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Peter Norlin 11 Nov 2014
DIGITAL X-RAY INSPECTION OF
WELDS
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BACKGROUND • Digital technology has taken over visible light photography • Digital X-ray imaging is well in progress in medical and dental fields • The digital transition has been slower for non-destructive testing (NDT) of welds, at least for critical applications
- Requirements from older industry standards - Possibly conservatism in the field
This project’s objective Find out if current technology and standard development allows a digital transition to be made at OHB Sweden’s satellite production line (welding of tubes for propulsion systems). Project time 2012 - 2013
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Three X-ray images / weld ⇒ several hundred images for a propulsion system • No chemical development, which is
- Time consuming - Can be unreliable
• Examination – post-processing • Digital archiving and storage
POTENTIAL BENEFITS
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OVERVIEW
1. Introduction to digital X-ray detector
arrays 2.Standards 3.Survey of commercial systems 4.Detector tests 5.Summary and conclusions
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1. INTRODUCTION TO DIGITAL X-RAY DETECTOR ARRAYS
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DIGITAL DETECTOR ARRAYS: 1) INDIRECT DETECTION
CCD x-ray detector chip, 4.9 cm x 8.6 cm, 7K x 4K matrix, Siemens OPDIMA
CCD or CMOS arrays: Light e-
Scintillator: X-ray light +
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Columnar structure of a CsI scintillator. Light generated in the material is guided through the needle-like structures (~ 5 um diameter) onto the photodiodes of the detector, thus reducing detrimental scattering effects.
CsI doped with Thallium
Examples of scintillator materials used in indirect x-ray detectors
Scintillator crystal
Emission wavelength
nm CsI:Tl 540 NaI:Tl 415 Gd2O2S:Tb aka Gadox or GOS
544
SCINTILLATORS
Cherepy et al., IEEE Trans Nucl Sci, 2009
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LARGE AREA DETECTORS
TFT-arrays a-Si on glass substrates
Flat panel detector, 43 cm x 43 cm, 143 um pixel size (from www.trixell.com).
Scintillator: X-ray light +
No X-ray optics ⇒ need detector of same dimensions as the object under study (typ. human body parts)
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TFT FLAT PANEL DETECTOR ARRAYS
Indirect detector with scintillator layer on top Top view of the thin film
layer pixel structure.
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2) DIRECT DETECTORS
amorphous Se - ZSe = 34 (ZSi =14) - Conversion: X-ray e-
- Large area depositon
Anrad GR17 a-Se based flat panel detector (43 cm x 43 cm)
TFT-arrays a-Si on glass substrates
+
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PHOTON COUNTING DETECTORS Conventional detectors are charge (energy) integrating
The total X-ray energy absorbed by each pixel is registred
Photon (quantum) counting detectors
Each absorbed photon produces a charge pulse corresponding to that photon’s energy. Each pulse above a discrimination level produces a detector count.
From theoretical considerations: potential for improved noise/contrast (DQE)
Signal independent of electronic noise, if discrimination level fairly above the noise floor
With several discriminator levels, energy resolution, a.k.a. color imaging is possible
Increased ROIC complexity:
Amplifyers, discriminators, counters in each pixel area.
Count rate may be very high very fast electronics
Advances in microelectronics has made pixellated photon counting possible
Hybridization of sensor chip and ROIC through flip-chip technnology
Again, large area detector is problematic ... (from Czech Technical University in Prague)
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Typical pixel dimensions General medical X-ray: ~200 um – 100 um range Mammography and dental: ~100 um – 50 um – 20 um range
SPATIAL DIGITIZING - PIXELS
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SIGNAL DIGITIZING - GREY LEVELS
Digital detectors 12 -16 bit A/D conversion 212 - 216 = 4 000 - 65 000 grey levels Human eye Can distinguish ~200 different grey levels Film Grey levels are fixed once and for all at the development process. in practice 200 fixed levels. Digital detectors Display 28 = 256 grey levels adapted to the range of the human eye. However the conversion from 65 000 to 256 grey levels can be tailored by adjusting parameters such as brightness, contrast and gamma.
