Resolution and Stability Tests of Medipix-1 Pixel Detector Used for X-ray Dynamic Defectoscopy

1. Resolution and Stability Tests of Medipix-1 Pixel Detector Used for X-ray Dynamic Defectoscopy Jan Jakubek a, Daniel Vavrik b, Stanislav Pospisil a, Jan Visschers c a IEAP CTU Prague, b ITAM CAS Prague, c NIKHEF Amsterdam X-ray Dynamic Defectoscopy (XRDD) Direct observation of damage development in time dependent gradually stressed materials by means of X-ray transmission measurements.

2. Stressed Al specimen With prefabricated slit Hamamatsu rtg tube Region of interest Medipix Our Basic Experimental setup • For sharp images we need point X-ray source => we use Hamamatsu microfocus X-ray source L8601-01 with 5 mm focal spot size. • As X-ray imaging device the Medipix-1 Si detector were used with standard readout setup (Muros, two NI cards, PC and Medisoft) attached to water cooling system with controlled temperature 10-30 °C (step 0.1°C)

3. Projection of straight Edge Slit (0.3 mm wide) Al specimen (5 mm) Stress Field of vision Pb marks (1 mm wide) Projection of the slit is too wide ! Flat area between slit and mark seems to be skew !

4. 1 1 I(x) q(x) x x 0 -d/2 d/2 Image of Ideal Edge Taken by Ideal Camera Let’s assume ideal camera: - each pixel integrates all light impacting its area. - there is no insensitive areas between pixels - there is no overlaps between adjacent pixels - pixels of equal area and shape Let’s assume ideal edge and lighting: - parallel rays - no light can pass through the material behind the edge - no scattering, no diffraction We measure values of function I’(xi) in the pixel centers xi Where I’(x) is convolution of light intensity I(x) and “sampling” function q(x). where d is pixel size

5. Edge Profile Measurement With Sub Pixel Resolution Pb (1 mm) Detector matrix Count versus pixel order Each row holds the samples of the same function but shifted. Shift is given by angle of the edge. I can combine rows to get more precise edge profile:

6. Measured Edge Profile – nearly ideal case Lead plate (1 mm) Hamamatsu rtg tube at 35kV Medipix Ideal response Measured edge profile (normalized) It’s derivative

7. Edge profile – more real case Led plate (1 mm thick) Hamamatsu rtg tube at 35kV Medipix Al shielding (5 mm thick) ?

8. Scattering ? If the reason is scattering, the effect should depend on threshold (photons of lower energy). => We repeated measurement for different threshold values. The differences of edge profiles from the ideal one:

9. How to reduce influence of the “tails” around edges? • If possible use monochromatic X-ray source (often not available). • Precise tune the threshold level. • Prevent occurrence of big “parasite” contrasts in image near regions of interest. In case of our specimen for XRDD we filled the slit by absorbing tin paste.

10. Temperature Stability – Leakage Current

11. Temperature Stability – Noisy and Dead Pixels Noisy pixels = pixels which noise is more then two times bigger then dispersion of Poisson distribution Dead pixels = pixels giving les then half count of events. Conditions: Temperature 18°C, rtg at 35 kV, Vth=1.5V (minimum threshold was 1.35V)

12. Temperature Stability – Noisy Pixels, GaAs detector

13. Appendix I – Threshold scans

14. Appendix II – Edge Profile Versus Threshold Level Conditions: Temperature 18°C, rtg at 35 kV.