X-radiation
X-radiation
X-rays are part of the electromagnetic spectrum. X-radiation
(composed of X-rays) is a form of electromagnetic radiation. Xrays have a wavelength in the range of 10 to 0.01 nanometers,
corresponding to frequencies in the range 3 × 1016 Hz to 3 × 1019
Hz and energies in the range 120 eV to 120 keV. They are
shorter in wavelength than UV rays and longer than gamma rays.
In many languages, X-radiation is called Röntgen radiation, after
Wilhelm Conrad Röntgen, who is generally credited as their
discoverer.
X-rays from about 0.12 to 12 keV (10 to 0.10 nm wavelength)
are classified as "soft" X-rays, and from about 12 to 120 keV
(0.10 to 0.010 nm wavelength) as "hard" X-rays, due to their
penetrating abilities.
X-rays are a form of ionizing radiation, and exposure to them
can be a health hazard.
Electromagnetic spectrum
Electromagnetic spectrum
Bremsstrahlung and characteristic X-rays
• There are two different atomic processes
that can produce X-ray photons. One is
called Bremsstrahlung and is a German
term meaning "braking radiation“.
• The other is called characteristic X-rays
(K-shell emission). They can both occur in
the heavy atoms of tungsten. Tungsten is
often the material chosen for the target or
anode of the x-ray tube.
Bremsstrahlung X-rays
• Electrons are scattered
elastically and inelastically
by the positively charged
nucleus. The inelastically
scattered electron loses
energy, which appears as
bremsstrahlung. Elastically
scattered electrons (which
include backscattered
electrons) are generally
scattered through larger
angles than are inelastically
scattered electrons.
Characteristic X-rays
• An incident electron
ionizes the sample atom by
ejecting an electron from
an inner-shell (the K shell,
in this case). De-excitation,
in turn, produces
characteristic X-radiation
(above) or an Auger
electron (below).
Secondary electrons are
ejected with low energy
from outer loosely bound
electron shells, a process
not shown.
Characteristic X-rays
• When outer-shell electrons drop into inner shells, they
emit a quantized photon "characteristic" of the element.
The energies of the characteristic X-rays produced are
only very weakly dependent on the chemical structure
in which the atom is bound, indicating that the nonbonding shells of atoms are the X-ray source. The
resulting characteristic spectrum is superimposed on the
continuum. An atom remains ionized for a very short
time (about 10-14 second) and thus the incident
electrons that arrive about every 10-12 second can
repeatedly ionize an atom. However, not all outer-shell
electrons can fall in to produce X-rays.
X-rays are generated by
an X-ray tube. It works
with a very good
quality vacuum. The
electrons are produced
by thermionic effect
from a metal filament
heated by an electric
current. The filament is
the cathode of the tube.
The
high
voltage
potential is between the
cathode and the anode,
the electrons are thus
accelerated and then hit
the metal target anode.
eU 
X-ray tube.
Coolidge side-window tube (scheme)
K: filament (-)
A: anode (+)
Win and Wout: water inlet and outlet of the
cooling device (C)
mV
2
2
The voltages used in diagnostic X-ray tubes, and thus the highest
energies of the X-rays, range from roughly 20 to 150 kV. The anode is
usually made out of tungsten or molybdenum. The high energy
electrons interact with the atoms in the anode.
Two atomic processes of production of x-ray photons.
One is called Bremsstrahlung, which is a fancy German name meaning
"braking radiation." The electron (much lighter than the nucleus) comes
very close to the nucleus and the electromagnetic interaction causes a
deviation of the trajectory where the electron looses energy and an X-ray
photon is emitted. High energy electron beam striking high-Z material
produces X-rays and Heat. 99% or more of the electron beam energy is
deposited as heat! Less than 1% of electron beam energy is converted in
X-rays! The ideal situation would be if most of the electrons created x-ray
photons rather than heat. The energy of the emitted photon can take any
value up to a maximum corresponding to the energy of the incident
electron. The highest X-ray energy is equal to an electron kinetic energy:
mV
2
2
 h  max
h  max  eU
hc
 min
 eU
The total flux of X-radiation is:
  kIU Z
2
k  10
9
V
1
I - tube current
U – anode voltage
Z – atomic number
The anode has two primary functions: (1) to convert electronic
energy into x-radiation, and (2) to dissipate the heat created in the
process. The material for the anode is selected to enhance these
functions.
Another atomic process is X-ray fluorescence. If the electron has
enough energy it can knock an orbital electron out of the inner
electron shell of a metal anode atom, and as a result electrons from
higher energy levels then fill up the vacancy and X-ray photons are
emitted. This process produces an emission spectrum of X-ray
frequencies, sometimes referred to as the spectral lines. The
spectral lines generated depend on the target (anode) element used
and thus are called characteristic lines. Usually these are transitions
from upper shells into K shell (called K lines), into L shell (called
X-ray Continuum and Characteristic
radiation spectra