Operating Manual

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The scanner comprises an X-ray tube and a detector consisting of a number of elements as
illustrated in figure 13-17. A collimator reduces the beam of rays to 0.5 mm in diameter, so
that it cannot irradiate the detectors directly.
When a photon and an electron collide in the material, the primary X-radiation is scattered
as somewhat softer radiation in all directions, and thus partly also back from the material to
the scanner. This secondary radiation is then caught by the detector through a specially sha-
ped diaphragm, see figure 20-17. The detector is made up of 20 or more detector elements
marked A’, B’, C’ etc. each of which measures the quantity of back scattered radiation from
at a certain depth (A, B, C) in the object, as figure 20-17 shows. Each sensor element is, say,
focussed at a certain depth.
The cylindrical scanner measures only 7 x 7 cm and scans the object in a grid. By linking the
scanning system with a data processor, a comprehensive “Compton image” of the object
develops and any possible defects in it. The Compton backscatter technique is for instance
frequently applied to honeycomb constructions and composite materials and has a penetra-
tion depth of approximately 50 mm.
The method is (still) fairly slow; scanning a 50 cm
2
surface takes approximately 5 minutes.
An added advantage is however that the depth position of defects becomes known
immediately as a result of the “quasi-focussing” of each individual detector element.
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Such 3D CT systems are primarily intended for defect analysis. Scanning of a girth weld
typically takes about one hour. Inspection and reconstruction of one cross section takes less
than 10 minutes. An example of such a system is the so-called TomoCAR***, see “acknow-
ledgments” at the end of this book. This system uses a CMOS line array and is capable to
inspect (analyse) pipe diameters of up to 500 mm with a total irradiated thickness of up to
50 mm (2 x 25 mm). To analyse a defect in a circumferential weld the X-ray tube is at one
side of the pipe and the detector is located diametrically (at 180°) at the other side. Both are
moving synchronously. Condition for use is that there is enough space in the vicinity of the
pipe/weld for the scanner to move.
17.5 Neutron radiography (neutrography)
Neutrons, which are atomic particles without an electric charge will penetrate most materi-
als, are attenuated in passage, and so can be used to produce “radiographs”. There are vari-
ous kinds of neutron energies, but only the thermal and cold neutrons are suitable for NDT
applications. Contrary to ionising radiation in the keV and MeV range, neutron absorption
is higher in light than in heavy materials. Neutrons will be strongly influenced by hydroge-
nous materials, plastics (all types), explosives, oil, water etc., even when these materials are
inside metal containments made of lead, steel or aluminium.
There are many potential applications for neutrography, but its practical use is limited to a
large extent by the lack of suitable, portable neutron sources. A neutron “window” in an ato-
mic reactor is by far the best source, but such facilities are not commonly available. The only
neutron-emitting radioactive source is Californium
252, which is extremely costly and has a
half-life of only 2.65 years. An X-ray film also reacts to neutron energy, but useably results
are not obtained until it is combined with gadolinium or cadmium intensifying screens. The
Agfa D3SC (SC = single coated) film is frequently used for this purpose. The secondary radi-
ation generated in the intensifying screens brings about the image formation.
Another filmless application of neutron radiography in NDT is moisture detection in insula-
tion. This portable equipment that is on the market uses a very weak neutron source.
With the aid of this neutron backscatter method, the presence of water, actually that of
hydrogen atoms, is established. The presence of moisture is generally an indication of exter-
nal corrosion in a pipe, or the likelihood that corrosion will occur in the near future.
The portable real-time equipment as described in section 17.3, or flash radiography descri-
bed in section 18.7, can in some cases confirm the presence or absence of corrosion without
removing the insulation.
17.6 Compton backscatter technique
The Compton backscatter technique, see section 2.6, benefited from the introduction of
computer technology into NDT equipment, just as most other methods discussed in this
chapter. This method is very attractive for objects with access from one side only.
It is now an accepted NDT-technique for plastics and light metals [2].
Fig. 20-17. The Compton back scatter technique
X-ray beam
diaphragm
detector
collimator
object
detector