X-rays can be produced
by other methods, particularly from cyclotron-style equipment
using the Bremsstrahlung effect. Additionally some radioactive
sources produce x-rays. However, these are not generally
suitable for security or industrial applications.
What affects the characteristics of
the x-rays?
A number of parameters affect the characteristics of the
x-rays produced by an x-ray tube and determine the spectrum.
The characteristics affected are:
X-ray
energy
This is determined by the KV across the x-ray tube. Increasing
KV leads to increasing x-ray energy and results in x-rays
that are more penetrating.
Flux
density
The number of x-ray photons produced is determined by the
current (amps) through the x-ray tube.
X-ray
spectrum
When electrons hit the anode, the energy is absorbed. 95%
is dissipated as heat, but the remaining 5% produces x-rays
from the anode material. The material from which the anode
is made will determine the spectrum, or distribution, of
x-ray energies emitted. Anode materials typically used in
x-ray tubes are copper, tungsten, molybdenum and silver.
X-rays range in wavelength from10 to 0.01 nanometres, corresponding
to frequencies in the range 30 to 30 000 PHz (1015Hz)
X-ray beams and how they
are created
When x-rays are produced
in an x-ray tube they travel in many directions depending
on the shape of the anode (sometimes called the target,
but not to be mistaken with the intended subject to be inspected
by the x-ray, which can also be termed “target”).
The x-ray tube is put in a steel or lead container to stop
the x-rays escaping, in random directions, and a small hole
(aperture or collimator) is introduced which releases them
(collimates) in the required direction to form the primary
beam. The shape of this aperture will determine the shape
of the primary beam. Many different types of shape can be
created, but the ones most commonly used in security and
industrial applications are cone beams and fan beams.
When the x-ray beam enters
an object it will interact with the material in one of three
basic ways. It could pass through it completely unhindered,
it may become totally absorbed or it may in some way interact
with the material and be scattered, that is leave the material
at a different angle and energy to that of the incident
primary beam. The proportion of x-rays that pass through,
are absorbed or are scattered will be dependent on the energy
of the x-rays and the material they pass through.
 |
In general scattered
x-rays are created by any material that the x-ray
beam hits, the target, air, x-ray shielding such as
steel or lead, and travel in all directions. As a
result, any x-ray system will produce x-rays from
a number of points and surfaces. These scattered x-rays
are important, because they can affect the quality
of the x-ray image. They are also a safety issue as
they can cause significant exposure to personnel.
Therefore any system designed to screen items and
personnel from exposure to the primary x-ray beam
may also have to consider screening of these scattered
x-rays. |
Containing and stopping x-rays
Different materials absorb
x-rays in different amounts at different energies. This
principle is used in x-ray imaging. In general absorbtion
increases with density so materials such as lead, which
absorb x-rays very effectively, are widely used to shield
people against harmful effects.
X-rays with more energy
(KV) are harder to stop and so require more lead to effectively
contain them. Therefore the thickness of lead required is
dependent on the x-ray energy.
How are x-rays detected?
Different types of x-ray detector
| Historically, photographic film
was the most widely used detection medium in x-ray imaging
applications. It has been used since the discovery of
x-rays at the end of the nineteenth century. However,
the principal disadvantage of x-ray film is its low
sensitivity due to poor absorption. Only about 1% of
the incoming x-rays are absorbed in the film and hence
imaged. In addition the film needs to be chemically
developed before it can be viewed. A major advantage
of film, of course. is that the detection area can be
comparatively large. |
 |
Over the last 30 years
a wide range of electronic (digital) x-ray detection devices
have been developed which are now steadily taking over from
film in many applications. The advantages of digital x-ray
imaging systems compared to the photographic film are the
higher sensitivity due to increased absorption and the avoidance
of time and material consumed in chemical processing. The
image is immediately available in digital systems, which
allows real-time operation of equipment. Because the image
is available digitally, it can be processed on the computer,
for example, adding colour to identify areas of interest,
or measure specific features.
