Operating Manual

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Linear detectors
Linear detector arrays (LDAs) based on CMOS tech-
nology, as shown in figure 12-16, are commonly
used in applications where a mechanical means pro-
vides a relative motion between the object being
inspected and the X-ray beam. LDAs can be made in
virtually any length. In practice active lengths are
available up to over 1 metre and energy ranges from
100 kV to several MeV.
A common well-known application is airport luggage inspection, where a conveyor belt
carries objects through a fan-shaped (collimated) X-ray beam and past a linear detector
array as illustrated in figure 13-16. A series of row-like subimages from successive locations
is then assembled to form a two-dimensional radiograph for interpretation.
In NDT, similar linear arrays
with small sensor elements
are typically used in high
speed testing machines for
production inspection appli-
cations that incorporate
either a manipulator or a
conveyor to move parts past
a stationary X-ray tube de-
tector arrangement similar
to figure 13-16.
A relatively new application is the inspection of circumferential welds (so-called girth-
welds) during construction of pipe lines, either cross country or on lay barges, see
section 16.11. For such systems CMOS type linear arrays are in use because of their effi-
ciency, fast response and erase properties (< 0.2 msec) and last but not least their robust-
ness; an essential requirement for application under adverse field conditions.
Linear (or curvilinear) arrays are also commonly used in CT applications .
Direct linear arrays using CdTe (Cadmium Telluride) and other semiconductor materials
are now available, but most commonly linear arrays are of the indirect type with a scin-
tillator material to convert incident X-rays into visible light and crystalline silicon photo-
diodes measuring the light. These analogue signals are subsequently digitised and con-
verted into grey levels.
2D detectors
The “simplest” type of DR detector used in NDT is a two-dimensional array of detection
“pixels” to measure incident X-ray intensity to directly create a radiographic image
without the need for any motion of the component. Small 2D detectors typically use a
photo detector array made from a crystalline silicon integrated circuit, optically mated to
a powdered scintillator screen.
Both Charge-Coupled Devices (CCD’s) and Complimentary Metal-Oxide Semiconductor
(CMOS) devices are used. Typically the screens use a powdered Gadolinium
OxySulfide (GOS) material to convert the incident X-rays to visible light. These
devices can have very good spatial resolution, but are often used with thin scintillator
screens that can limit X-ray absorption efficiency (detection) over the full range of Xray
energies used in common NDT applications. Because they are made from singlecrystal
silicon wafers, they are also limited in size. Thus detector designs that cover a
larger area either require tiling of multiple devices or an x-y motion of a single small
device to simulate over time the effect of a larger device.
Larger 2D detectors (up to the size of common X-ray films) are usually made from photo
diode arrays of amorphous semiconductors. Some early direct detector products were
made from relatively thick film of amorphous selenium, but these direct radiography
detectors are no longer widely available for NDT applications.
More common are the indirect devices with photo detector arrays made from very
thin film of amorphous silicon, its schematic is shown in figure 11-16.
These detector panels are available both with GOS screen scintillators and with thick
layers of needle-crystal Caesium Iodide (CsI) grown directly on the photodiode arrays.
The thicker scintillator layer in the CsI devices typically provides better absorption of the
incident X-rays, and thus better imaging efficiency.
16.5.2 Fill Factor
Lateral resolution of a 2D digital
detector array is determined
by the packing density of
the
individual sensor elements
(pixels).
The denser the better.
This packing density is known as the “Fill Factor” and is illustrated in figure 14-16. This fac-
tor is dependent on the minimum possible spacing between individual elements. The Fill
Factor can be a reason to select a CMOS type detector for a particular application. For CMOS
this factor (active portion) is up to 90%, for amorphous materials up to 80%.
Fig. 12-16. CMOS linear detector array
(courtesy Envision)
Fig. 13-16. Inspection set-up using a linear array
Fig. 14-16. Fill Factor for amorphous silicon and CMOS
X-ray tube
Object
Workstation
Linear diode array
and electronics
Collimator
Moving direction
Length 640 mm
~7500 pixels
Amorphous silicon CMOS