In
1895, German physicist Wilhelm Conrad Roentgen was studying cathode rays
inside of a gaseous discharge tube put under high voltage. The tube was
enclosed in a black cardboard box, but Roentgen noticed that each time the
tube was turned on a barium-platinocyanide screen lying next to the box
glowed with fluorescent light. Roentgen
called this “new” form of
electromagnetic radiation “X-rays” because he did not know what they were.
Today we know that X-rays are similar to normal light, except that X-ray
photons have more energy and a smaller wavelength than visible light rays,
the shorter the wavelength (1.00*10-6cm – 1.00*10-10cm)
the stronger the power. X-rays are also known as Roentgen rays.
Today’s X-ray machines are simply composed of an X-ray tube and a
target. The tube contains an electron gun which fires electrons at a
target, usually tungsten, and one of two atomic processes prompted by the
high-energy electrons form an X-ray picture. The two atomic processes are Bremsstrahlung and K-shell emission.
Bremsstrahlung is German for “breaking radiation”. In
Bremsstrahlung, the electron fired at the tungsten target “breaks” as it
comes around the nucleus of the tungsten atom. The energy lost as the
electron slows down is emitted as X-rays.
A K-shell is the lowest energy state of an electron. In K-shell
emission, the electron shot out of the X-ray tube forces an electron in the
K-shell out of its energy state. An electron from a higher energy state
must fall into the K-shell to replace it. Energy is released by the falling
electron in the form of X-ray photons. K-shell emission creates a cycle, as
electrons are displaced and more electrons must drop down from higher energy
levels-therefore releasing more energy in the form of X-rays-to replace the
displaced electrons. X-ray photons released by K-shell emission have a
single wavelength of a greater intensity than the X-ray photons released by Bremsstrahlung, where the photons have various wavelengths. For this
reason, K-shell emission is preferred over Bremsstrahlung.
To make the image, the X-rays are directed to a photographic plate.
The plate is white, but areas of the plate touched by X-rays turn darker in
color. X-rays can travel through skin and muscle tissue, but can not travel
through bones; therefore bones appear white in an X-ray picture. This makes
X-rays useful for studying bone structure and finding bone fractures.
Like all forms of radiation, X-rays can damage tissue. X-rays that
are absorbed by the body cause electrons to become detached from atoms and
molecules, this is known as ionization. The body is capable of repairing
damage done by X-rays, but a problem arises when the DNA of a cell is
damaged. If the DNA is marred, defected cells will be created; cancer is a
collection of cells with damaged DNA that multiply out of control. The
occasional X-ray will not permanently damage your body or cause cancer.
X-ray technicians and dentists use X-rays in a high enough dosage that they
can see what is happening to your bones and teeth, but in a small enough
dosage that the body is not permanently damaged.
To protect the body against unnecessary exposure to radiation, lead
aprons, also called gonad shields, are draped over the body. These
shields
are especially used to protect the reproductive organs from X-ray
radiation. If cells in the reproductive system are damaged, the damage may
be passed on to the patient’s children. This is also why X-rays are not
given to pregnant women, as the human fetus is seriously vulnerable to
radiation. To protect themselves from unnecessary radiation, X-ray
technicians step out of the room or behind a protective screen while the
X-ray machine is in operation.
In the medical field, X-rays are useful for more than diagnosis.
X-ray
radiation is used as a treatment for cancer tumors. In the fields of
physics and chemistry, X-ray diffraction is used to study the composition
of crystalline substances. The patterns made by X-rays diffracted
through crystals can identify the element or compound. Similar
methods of diffraction are also used for powders that contain somewhat of
a regular structure. Not only can X-rays help to identify crystals
and powders, but X-ray spectroscopy can help to classify chemical
elements
and their isotopes. Even very small organisms can be studied with
X-rays; an electron microbe emits a very narrow beam of electrons to
produce X-rays over an area as small as 1mm2.
In an industrial setting, X-rays are used to detect flaws in metallic
products without risking damage to the object. However, the equipment
needed to produce the high energy X-rays necessary for this sort of work is
bulky and costly, so gamma rays emitted by radioisotopes contained in
compact containers are often used. X-rays are useful for analyzing the
unique brushstrokes of famous artists and the age of the paintings,
therefore verifying whether or not a painting is genuine. X-rays can also
be used to verify whether or not jewels are genuine. Finally, X-rays are
very useful in airport customs, where they are used to spot explosives or
contraband in luggage.
