FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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for light weight and resistance to shock [14]. Advances of particular interest
in transportation and energy include a portable linear accelerator [15,16],
a newly available, medium energy radioisotope source [17,18] and new microfocus
X-ray tubes that permit inspections with high geometric magnification
[19,20].
The portable linear accelerator, the Minac, is a lightweight (about 100 kg)
X-ray machine that provides an output of 100 R/min. at 1 meter at an X-ray
energy of 3.5MeV. The unit is portable and has been used for a variety of
field applications including inspections of pumps, valves and piping in nuclear
power plants. Wall thicknesses up to about 20 cm of steel have been inspected.
A new modification, called the Shrinkac, is extremely small (about
25 cm on a side). It is coupled to the main radiation head by a flexible waveguide
up to 6 m in length. This provides excellent versatility for placing
the radiation head in the tight quarters often found in field situations.
At the low energy end of the source scale is the radioactive gamma source
ytterbium-169. This source yields gamma rays in the energy range 53 to 310kV,
with much of the radiation intensity in the 180 to 240kV energy range [17,18].
Therefore, this source is useful for many medium thickness inspections in
which the higher energy of conventional sources such as iridium-192 or cobalt-
60 makes them unsuitable. The first half-value-layer for 1 °^Yb is 4.3 mm of
steel (about 180kV effective). The radiation output is 0.125 R/hour at 1
meter/Ci. Sources up to 10 Ci in size are available. The short half-life,
31 days, is a disadvantage but the portability and medium energy make it useful
for jobs such as panoramic inspections of welds in tubes, a common inspection
problem in the transportation and energy fields.
X-ray units with very small focal spots, 100 jam or less, permit a geometric
magnification of the X-ray image of 5, 10, 20X or more. In addition to providing
magnified views so that small detail, such as porosity in cast turbine
blades, can be seen more readily, these microfocus X-ray images also display
improved contrast because less scattered radiation from the inspection object
reaches the detector. Several commercially available units claim focal spots
in the 1 to 10 um range and yield X-rays up to 160kV in energy. Rod anode
units are available in rod diameters from 8 to 15 mm and lengths from 300 to
1000 mm. Such units are especially useful for panoramic radiography of tube
to tubesheet welds in heat exchangers and other similar applications.
The relatively low energy of these microfocus systems and the excellent detail
and contrast that can be obtained make them particularly useful for inspections
of nonmetallic materials and components such as composites and ceramics
[21], Ar. example of a microfocus X-ray inspection of a ceramic sample showing
small inclusions is shown in Fig. 2. In this case the microfocus X-ray was
combined with a real-time detection system so that the part could be oriented
to give the best inspection view; microfocus X-ray and real-time detection
can provide excellent inspection results.