Equalization of Medipix2 imaging detector energy thresholds using measurement of polychromatic - PowerPoint Presentation

1. Equalization of Medipix2 imaging detector energy thresholds using measurement of polychromatic X-ray beam attenuation Josef Uhera,b, Jan Jakubekca CSIRO PSE, Lucas Heights, NSW, Australia, b CSIRO MDU, NRF, Australiac IEAP-CTU, Prague, Czech Republic

2. CSIRO.OutlineMedipix detector descriptionEnergy threshold equalization using noise floorEnergy threshold equalization using fits of X-ray attenuation curvesExamples of applicationMicrofocus X-ray tubeSampleImaging detectorX-ray transmission was used in measurements

3. CSIRO. Detectors of Medipix familyImages from www.utef.cvut.czMedipix detectors:array of 256 x 256 pixelspixel size 55 x 55 mmtotal area 1.5 x 1.5 cm2 Planar 300 mm thick silicon pixel detector (GaAs and CdTe also available) Bump-bonded to a readout chip containing amplifier, discriminator (low and high threshold) and counter for each pixel3-bit fine threshold adjustment in each pixel

4. Threshold equalization3-bit fine threshold adjustment in each pixel values minimizing the threshold level spread has to be foundthe common technique is based on finding the noise floor in each pixelthreshold can be set just above the noise floorCSIRO. Adjustment bits = 0Adjustment bits = 7Optimized adjustment bits

5. Signal-to-thickness calibration (STC)CSIRO.Replacement of the flat-field correctionSTC converts the measured number of counts into equivalent thickness of the calibrating material (Al)Measured signalEquivalent thicknessX-ray attenuation in Al foils is measured

6. Signal-to-thickness calibration (STC)CSIRO.X-ray tube running at 40kVp, energy threshold set to ~13 keV i.e. above k-edges of Ni and Cu and below k-edges of Ag and SnSTCraw imagecorrected imagesample photoAlNiCuAgSn13 keVthresholdSnAgNiCu

7. STC and threshold close to k-edge of SnCSIRO.Aluminum ~3 mm thick Tin foil 25µm thickSTC corrected imageImage histogramDetected X-ray spectrumX-ray tube running at 40kVp, energy threshold set to ~27  keV (Sn k-edge is at 29.2 keV)Can we further reduce the threshold spread??Note:The spectra do not include the detector response.

8. Better threshold equalizationCSIRO.Equalization using monochromatic X-rays: + reduces the total threshold spread by 30% － requires change of the X-ray setup arrangement － count rates are low => timely procedureEqualization using polychromatic X-raysmeasurementfitAnalytical function calculating the detected signal:Requires:mass attenuation coefficientsX-ray spectrumMedipix detector response matrixAttenuation curves at different thresholdsFree parameters of the fit:amplitudethreshold6 keV12 keV20 keV30 keV

9. X-ray spectrumCSIRO.Measured using Silicon Drift Detector:AmpTek SDD X-123Dsensor thickness 450 µmBe window 12.5 µm thickcollimator ∅ 1 mmX-ray tube:Oxford InstrumentsMonoblock XRS-75-200DW targetInherent filtration @75kVp: 1.5 mm AlThe measured spectrum was corrected on the SDD detector responsePrecise knowledge of the spectrum is essential for this technique!

10. A function that calculates the charge sharing in the detector was developed.Its parameters (charge spread) are obtained from fit of experimental data.Needs to be constructed only once for the given detector bias.Medipix detector response matrixCSIRO. PhotonPixel cellsThe electric charge deposited in one pixel spreads into adjacent pixels. The hit pixel “sees” less charge!Pd kαPd kβThe charge sharing produces a large tail in the measured spectrum.IEEE NSS 2010, Knoxville, Tennessee, USA

