Digital Radiography for High-Energy NDT Applications

By Lior Pick, Ron Pincu and Ofra Kleinberger 

 

Introduction


Portable digital radiography surpasses film and film replacements by providing many additional benefits to users (Light, 2008). The main technologies for film replacement today are computerized radiography and process-free films. Digital radiography allows for creating images upon request for immediate analysis, yet there is no compromise on image quality due to speed and no repositioning is required. A reliable and portable digital radiography system enables reduction of work time and costs while increasing the profits of nondestructive testing (NDT) service providers.

 

This advanced technology has been recognized by new NDT standards and codes (ASME, 2010; ASTM, 2007; BSS, 2003; BSS, 2003). It enables detailed results and provides the basis for high-level on-site NDT analysis. Most of the detectors available with this kind of technology, however, usually work with limited levels of energy. Portable amorphous silicon (a-Si) flat-panel based systems are available that have been specially designed to work with digital radiography and high-energy levels/isotopes.

 

The special structure of a digital detector array (DDA) imager, in which electronic components are not placed behind the imaging area, allows for almost no backscattering, and works in high levels of energy with only a minimal external shield. The portable systems are small, can be conveyed to the inspection site and operated by one person. The systems can operate on batteries for an entire workday, so there is no need for external power in the inspection area. These characteristics make for truly mobile and reliable inspection equipment.

 

This paper presents case studies that work with digital radiography systems that were combined with high-energy levels from various locations around the world. For applications such as small bore pipes,welding, boilers and shipyard NDT, portable a-Si based systems are a true laboratory experience in the field, enabling immediate high-quality results for analysis on-site, and offering mobility and efficient testing anywhere.

 

Digital Flat-panel Technology The a-Si digital flat-panel is composed of a scintillator and an array of a-Si photodiodes (Thales, 2011). The X-ray tube sends a beam of X-ray photons through a target. The photons that are not absorbed by the target reach the a-Si detector and strike the layer of scintillating material that converts them into visible light photons. The light photons reach the photodiodes, which convert them into electrons that activate the pixels in the a-Si. The electronic data that are generated from this process are converted to a digital signal that is received by the computer. The software then converts this information into a high-quality image (shown in Figure 1).

 

Digital Radiography Advantages in Field NDT Digital radiography can be a sufficient, and even improved, replacement for X-ray film or other film replacements. In fact, digital radiography can provide advantages that do not exist with other technologies, as well as enhance the work experience of the NDT technician (Pick and Kleinberger, 2009).

 

Among these advantages is the ability to produce images in near real-time. With just a click of the mouse, a high-quality image is immediately available on the computer screen. A few more simple steps with the software, such as window leveling, sharpening or histogram equalization, and the images are ready for advanced analysis. The X-ray source can also be controlled by the system, enabling the best synchronization between the actual shooting of X-rays and the time the digital imager reads the data stored in the a-Si plate. As this process can be immediate, the data remain unspoiled. The speedy image acquisition process does not cause any compromise to image quality. The sensitive digital imager can provide images of the highest quality on the spot. At last, the professional user can make the most of the capabilities of X-rays.

 

By achieving an image that is immediately visible on the laptop screen, the results are immediately known and there is no need to remain blind on-site. Repeated shots are cost-free and thus the best position to take an X-ray image is determined on the spot. The operator can immediately reposition the X-ray source, correct the distance from the imager, change exposure time or place the imager in a better location to create the perfect image; all according to the previous results that appear immediately on the screen. There is no need to go back to a laboratory to develop or scan results.

 

Despite these advantages, in those applications where high energy is required, the NDT market tends to avoid the transition to digital radiography. The main problem experienced is the shorter lifetime of the DDA imager if it is exposed to high levels of energy. This shorter lifetime makes it difficult to return the investment made in the imager itself within the stated timeframe. This paper will investigate this problem in more detail and suggest a possible solution. 

 

The Problem: High-energy NDT and Digital Detector Array Imager Lifetime

 

High-energy NDT is defined as tests that require the use of X-ray energy levels that are higher than 160 kV, or tests that involve the use of isotopes (for example, the Ir-192 isotope, which generates approximately 450 kV on average).

