|NDT.net - December 2002, Vol. 7 No.12|
Radiography is a helpful method to identify the inherent structure of unexploded ordnances (UXO’s). Portable Computed Radiography (CR) systems on the basis of phosphor imaging plates , have attracted attention for NDT-applications during the last few years. A portable IP system “ACR 2000” has been tested for X-ray inspection of UXO's in the field. Digitised radiographs are presented to explain the needs and demands for a reliable characterisation of UXO's. Computed radiographs of UXO’s are compared with film radiographs exposed in the same way, to evaluate the reliability and to identify advantages and disadvantages of the CR system for UXO inspection. Film based and CR systems provide sufficient information for identification of chemical warfare agents in UXO's, which is an essential problem for explosive ordnance disposal. The advantages and the potential of the portable CR system is discussed on the basis of experiences gathered during different field trails.
The application of an IP system is shown schematically in Figure 1 The imaging plate is a flexible area sensor which contains a photostimulable phosphor layer. The exposure of an CR radiograph, taken from an UXO with an imaging plate is performed in a manner similar to the X-ray film technique. Like a film, imaging plates store the exposure pattern as a latent image, when they have been exposed to penetrating radiation. In contradiction to the film material the latent image is retained by means of photostimulable storage phosphors. The latent image must be transformed and computed to a digital image by stepwise scanning of the exposed imaging plate with a laser beam in the digitiser. Photo Stimulated Luminescence (PSL) is released (blue light) upon laser excitation (red light). The PSL intensity is proportional to the dose stored in the IP during the exposure. While scanning, the PSL signal of each plate position is collected in a light guide and is converted to an electric signal by means of a photomultiplier tube(PMT). Then, the signal is converted to a digital signal (e.g. 12 bit for ACR2000 system of AGFA-NDT)) and stored in a digital storage media. The scanning intensity of the laser and the reading sensitivity of the photomultiplier tube can be selected according to the application. The stored data are displayed on a monitor or they are printed to film. Although monitors are only capable of displaying 256 grey-levels (8 bits), the digital images look like a film radiograph on a light box. The quality of the digital images can be improved by contrast stretching and brightness shifting to make them better viewable. Additional improvements, manipulation, annotation and data analysis by means of image processing are possible on the computer, which makes the evaluation of the image easier for the operator. The internet tools can also be used to send the images via the network to other experts for consultation, which is sometimes needed to make the correct decision with minimal time loss. A complete erasure of the IP has to be performed, before it can be used for the next exposure. For this procedure the IP is irradiated by an intensive light source.(Fig. 1). The intensive light discharges the metastable colour centres which could not be excited for the stimulated luminescence during the scanning process of the imaging plate. The latent picture is completely removed by this way. An imaging plate can be reused up to about 1000 times until physical destruction of the protective coating layer leads to damages of the plate.
|Fig 1: Scheme of the imaging plate cycle process.|
The field trail was performed to evaluate the capabilities of the portable CR-unit "ACR 2000" of AGFA-NDT which was employed for the UXO-clean up area "Krampnitzer Heide". Phosphor imaging plate radiography as well as film radiography were performed for inspection of UXO’s, gathered from the cleared area. The X- ray equipment and experimental set up used for the field trail are shown in Figure 2. A special object holder had to be designed and prepared for the x-ray inspection for two reasons: Firstly, to prevent the contact between the X-ray kit and the poisonous UXO surface. Secondly, to arrange the UXO in a tilted position, what means some degrees inclined from the vertical direction in order to verify the liquid surface of the chemical agent inside the UXO. A 300 keV X-ray tube, Philips G301, was used for the experiments. The portable IP scanner, portable PC and eraser unit were set up in a mobile working unit. A petrol generator was used for the power supply. Exposure charts for the portable IP system were not available for the study. Therefore, first exposure time guidelines were established by radiographing steel step wedges in the laboratory in preparation of the field trail. The exposure times selected in the experiments are related to the estimated wall thickness of the UXO.
Fig 2: Experimental set up for in field X-ray inspection of UXO at the contaminated clean up area
(State of Brandenburg; Germany).
