|
X rays are produced when
fast moving electrons
hit a piece of metal (called the target).
Electrons are thermionically emitted by the
filament (cathode).
The accelerating voltage is about 100kV.
Less than 1%
of the kinetic energy of the electrons is converted into x rays so
the anode (target) must be cooled
during operation.
X rays are not deflected by electric or
magnetic fields but can be diffracted suggesting that they have
wave-like properties.
X rays are electro-magnetic radiations having
wavelengths in the range 10-11m to 10-8m.
X rays cause certain substances to
fluoresce, they affect
photo-graphic emulsions and
can ionise atoms. These three
properties can be used to detect x rays.
The intensity of the beam of x rays (Wm-2)
depends on the number of electrons
hitting the target per unit time. This depends on the
temperature of the filament.
The
penetrating power of the beam of x rays depends on the
kinetic energy of the electrons.
This depends on the accelerating voltage.
Quality & intensity of an X-Ray Beam
The intensity of the beam of x rays (Wm-2)
depends on the number of electrons
hitting the target per unit time ie the number of electrons flowing
through the tube the tube current. This depends on the
temperature of the filament.
The penetrating power of the beam of x rays
depends on the kinetic energy of the
electrons. This depends on the
accelerating voltage.
A useful analogy of
this can be made with light. Intensity can be equated to colour and
penetrating power to brightness.
Increase of the accelerating voltage applied
between filament and target is found to increase the penetrating
power of the Xrays. Since the maximum loss of kinetic energy at a
single collision is now higher (=eV), the highest frequency emitted
is also higher as expected. Thus the quality of the emitted X rays
is altered. These are called ‘hard’ Xrays.
It is
found that an increase of the heating voltage increases the
intensity of Xrays without any change in the hardness or penetrating
power. The high intensity spikes characteristic of the target
material are also of unchanged wavelengths
Effect on output of increasing Kv note the greater amount /
intensity of radiation as well as the number of higher enrgy
xrays |