11. Noise optimized adjustment bitsEqualization using the X-ray attenuation curvesCSIRO.Adjustment bits = 0Adjustment bits = 7STC optimized adjustment bitsX-ray attenuation in Al is measured for both extreme values of THL adjustmentMeasured attenuation curves are fitted in individual pixels and threshold value in keV is determinedOptimal THL adjustment value is determinedFWHM:1.4 keV → 1.1 keVFWTM:2.7 keV → 1.6 keVNote:The upper threshold can be equalized relative to the lower threshold

12. STC and threshold close to k-edge – new equalizationCSIRO.Aluminum ~3 mm thick Tin foil 25µm thickSTC corrected imageImage histogramDetected X-ray spectrumX-ray tube running at 40kVp, energy threshold set to ~27  keV (Sn k-edge is at 29.2 keV)Aluminum ~3 mm thick Tin foil 25µm thickSTC corrected imageImage histogramDetected X-ray spectrumFWHM:723 → 634 µmFWTM:1671 → 1099 µmoldnew

13. New equalization – raw images before the STCCSIRO.Aluminum ~3 mm thick Tin foil 25µm thickRaw image with noise equalizationImage histogramX-ray tube running at 40kVp, energy threshold set to ~27  keV (Sn k-edge is at 29.2 keV)FWHM:6647 → 4058µmFWTM:12690 → 7587 µmRaw image with attenuation fit eq.FWHM:5146 → 3368 µmFWTM:9220 → 6298 µm

14. No Medipix response matrix?CSIRO.The attenuation curve shape should be dominated by the material of sample, sensor, the X-ray spectrum and threshold. Can we omit the response matrix?Attenuation curve fitFit residualsTHL adj bit set to 7pixel 111,87THL adj bit set to 0pixel 111,87measurementfit with no resp. matrixfit with resp. matrixmeasurementfit with no resp. matrixfit with resp. matrixthreshold from fit: 19.1 keV vs 17.5 keVthreshold from fit: 27.0 keV vs 26.2 keV

15. THL adjustment with and without the response matrixCSIRO.Distributions of STC calibrated values for Sn foil 25 µm thickCan we omit the response matrix?Distribution of differences of the adjustment with and without the response matrixadj. without resp. matrixadj. with resp. matrix

16. k-edge imaging - examplesCSIRO.SnAgPdMoSnAgPdZrCdSnAgCuNiX-ray tube: 50kVp, 500µAThreshold scan: 7÷40 keVEnergy step: ~1.4 keVTotal measurement time: 15 minK-edges identified by relative differences of the measured spectrum and a spectrum under Al.Mo 25-75µmk = 20.0keVPd 25-75µmk = 24.3keVAg 10-30µmk = 25.5keVSn 25-75µmk = 29.2keVNi 20-60µmk = 8.3keVCu 20-60µmk = 9.0keVAg 10-30µmk = 25.5keVSn 25-75µmk = 29.2keVZr k = 18.0keVPd k = 24.3keVAg k = 25.5keVCd k = 26.7keVSn k = 29.2keVThicknesses:10 – 200µmMoSnPdAgNiSnCuAg

17. ConclusionAdvantages of the presented equalization method:better energy threshold equalization compared to the noise based techniquecomparable performance to the monochromatic source based equalizationsimple to implementeasy to scale to large area detectorseasy automatization of this methodrough threshold energy calibrationsignal-to-thickness calibration for raw image correctionprecise calculation of the detector signal can be used:to calculate signal-to-thickness calibration for different materials other than the one used in the measured calibrationbetter beam-hardening correctionin iterative CT reconstruction techniques (the Expectation Maximization method)CSIRO.

18. Contact UsPhone: 1300 363 400 or +61 3 9545 2176Email: enquiries@csiro.au Web: www.csiro.auThank youProcess Science and EngineeringOn-Line Analysis & ControlJosef UherPhone: +61 2 9710 6720Email: Josef.Uher@csiro.au

19. Backup slidesCSIRO.

20. Detection of k-edge in the spectrumCSIRO.Spectrum behind AlSpectrum behind Sn∆EIAl/ISn=1IAl/ISn>1Spectrum of k-edges is generated by running the window across the measured spectrum.Only values above and below the edge can be measured if the elements in the sample can be anticipated.