to interpret the interference patterns. This method became known as
X-ray crystallography. It allowed scientists to study the structures of crystals with
unsurpassed precision.

Crystallography has become an important tool for scientists, particularly those striving to
synthesize chemicals (produce them artificially). By analyzing the information within a
crystal's interference pattern, enough can be learned about that substance to create it
artificially in a laboratory, and in large quantities. This technique was used to isolate the
molecular structures of penicillin, insulin, and DNA (deoxyribonucleic acid).



Source Citation: "X-ray machine." U*X*L Science. U*X*L, 1998. Reproduced in
Discovering Collection. Farmington Hills, Mich.: Gale Group. October, 2001.
http://galenet.galegroup.com/servlet/DC/
 

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