|
3.
X
- Rays
In
the latter half of the year 1895, a German scientist called Roentgen was
working in his laboratory at the Physical Institute of the University of
W¸rzburg,
Germany, experimenting with a type of discharge tube called Crookeís tube.
The tube displayed a fluorescent glow when a high voltage current was passed
though it. When he shielded the tube with heavy black cardboard, he found
that a greenish fluorescent light could be seen on a fluorescent screen kept
some 9 feet away. Roentgen concluded that a new type of ray was emitted from the
tube that could pass through the black covering. The rays could pass through
most substances, including the soft tissues of the body, but left the bones
and most metals visible. One of his earliest photographic plates from his
experiments was that of a film of his wife, Bertha's hand with a ring. Roentgen named the invisible radiations as X-rays (or unknown rays).
Bertha's hand with a
ring
Detailed
experiments showed that when cathode rays or electrons are stopped, X-rays
are emitted. The kinetic energy
of the electrons is transferred to the anode material of the discharge tube.
The atoms of the anode emit X-rays. These
days the origin of X-rays is well known and tabulated. When the cathode rays
strike the atoms in the anode, the electrons of these atoms jump to higher
excited orbits. When the electrons jump back to their original orbits,
they emit X-rays. These are called characteristic X-rays. Each element emits
specific X-rays only. X-rays are electromagnetic radiations and have
energies of the order of a few eV to a
few 10s of keV. Their energies are much less than -
radiations.
Production
of X-rays
X-rays
are produced by modified discharge tubes that are called Coolidge tubes.
Cathode is in the form of a filament, which emits electrons on heating.
Anode is made up of solid copper, molybdenum or any other material whose
X-ray energies are useful for further applications. Anode is also called the target. A high potential difference between
the cathode and anode is maintained by an external high voltage source. A
separate battery that supplies high current heats the cathode filament. The
filament is generally made out of tungsten and is coiled to provide high
resistance to the passing current.
The
electrons emitted from the filament, experience the potential difference and
are accelerated towards the anode. Thus the electrons gain energy. When such
electrons hit on the anode surface, they are stopped. They transfer their
kinetic energy to the electrons of the anode material. These then give rise
to the X-rays.
The
intensity of the X-rays depends on the number of electrons striking the
anode or the intensity of the electron beam.
High
energy X-rays are called hard X-rays and they have very high penetrating
power. Low energy X-rays are called soft X-rays and they have low
penetration powers.
| Properties
of X-rays |
-
X-rays
are invisible electromagnetic radiations. Their wavelength is less than
that of the visible light photons and is larger than the
-ray photons. They are generally of the order of a few 10s of nano-meters.
-
X-rays
posses all properties of visible light photons, such as reflection,
diffraction, rectilinear propagation, etc. Refraction of X-rays is not
yet seen.
-
Being
electromagnetic waves, they travel at the speed of light 3 x 108
m/s.
-
Since
they are not charged particles, they are not affected by either electric
or magnetic fields.
-
X-rays
can ionize materials through which they pass such as gases.
X-rays like -rays produce fluorescence and affect photographic plates.
-
X-rays
are highly penetrating. They can easily pass through thin sheets of
paper, metal foils, tissues etc.
-
X-rays
are stopped by high density materials such as lead, bones, etc.
|
Uses
of X-rays
1.
Medical
uses : X-rays are widely
used by radiologists to see if the bones are broken or not. They can be used
to see if a foreign object such as a bullet has lodged in a body, if there
are any other defects such as kidney or gall bladder stone is present or
not. X-rays are also used to test the density of the bones.
2.
Industrial
uses : Hard X-rays are routinely used to see if there are any faults
deeper in any structure such as pipes, machine parts, airplane bodies, etc.
X-rays are used to see the characteristic qualities of gemstones, the
genuineness and purity of precious metals such as gold, silver, etc. X-rays
are also used to detect the percentage of certain elements in an ore before
the ore is taken for commercial mining.
3.
Scientific
uses : X-rays undergo
diffraction. This is useful in studying the nature of the crystal and the
arrangements of atoms within the crystal. X-rays are also extensively used
to study the way chemical reactions occur. Characteristic X-rays give
information about the structure of atoms.
Differences
and similarities between cathode rays and X-rays
Both
cathode rays and X-rays were discovered when discharges from a discharge
tube were studied in details. But the rays are completely different from
each other.
| Differences
:
|
-
Cathode
rays are charged particles. They are negatively charged electrons.
X-rays on the other hand are electromagnetic radiations. X-rays have no
charges.
-
Cathode
rays emanate from the cathode itself. X-rays are emitted when
high-energy electrons are stopped.
-
Cathode
rays have low penetrating powers. X-rays have high penetrating powers,
although -rays
are much more penetrating than the X-rays.
-
Cathode
rays travel at the speed given by the potential difference between the
anode and the cathode. X-rays always travel at the speed of light.
-
Cathode
rays are deflected by electric and magnetic fields. X-rays are
unaffected by both the electric as well as the magnetic fields.
|
| Similarities
:
|
-
Both
rays are capable of ionizing materials though which they pass,
especially gases.
-
Both
rays cause fluorescence when they strike any fluorescence material such
as zinc sulphide.
-
Both
rays affect photographic plates.
|
|
