are just like any other kind of electromagnetic
radiation. They can be produced in parcels of energy called
photons, just like light. 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 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.
Both ways of making X-rays involve a change in the state of electrons.
However, Bremsstrahlung is easier to understand using the classical
idea that radiation is emitted when the velocity of the electron
shot at the tungsten changes. The negatively charged electron
slows down after swinging around the nucleus of a positively charged
tungsten atom. This energy loss produces X-radiation. 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. In the interaction, many photons of different
wavelengths are produced, but none of the photons have more energy
than the electron had to begin with. After emitting the spectrum
of X-ray radiation, the original electron is slowed down or stopped.
Bremsstrahlung Radiation X-ray
tubes produce x-ray photons by accelerating a stream of electrons
to energies of several hundred kilovolts with velocities of several
hundred kilometers per hour and colliding them into a heavy target
material. The abrupt acceleration of the charged particles (electrons)
produces Bremsstrahlung photons. X-ray radiation with a continuous
spectrum of energies is produced with a range from a few keV to a maximum
of the energy of the electron beam. Target materials for industrial
tubes are typically tungsten, which means that the wave functions
of the bound tungsten electrons are required. The inherent filtration
of an X-ray tube must be computed, which is controlled by the amount
that the electron penetrates into the surface of the target and
by the type of vacuum window present.
The bremsstrahlung photons generated within the target material
are attenuated as they pass through typically 50 microns of
target material. The beam is further attenuated by the aluminum
or beryllium vacuum window. The results are an elimination of
the low energy photons, 1 keV through l5 keV, and a significant
reduction in the portion of the spectrum from 15 keV through 50
keV. The spectrum from an x-ray tube is further modified by the
filtration caused by the selection of filters used in the setup.
The applet below allows the user to visualize an electron accelerating
and interacting with a heavy target material. The graph keeps
a record of the bremsstrahlung photons numbers as a function of
energy. After a few events, the "building up" of the
graph may be accomplished by pressing the "automate"
K-shell Emission Radiation Remember
that atoms have their electrons arranged in closed "shells" of different energies. The K-shell is the lowest energy state
of an atom. An incoming electron can give a K-shell electron enough
energy to knock it out of its energy state. About 0.1% of the
electrons produce K-shell vacancies; most produce heat. Then,
a tungsten electron of higher energy (from an outer shell) can
fall into the K-shell. The energy lost by the falling electron
shows up in an emitted x-ray photon. Meanwhile, higher energy
electrons fall into the vacated energy state in the outer shell,
and so on. K-shell emission produces higher-intensity x-rays than
Bremsstrahlung, and the x-ray photon comes out at a single wavelength.
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 non-bonding shells of atoms are the
X-ray source. The resulting characteristic spectrum is superimposed
on the continuum
as shown in the graphs below. An atom remains ionized for a very
short time (about 10-14 second) and thus an atom can be repeatedly
ionized by the incident electrons which arrive about every 10-12