 

Historically, the DDA imager lifetime is calculated by recording the time of the first dose-related failure in the imager. These first failures are typically caused by problems in the electronic components of the imager. Electronic parts are relatively low-cost among the components of the imager, and can be easily replaced. The a-Si layer inside the imager can continue to function much longer than the electronics, even if it is exposed to high doses of energy.

 

The first step towards enabling the transition to digital radiography in high-energy NDT applications is to understand that the true lifetime of an a-Si plate is much longer than the typical lifetime of the imagers. It should be understood that faulty electronic parts can be replaced at a relatively low cost, allowing the imager to operate with the original a-Si panel for a longer period of time. 

 

The Solution: Avoid High-dose Exposure of Electronics

 

The major problem when working with DDA imagers in high doses is the failure caused to electronic parts due to exposure to high-energy radiography. The key to avoiding electronic failures related to the radiography dose is to shield the electronics. The solution therefore needs to be found in the early stages of imager planning. There are two main technological solutions that must be incorporated into the initial design of the imager.

 

• A layer of shielding material (such as lead or tungsten) should be placed inside the imager between the a-Si plate and the electronic components. Thus, the a-Si plate absorbs most of the X-rays and translates them to an image, and any X-rays that go through it are blocked by the internal shield. The electronics are fully protected. The advantage to this solution is that the shield is integrated into the imager.

 

• Moving the electronics from behind the a-Si plate. When the electronic parts are located to the side of the a-Si plate, they are not directly impacted by the X-rays shot towards the imaging area. Figure 2 shows graphics of a digital panel that has the electronics located behind the a-Si plate (Thales, 2011). An external shield can easily provide foolproof protection of the electronic parts of the imager at a low cost, with an adequately cut lead or tungsten plate (shown in Figure 3). The main advantages to this solution are that the original weight of the imager is maintained and the backscattering effect is reduced to a minimum, almost nonexistent. A disadvantage may be found in the increase of the surface area of the panel.

 

In regard to the first solution, there are panels that already have internal shielding for the electronic parts, but this is a thin layer of metal that blocks most of the X-rays (but cannot provide complete protection), is limited to 160 kV and serves to maintain the declared lifetime of the panel, (which is calculated as the time to first dose related fault, and which typically occurs in the electronic parts). This is not the kind of internal shield that will serve as a solution for high-energy applications. A thicker layer of shielding, which is also heavier, is required.

 

The two main disadvantages to the first solution are imager weight and backscattering effect. The resulting imager weight is significantly increased. The shielding material layer adds a disproportionate amount of weight to the imager compared to its original weight without the internal shield. Also, in applications where high energy is not required, the NDT technician has to always work with the heavier resulting panel.

 

With the second solution, the shield plate throws back many X-rays that reach it because of its proximity to the a-Si plate, causing a heavy backscattering effect. The scattered X-rays then return and light up the a-Si pixels once more. The resulting images may be unclear due to the excess of X-rays.

 

A detailed view of the effects caused by the second solution show that it contains many advantages.

 

• The original weight of the imager is maintained. This makes its placement in various locations a simple matter. Also by maintaining its original weight, it remains easy to transport to each inspection site. In most applications where high energy is not required, one continues to work normally, with a lightweight panel and without shielding.

 

• The a-Si based DDA imager is sensitive enough to enable great results in low doses. Sometimes, when working with digital radiography, it may not be necessary to use the energy level one is accustomed to from working with film. High-energy dose levels can be avoided, which increases operator safety.

 

• When shielding is required, a simple external shield will do. Imaging area is not lost because only the sides of the imager (where the electronics are located) require protection.


• The thickness of the external shield can be determined according to the energy level used. This means its weight can be optimized to the kind of work required.

 

• The imaging area can start from the edges of the panel (on the corner where the a-Si plate is located).


• The imager is thin. Because the electronics are located on the side, the entire depth of the imager can be reduced.


• An almost entirely backscatter-free panel is created because there is nothing behind the a-Si plate that will cause radiation to return to it. This reduces the inherent noise in the images, increasing the signal-to-noise ratio and image quality.


• All of the inherent advantages of digital radiography mentioned in the beginning of this paper (high-quality images upon request, analysis anywhere, no repositioning and no compromise on quality of results) can all be available in high-energy applications.

