The experimental setup used for the X-ray backscatter imaging
of aerospace materials is shown in Fig. 4. An industrial X-ray tube
(Comet MXR-600 HP/11) with a maximum voltage of 600 kV and
anode current of 2.5 mA was used to irradiate the object being
inspected. The target of the X-ray tube was made of tungsten, with
5 mm thick beryllium window. The focal-spot size was 1 mm.
The X-ray backscatter camera was placed near to the source
side forming an angle of about 451 between incident and
scattered radiation directions for imaging (see Fig. 4). The width
of the twisted slit collimator is 1.4 mm. The backscatter X-ray
imaging was carried out using an X-ray backscatter camera
equipped with a digital flat panel detector with high spatial
resolution (200 mm).
The reason for selecting digital detector array for measuring
backscatter radiation in this study is due to its high signal-to-noise
ratio (SNR) and fast image acquisition time ( $ 180 s) as compared
to the conventional imaging plates [24,25]. The X-ray imaging
area on the amorphous-silicon based digital flat-panel detector
(VIDISCO Rayzor-X) is 22.2 Â 22.2 cm2 and it consists of 1560 Â
1560 pixels with a pixel size of 143 mm. In the present experi-
ments, the actual backscatter imaging area on the detector is
limited by the slit collimator geometry, mainly its width. The
backscatter image data was acquired using digital X-ray imaging
software (XbitPro) [26]. The backscattered signals were digitized
at 14 bit gray level (i.e. gray value range between 0 and 16, 384) for
obtaining better image quality. In order to reduce the scattered
radiation from surroundings (i.e. noise in the image), we posi-
tioned a 3 mm thick Cu plate followed by a 5 mm thick Pb plate at
the rear side of the digital detector array (Fig. 4).