X-ray Dynamic Defectoscopy (XRDD) Direct observation of damage development in time dependent gradually stressed materials by means of X-ray transmission measurements.

Jan Jakubek a, Daniel Vavrik b, Stanislav Pospisil a, Jan Visschers c

a IEAP CTU Prague, b ITAM CAS Prague, c NIKHEF Amsterdam

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

Defectoscopy

Our Basic Experimental setup

Hamamatsu rtg tube

Medipix

Stressed Al specimenWith prefabricated slit

Region of interest

• For sharp images we need point X-ray source => we use Hamamatsu microfocus X-ray source L8601-01 with 5 m 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)

Projection of straight Edge

Stress

Slit (0.3 mm wide)

Pb marks(1 mm wide)

Al specimen (5 mm)

Field of vision

Projection of the slit is too wide !Flat area between slit and mark seems to be skew !

Image of Ideal Edge Taken by Ideal Camera

Edge Camera Taken image One column response

I(x) x

I Light Intensity

x

cnt

I’(x)

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 xiWhere I’(x) is convolution of light intensity I(x) and “sampling” function q(x).

where d is pixel size

I(x)

x

1

0

I’(x)=(I o q)(x)

x 0 d/2 d/2

q(x)

x

1

-d/2 d/2

Edge Profile Measurement With Sub Pixel Resolution

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:

Pb (1 mm)

Detector matrix

Count versuspixel order

It’s derivativeMeasured edge profile (normalized)

Ideal response

Measured Edge Profile – nearly ideal case

Hamamatsu rtg tube at 35kV

Medipix

Lead plate (1 mm)

Edge profile – more real case

Hamamatsu rtg tube at 35kV

Medipix

Led plate (1 mm thick)

Al shielding(5 mm thick)

?

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:

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.

Temperature Stability – Leakage Current

Leakage Current versus Temperature

0

0,02

0,04

0,06

0,08

0,1

0,12

0,14

0,16

0,18

0,2

15 17 19 21 23 25 27 29 31

Temperature [°C]

Leak

age

curr

ent [

A

]

Temperature Stability – Noisy and Dead Pixels

Noisy pixels = pixels which noise is more then two times bigger then dispersion of Poisson distributionDead pixels = pixels giving les then half count of events.

Number of dead resp. noisy pixels in dependence on the temperature

0

10

20

30

40

50

60

70

18 20 22 24 26 28 30 32

Temperature [°C]

Coun

t Dead

Noisy

Conditions: Temperature 18°C, rtg at 35 kV, Vth=1.5V (minimum threshold was 1.35V)

Temperature Stability – Noisy Pixels, GaAs detector

Number of noisy pixels in dependence on the temprature

0

50

100

150

200

250

5 10 15 20 25 30 35

Temperature [°C]

Noi

sy p

ixel

s [c

ount

]

2 times

1.3 times

Appendix I – Threshold scans

Appendix II – Edge Profile Versus Threshold Level

Conditions: Temperature 18°C, rtg at 35 kV.