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MAGNIFICATION TECHNIQUE
Issues: • Microfocus source needed higher cost (?) • Larger detector area needed • Room for larger object-to-detector distance needed
Increased object-to-detector distance Magnified image
Standard source unsharp image
Microfocus or nanofocus source sharp image
Principle
Result: The requirement for <40 um or <50 um pixel size can potentially be relaxed
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2. STANDARDS
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DIGITIZING OF STANDARDS 1997 SS-EN
1435 Non-destructive testing of welds - Radiographic testing of welded joints (Film)
2005 EN14784-2 Non-destructive testing - Industrial computed radiography with storage phosphor imaging plates – Part 2: General principles for testing of metallic materials using X-rays and gamma rays (CR)
2010 ASTM Set of standards for DDA 2010 EN
ISO/FDIS 17636-2
Non-destuctive testing of welds – Radiographic testing – Part 2: X- and gamma-ray techniques with digital detectors (CR and DDA)
Feb 2013
SS-EN ISO 17636-2
Non-destuctive testing of welds – Radiographic testing – Part 2: X- and gamma-ray techniques with digital detectors (CR and DDA), 51 pp
Feb 2013
SS-EN ISO 17636-1
Non-destuctive testing of welds – Radiographic testing – Part 1: X- and gamma-ray techniques with film (Film)
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REQUIREMENTS IN EN ISO 17636-2 MINIMUM IMAGE QUALITY VALUES CONTRAST SENSITIVITY AND SPATIAL RESOLUTION
= 50 um = 63 um
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IMAGE QUALITY INDICATORS (IQI)
Single wire IQI for verification of contrast sensitivity
Double wire IQI for verification of spatial resolution
W13 W19 200 um 50 um
D1 D13 800 um 50 um
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3. SURVEY OF COMMERCIAL SYSTEMS
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DEDICATED SYSTEMS FOR TESTING OF PIPES AND WELDS
Typical example
Satellite propulsion system application 3.2 – 0.9 mm OD 0.1 – 0.9 mm wall thickness
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MEDICAL SYSTEMS Mammography • Probably the most demanding of medical imaging tasks in terms of spatial resolution and contrast resolution • Pixel size: ~25-80 um • Typ area: 18 x 24 cm2 – 24 x 30 cm2
• Energy: ~17-30 kV, (trade-off contrast/dose) • Size and weight appear to be large for inspection of fully integrated satelite systems
Dental imaging • Pixel size 20-50 um • Typ area: ~40 mm x 30 mm • Energy: ~50-100 keV, Typical ~65 keV ?
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SUMMARY OF SURVEY 1st survey 15 suppliers (Mammo, dental, NDT, general) 2nd survey 22 dental X-ray suppliers, 11 with Swedish representation Visit to SweDental For the next phase - experimental evaluation – Four detectors were selected
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4. DETECTOR TESTS
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DETECTORS SELECTED FOR TEST
XCounter PDT25
Rad-icon Imaging (Dalsa) Radeye
Instrumentarium Snapshot
Hamamatsu S10811-11
Application Dental, mammo, general
NDT Dental Dental (NDT, general)
Detector type CdTe/Cd(Zn)Te, direct hybrid, photon counting
Gd2O2S / CMOS CMOS, scintillator not specified
CCD, scintillator not specified
Pixel pitch, µm 100 48 / 22.5 19 20
Energy spec, kV 10-160 /10-90 60-70 typ
A/D resolution, bit 32 16 16 12
Detector window, mm2
25 x 25 25 x 49 30 x 20 34 x 24
Price kSEK 65 (incl s/w) 52/56 (+19) 55 (incl s/w) 29
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PIPE SAMPLES
OD, mm Wall thickness, mm Stainless steel 6.3 (1/4”) 0.9 Titanium 3.2 – 9.5 (1/8”-3/8”) 0.1-0.7
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EXAMPLES OF TEST RESULTS
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SUMMARY OF TEST RESULTS • All detectors passed the test regarding image quality as specified in ISO 17636 • Although: XCounter uses magnification; Contrast sensitivity was never evaluated with Instrumentarium • All detectors could identify pore defects in pipe samples
In other words: Digital X-ray detector technology can replace film for pipe weld inspection in space applications. • Hamamatsu’s detector was selected for extended verification by OHB
Good support from Hamamatsu representatives Hamamatsu not primarily a dental company Also supplier of X-ray sources Price
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IMPLEMENTATION AT OHB SWEDEN IN KISTA X-ray source, 40-130 kV (Hamamatsu)
Welded pipe sample
Digital detector (Hamamatsu) inside metal fixture
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IMPLEMENTATION AT OHB SWEDEN IN KISTA
Computer display used for digital X-ray image inspection
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Industrial value Digital system outperforms the old film-based system in several aspects: • Faster cycle times • Lower dose rates • Digital storage • File sharing • Image processing • Less expensive • Equal or better image quality
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5. SUMMARY AND CONCLUSIONS • An applicable ISO standard for radiographic non-destructive testing of welds with X-ray techniques and digital detectors was published in 2013.
• After a market survey, testing of four different digital detectors was carried out with positive results. Digital dental detectors were found to have close to ideal properties for the application.
• OHB Sweden’s primary customer, European Space Agency (ESA), approved the use of the digital ISO standard and the replacement of film with digital detectors in the weld inspection process.
• In connection with the setup of a new production facility in Kista, OHB decided to make the switch to a completely digital X-ray inspection system using a detector from Hamamatsu.
The project’s objective:
Find out if current technology and standard development allows a digital transition to be made at OHB Sweden’s satellite production line.
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MAIN CONTRIBUTORS AT OHB SWEDEN
Bjarne Andersson Robin Linde
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