There are many ways to
detect x-rays, all of which detect incoming photons by their
interaction with the detector material. That interaction
produces a signal which can be in the form of an electric
current, a low-energy photon (typically visible light) or
heat. The most widely used types of detectors are described
below.
Proportional
counter arrays
Proportional counters are large area detectors. They are
filled with gas that produces an electrical charge when
an x-ray passes through. The photon's energy is determined
from the strength of the electrical signal; its time from
the arrival of the x-rays and the shape of the electrical
signal.
Microchannel
plates
Microchannel plate detectors are also large-area detectors.
They are basically x-ray photomultipliers (a device which
detects dim light by producing a cascade of electrons).
They are composed of layers of reactive material divided
into narrow channels. The energy and location of incoming
x-ray photons are determined by the strength, channel location
and time of the electrical signal produced by the photon's
interaction with the detector.
Charged
coupled devices (CCDs)
In contrast to proportional counters and microchannel plates,
CCDs are small-area detectors and require the photons to
be focussed onto the detector plane. CCDs are made of silicon
doped with impurities to create sites with different conductivities.
Incoming x-rays then interact with the silicon and impurities
to create a "cloud" of electrons. A voltage is
applied across the CCD, and this cloud of electrons follows
that voltage to the end of the CCD chip. From the charge
of the electron cloud, the photon's energy is determined.
Since regular readouts are performed, the timing can also
be determined.
Semiconductor detectors
Several types of semiconductor detector exist, and more
are under development. The main types include energy-resolving
semiconductor detectors made from silicon or germanium detectors
which are good as energy-resolving detectors of single photons
(about 150 eV at 5.9 keV), and current-mode semiconductor
detectors - semiconductor diodes used in current mode to
measure x-ray flux, they offer very linear responses and
thin entrance windows.
Amorphous
silicon flat panel detectors
A further development of the CCD technology is large area
flat panel image sensors based on amorphous silicon. These
were originally developed for x-ray imaging in medical applications.
As they provide both high resolution and high dynamic range,
these expensive image sensors are well suited to certain
high-end of industrial applications.
Scintillators - improving
detector effectiveness
Several of the detectors
above, such as amorphous silicon flat panels and CCDs, incorporate
a scintillator screen in order to increase their efficiency
in capturing x-rays. Scintillators are far more efficient
at stopping x-rays than the semi-conductor or CCD which
are far more efficient at absorbing light than x-rays. When
an x-ray strikes the scintillator, it is converted to light.
The light emitted (via a process called fluorescence) is
then absorbed by the detector and converted to an electronic
image.
Different types of scintillators
are used depending on the energy of the x-rays to be detected.
The scintillators can also be used in different configurations.
In one possible configuration the scintillator is bonded
directly onto the electronic device such as the CCD or amorphous
silicon panel which increases the collection efficiency.
More traditionally, a fluoroscopic screen has been used.
The x-rays are absorbed by the screen and turned into light
creating an image, which can be captured using a standard
2D camera.
Different configurations of x-ray
detectors
Detectors for digital
x-ray imaging systems may have either linear (line scan)
or area configurations.
Line scan x-ray systems
Line scan technology uses
a thin linear array of semiconductor detectors to acquire
the image line-by-line either as the object passes by on
a conveyor, or, in the case of some systems as the detector
is moved across the target.
Click here
for an Linescan animation |
Click here
for an moving array animation |
| Producing an image using a line
scan may use a thin curtain of x-rays to illuminate
the detector array. As the object passes through the
inspection area it is illuminated by the x-ray curtain
and the resultant line image formed on the detectors
is continuously read by the computer, building up a
complete image of the object line-by-line. This technique
also reduces the x-ray exposure to the object being
inspected by around 98%.
Area detectors are
two-dimensional and filled with rows and columns of
detectors. They require no scanning procedure. They
typically use large and expensive specialised “imaging”
chips, similar to those used in conventional digital
or movie camera.
The different configurations
are suitable for different applications. For example,
in baggage inspection machines, linear detectors are
used since the bags are transported past the detector
on the conveyor belt. In medical applications, an
area detector is normally used because a picture of
an area of the body is required and the patient must
remain still. |
 |