All imaging plate exposures were taken with an Agfa D7/C5 film system, to allow the identical radiographic set up needed to draw a comparison. An AGFA MD10-2TWQ image plate has been used for the exposure experiments. It was combined with a 0,2 mm thick front and back screen of lead to improve the contrast sensitivity. Also, the screens and the IP were combined in a cartridge to ensure direct contact. In contradiction to the IP system the film was combined with two 0.1 mm intensifying lead screens. Exposure experiments have been performed before the in field test to adjust the ACR 2000 scanner parameter in an appropriate manner. Evaluation of the IP und film images made an assignment between density of an Agfa D7/C5 film system and a certain grey value of an AGFA MD10-2TWQ / ACR 2000 system possible. The grey value of 3500 digits corresponds to D-D0 = 2 when the photomultiplier tube gain is set to 50. This corresponds approximately to a signal to noise ratio of 55 (related to a linear dose response; see standard proposal )
The intensity resolution of the imaging plate system was limited by the grey level-scale of 4096 values (12 Bit). The spatial resolution was set to 300 dpi.
All computed radiographs were taken with the logarithmic characteristic of the amplifier electronic and interpreted with a home made software with 16 Bit image processing.
Representative images of UXO's are shown in Figures 3 to 6. Each radiograph is shown for both IP system and film system radiography to allow a better comparison of the images.
Fig 3: Radiograph of a German 7,7 cm shrapnel shell
U= 295 keV; I= 5 mA; t= 9 min; FOD= 1 m;|
a) D7/C5; 0,1/0,1Pb- f./b. screen
b) ACR 2000 / Agfa MD10-3 imaging plate system;
PMT gain 50; 0,2/0,2 Pb- f./b. screen
Figure 3 shows a German 7,7 cm shrapnel. It most likely contains steel or lead beads. They can be easily recognized in the image in both cases. The Figures 4a and 4b show unambiguously a very sharp contrast transition inside the shell interior. The contrast transition originates from the liquid surface of the chemical warfare filling inside the shell. It appears diagonal in the image because the shell was inclined at about 30 ° to the horizontal direction during the exposure. The UXO has been identified as a chemical weapon.
Fig 4: Radiograph of a German 15 cm mortar shell with liquid chemical warfare.|
a) D7/C5; I= 5 mA; t= 16 min; 0,1/0,1Pb- f./b. screen; U= 295 keV; FOD= 1 m
b) D8+U8 (salt screen); I= 1 mA; t=0,7 min; U= 295 keV; FOD= 1 m
c) ACR 2000 / Agfa MD10-3 System; PMT gain 50; I= 5 mA; t=16 min;
0,2/0,2Pb- f./b. screen; U= 295 keV; FOD= 1 m
d) ACR 2000 / Agfa MD10-3 System; PMT gain 120; I= 5 mA; t= 4 min;
0,2/0,2Pb- f./b. screen; U= 295 keV; FOD= 1 m.
The radiographs in Figures 4c and 4d show the liquid level more noisy and it can only be speculated. The drop in the image quality is related to changes of the intensifying screen/film system und the gain respectively. An Agfa U8/D8 salt intensifying film and photmultiplier tube gain of 120 was selected for this case. The change in the parameter required to reduce the exposure time down to one tenth of the regular exposure time. The image contrast has been reduced in such a way that the identification of the shell as a chemical weapon was difficult und uncertain. This is because of the high granularity of the film system, the higher inherent unsharpness and the lower signal to noise ratio in the imaging plate radiograph.
The importance of the image contrast und image unsharpness is discussed in more detail with aid of Figure 5. The contrast of the phase transition (liquid/gas) is not clearly perceptible in Figure 5a in comparison to Figure 4. The shell was shaken and exposed for a second time to increase the confidence for the statement of safety. The diagonal indication of the phase transition diminished in Figure 5b. The image led to the identification of the UXO as a practice mortar shell, which was probably filled with sand.
Fig 5: Radiograph of a German 10,5 cm practice mortar shell (Content: probably sand)
U= 295 keV; I= 5 mA; t= 20 min; FOD= 1 m; 0,1/0,1Pb- f./b. screen; D7/C5|
a) shell in inclined position
b) shell was shook before putting it in inclined position.
Figure 6 illustrates the importance of the material thickness sensitivity. Many inherent parts of the shell can be recognized by the operator, if it is large enough. Every recognizable part in the shell interior can be used by an EOD-expert for identification purposes. A radiograph can help to analyse the ordnance with regard to its function, provided that the operator can combine the image information with basic knowledge. The high-quality image enables the EOD service staff to recognise the potential danger posed by the UXO, to neutralize the hazards and to arrange necessities for transportation and disposal.
Fig 6: Radiograph of a German 7,6 cm mortar shell;|
U= 295 keV; I= 5 mA; t= 4,5 min; FOD= 1 m;
a) D7/C5; 0,1/0,1Pb - f./b. screen;
b) ACR 2000 / Agfa MD10-3 System; PMV gain 50; 0,2/0,2Pb - f./b. screen.
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