 

Relevant Applications

 

Applications that require high-energy levels usually require penetration of thick metallic components. Such NDT inspections include pipe welding and pipe erosion tests, welding in boilers or ship hulls, and quality control inspections in casting facilities. The following case studies contain various high-energy testing examples in which a portable digital radiography system was used.

 

Pipe Inspection: Reducing Dose


A pipe test was conducted by an NDT service company using an imager with Ir-192 (iridium) and Se-75 (selenium) sources alternatively. This test showed two interesting results.

 

• Testing that is usually conducted with Ir-192 at a specific level of activity can be also conducted with a lower level of activity. This means longer usage of the same source is achieved and good results are maintained.


• Testing that is usually conducted with Ir-192 can be done with the weaker Se-75 source, and produce images of higher quality (due to the better focal spot and lower radiation energy spectra).

 

A more specific example from these tests can be seen in Figures 4a and 4b. X-ray images of a 203.2 mm (8 in.) diameter pipe with 19 mm (0.75 in.) wall thickness (total wall thickness 38.1 mm [1.5 in.]) were taken with iridium and selenium isotopes. Table 1 organizes the condition details of the images in Figures 4a and 4b. In the image taken with the selenium isotope, an extra fifth wire is clearly visible. Both images were taken under the same setup conditions, with the exception of the isotope type and exposure times. 

 

Pipe Inspections: Reducing Exposure Time

 

Table 2 contains results of tests that were conducted by an NDT service provider on pipes on-site using isotope Ir-192 combined with a digital radiography system and/or film. The comparison clearly shows that exposure times have been cut tenfold. In a large test conducted in cooperation with a refinery in France, several pipe-welding samples with intentional discontinuities, such as slag, undercut, corrosion, porosity and cracks, were tested with a high-energy compatible a-Si panel in the laboratory with a portable pulsed X-ray source. Criteria for the success of the tests were the time taken to achieve an image and the visibility of the discontinuities and the image quality indicator wires. Table 3 organizes typical tested items and time results.

 

Further tests were conducted in the refinery itself with an Ir-192, 16 Ci gamma ray source (real piping in the field) (see the set-up example in Figure 5). Criteria for the success of the tests were the time to set up the detector and source on-site, time to take a good image, the quality of images in comparison with known images of the tested object, and analysis tools available on-site (Pincu and Kleinberger, 2009).

 

The tests in the refinery proved a reduction of exposure time from an average of 4 to 5 min. down to 8 to 16 s. The X-ray conditions were the same (X-ray/gamma ray source, 500 mm [19.69 in.] distance between imager/film to the source, sample or pipe inspected); the only difference was that the film was replaced with a DDA imager. Thirty-three images were taken in just 3 h. Reducing exposure times tenfold from minutes to mere seconds means a significantly faster rate of inspection that translates to shorter refinery shutdown periods and increased inspection efficiency.

 

Summary

 

It is possible to use digital radiography with high-energy levels, provided one has a suitable imager. Such an imager can also contribute to shortening exposure times and reducing dose levels, making many applications once considered high-energy easier and quicker to accomplish.

 

Additional inherent advantages to working with digital radiography technology are improved operator safety due to lower exposure (time and dose) and increased NDT profitability caused by the shortening of time to results (cost and time are considerably saved because one can take many images per day with a digital radiography imager). The true lifetime of the a-Si based DDA imager is long enough to allow fast return of investment even when conducting high-energy tests.

 

TABLE 1 : Imaging conditions in Texas 

 

conditions

Ir-192

Se-75

Ci (average energy in kV)

56 Ci

27.2 Ci

Exposure time (per image)

0.6 s

10 s

Averaging (to improve SNR)*

20 images

20 images

Total exposure time for averaged final image

12 s

200 s

Focal spot

 3.708 mm (0.146 in.)

3.531 mm (0.139 in.)

Distance between source and detector

 Contact technique 
~228.6 mm (9 in.)

 Contact technique 
~228.6 mm (9 in.)

 

 

TABLE 2: Isotope energy with digital radiography flat-panel versus isotope energy with film results 

 

Item Inspected

Pipe diameter

Material

Wall Thickness

Liquid Content

Exposure Time
Proprietary Digital Radiography Solution****

Exposure Time
Film***

 Fire water hose

208 mm
(8.19 in.)

