Continuous X-ray scanning in the Time Delayed Integration mode with photon counting detectors

Introduction

We describe the application of photon-counting detectors for online X-ray scanning where the object(s) continuously move along the imaging detector. The detector is in a form of a line placed perpendicularly to the direction of the object’s movement. The detector then generates an “endless” image, hence it makes this approach ideal, for instance, for cases where inspected products are running on a conveyor belt. However, it finds use also in weld inspection as shown in this article. The novel types of imaging detectors bring to this area image quality superior to the existing line detectors used for these purposes.

X-ray photon counting imaging detectors

Hawkeye Spectral Imaging provides Advacam’s so-called direct conversion hybrid photon counting (HPC) detectors. X-ray radiation is converted in a semiconductor or semi-insulator sensor directly into a measurable electric signal. Signals are processed and digitally stored in each pixel which is 55 µm size. This approach overcomes several limitations of common scintillator-based charge integrating imaging detectors (CIID), i.e. flat panels, CCDs, or CMOS line detectors. The photon-counting detectors eliminate most of the noise sources present in common CIIDs achieving a nearly arbitrary signal-to-noise ratio. 

The scintillators in CIIDs with a small pixel size (less than 100 µm) have to be deposited in thin layers in order to minimize the light spread and match the pixel size. The small scintillator thickness limits the sensitivity and increases measurement times. The HPC imagers can use thick sensors while not compromising the resolution. Therefore, they offer considerably higher X-ray sensitivity compared to CIIDs of the same pixel size.

The pixel electronics in the new detectors are designed radiation-hard extending further the lifetime of imaging detectors.

In addition, the direct conversion photon-counting detectors are capable of X-ray energy discrimination, i.e. only photons above certain energy allowing suppressing scattered radiation of lower energies.

Applications of HPC detectors were limited for a long time due to the availability of Silicon sensors only. The silicon crystal does not provide a sufficient detection efficiency for photons over 20 keV. Today, so-called high-Z CdTe or CZT sensors are available offering a significantly higher sensitivity.

Advacam produces photon-counting detectors with 1 mm thick CdTe sensors and prepares devices with 2 mm thick CZT sensors. The sensitivity of these sensors is considerably higher not only compared to Si but also scintillators commonly used in high-resolution flat panels (pixels of less than 100 µm).

Hardware based Time-Delayed Integration

The time-delayed integration (TDI) is a mode of the detector operation where the device produces a continuous X-ray image of an object that is moving along the detector. This mode is especially applicable for a conveyor belt-based inspection. It can be also used in scanners where the detector is moved by appropriate mechanics along the imaged object. 

Several Advacam’s detectors support this feature. The TDI is in this case hardware-based, i.e. the signal integration is done directly within the pixel electronics. In comparison with devices that rely on a high frame rate and the time-delayed integration in the computer, the hardware-based approach significantly reduces demands on the data throughput to the control computer and increases the maximum scanning speeds.

Experimental evaluation

The imaging quality of Widepix 1×5 MPX3 [1,2] imaging detector operated in the time-delayed-integration mode was tested on steel welds in collaboration with the Bundesanstalt für Materialforschung und -prüfung (BAM), Berlin, Germany . The setup parameters were:

  • X-ray tube at 120 kVp and 8 mA.
  • DIQI/IQI to detector distance was 16 mm.
  • The source-to-detector distance was 800 mm.
  • UR-3e robot for detector movement [3].
  • Robot scan speed 0.7 mm/s (TDI speed).

The detector used in this evaluation was Widepix 1×5 MPX3 with parameters:

  • A row of five Medipix3 chips.
  • 256×1280 pixels, pixel size 55 µm.
  • 70×14 mm sensor array size.
  • CdTe sensor 1 mm thick.
  • Discrimination thresholds were 10 keV for CSM and 60 keV for the signal.

The device was tested on BAM-5 and BAM-25 steel weld samples with IQI and DIQI attached.

The detector-to-sample distance was set as short as possible to minimize the geometrical unsharpness in order to evaluate only detector properties and not the influence of the X-ray tube spot size. 

The detector was operated in a charge-summing-mode (CSM) that corrects the spread of signal between neighboring pixels. The CSM mode significantly improves the detector spectral response and therefore also allows setting a high discrimination threshold to minimize the contribution of the scattered radiation. The reduction of scattered radiation increases the achieved contrast and therefore also detail detectability.

The detector running in the TDI mode was moved along the sample by a robotic arm running software developed by Radalytica a.s. for robotic X-ray inspection systems [3]. The speed of movement and the detector readout were synchronized. The detector then produced a scan along the axis of movement.

Results

The measured images were corrected using the signal-to-thickness calibration [4] where the number of photons is converted to an equivalent thickness of a calibration material. In this case, the calibration material was steel. 

The spatial resolution was measured using DIQI. The narrowest wire pair resolved was the D13 (50 µm wires, 50 µm apart).

The signal-to-noise ratio measured (SNRm) achieved was 148 in case of the 8.3 mm thick BAM-5 sample. SNRm of 190 was measured for the BAM-25 which is 6 mm thick steel. The SNRm was capped by the X-ray tube power. The detector has a 24-bit counter depth, therefore, allowing SNRm as high as 4000.

The resulting SNRn (normalized on the detector resolution) was 336 at 6 mm thick steel and 262 in the case of 8.3 mm thick steel.

Detector contrast was evaluated using the 10FEEN IQI. All wires including the wire 16 (0.1 mm thick) were resolved behind the 8.3 mm thick steel sample wall.

The full BAM sample scans are shown at the end of this document.

Conclusion

The hardware-based TDI mode integrated into Advacam’s detectors opens the possibility to benefit from the superior image quality of photon-counting detectors also in applications where continuous scanning is required. The detector can produce an “endless” image of objects passing along it.

The X-ray energy discrimination can be used in the TDI measurement as well. It helps to significantly reduce the amount of detected scattered radiation. Consequently, it further improves the already very high contrast of images.

The speed of TDI measurement can be as high as 0.5 m/s with the Widepix 1×5 MPX3 device. The “L” series of detectors offer speeds of up to 2 m/s.

The high speeds of measurement are backed up by the highly efficient CdTe sensors that detect up to an energy of about 60 keV 100% of photons. And at the energy of 140 keV still collect about 30% of photons. The 1 mm thick CdTe sensors were successfully operated with X-ray tubes operated even at 400 kVp.

The achievable spatial resolution corresponds to the pixel size that allows resolving 50 µm wire pair in DIQI.

The current detector portfolio allows scan widths from 7 to 21 cm. Further detector development focuses on extending the detector lengths up to 42 cm.

References

[1] R.Ballabriga, M.Campbell, E.Heijne, X.Llopart, L.Tlustos, W.Wong: Medipix3: “A 64 k pixel detector readout chip working in single photon counting mode with improved spectrometric performance,” Nuc. Instr. and Meth. in Phys. A, Vol. 633, Supplement 1, May 2011.

[2] Widepix 1×5 MPX3 CdTe

[3] J. Jakubek: ”Data processing and image reconstruction methods for pixel detectors,” Nuclear Instruments and Methods in Physics Research A 576 (2007) 223–234.

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