ST35

7.2 mm
(0.28 in.)

None

 30 s

  3 min.

 Glass fiber profile

700 mm 
(27.56 in.)

Glass fiber

~ 25 mm
 (0.98 in.)

None

 70 pulses**
  (~ 4.6 s)

 30 s

 Process water pipe

150 mm
(5.91 in.)

SS2343

Total one wall 
6 mm (0.24 in.)

Water

 20 s

 15 min.

 Steam cooler

250 mm
(9.84 in.)
plus insulation

10CrMo

Total one wall 
40 mm (1.58 in.)

None

 50 s

 1 h

Low pressure
steam pipe

400 mm
(15.75 in.) 
plus insulation

ST35

12 mm
(0.47 in.)

None

 30 s

 20 min.

 Fuel lye pipe

100/80 mm
 (3.94/3.15 in.)

SS2343

6 mm 
(0.24 in.) 

Lye 

 15 s

 10 min.

 

 *SNR = signal-to-noise ratio.
** test conducted with pulsed XRS-3 source.
*** film and Ir-192. Exposure time only, not including film developing.
**** a-Si panel and Ir-192. Time to image.

 

TABLE 3: Time to results 

 

Material

Outer diameter

Wall thickness

 Total wall thickness

Energy

Exposure time

Carbon steel 5355

60 mm 
(2.32 in.)

 2.9 mm 
(0.11 in.)

~6 mm 
(0.24 in.)

270 kV

4.3 s

Carbon steel 5355

60.3 mm 
(2.37 in.)

2.9 mm 
(0.11 in.)

~6 mm 
(0.24 in.)

270 kV

3.54 s

Carbon steel 5355

 88.9 mm 
(3.50 in.)

3.62 mm 
(0.14 in.)

~6.4 mm 
(0.25 in.)

270 kV

2.3 s

 

 

ACKNOWLEDGEMENT

The authors would like to thank Vidisco, Ltd., for providing images that were included as figures in this paper.

 

REFERENCES

ASME, ASME Boiler and Pressure Vessel Code, Section V, Article II, American Society of Mechanical Engineers, New York, 2010.
ASTM, E2597-07: Standard Practice for Manufacturing Characterization of Digital Detector Arrays, ASTM International, West Conshohocken,
Pennsylvania, 2007.
BSS, BSS 7044: Radiologic Inspection, Digital Radioscopic, Boeing Specification Support, Chicago, Illinois, 2003.
BSS, BSS 7045: Radiologic Inspection, Composite Structures, Boeing Specification Support, Chicago, Illinois, 2003.
Light, G., “Demonstration of Pulsed X-ray Machine Radiography as an alternative to Industry Radiography Cameras: Demonstration Pilot
Project,” Materials Evaluation, Vol. 66, No. 3, 2008, pp. 285–292.
Pick, L. and O. Kleinberger, “Technical Highlights of Digital Radiography for NDT,” Materials Evaluation, Vol. 67, No. 10, 2009, pp. 1111–1116.
Pincu, R. and O. Kleinberger, “The Transition from Conventional Radiography to Digital Radiography,” Materials Evaluation, Vol. 67, No. 5, 2009,
pp. 499–506.
Thales Group, “Digital Detectors” 2011, 1 Feb 2011.

Figure 1. Amorphous silicon (a-Si) flat-panel structure.
Figure 2. Flat-panel with electronics behind the a-Si plate.
Figure 3. Imager and designated external shielding drawings.
Figure 4. X-ray images of 203.2 mm (8 in.) diameter steel pipe, 19 mm (0.75 in.) wall thickness: (a) iridium; (b) selenium.
Figure 5. Set-up of an a-Si panel in a refinery and the corresponding X-ray image.

 

To see the relevant figures in the original article published in Material Evaluation - click here

 

CALL OUT QUOTES

The portable systems are small, can be conveyed to the inspection site and operated by one person.
Digital radiography can be a sufficient, and even improved, replacement for X-ray film or other film replacements.
The speedy image acquisition process does not cause any compromise to image quality.
There is no need to go back to a laboratory to develop or scan results.
Electronic parts are relatively low-cost among the components of the imager, and can be easily replaced.
Applications that require high-energy levels usually require penetration of thick metallic components.
The tests in the refinery proved a reduction of exposure time

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