The Influence of Edge Effects on the Detection Properties of Detector Grade Cadmium Telluride M.J. Bosma a, M.G. van Beuzekom a, S. Vähänen b, J.Visser a a. National Institute for Subatomic Physics, Nikhef, Amsterdam b. VTT Technical Research Center of Finland, Helsinki IEEE RTSD R18 3 October 28, 2011, Valencia, Spain
Tomorrow s digital radiography Medipix3 Back contact X ray photon Semiconductor sensor Sensor pixel Solder bump Pixel readout pad Medipix readout chip research group Medipix3 features ~1600 transistors per pixel: Charge summing circuitry Simultaneous counting and read-out no dead time Energy dispersion mode: 7 energy bands 2
Active detector area x 200 2.8 cm 35 x 40 cm 2 3 inch 2.8 cm Medipix quad module with Relaxd read-out 3
Tile ability: Sensor edges Edge effects: Charge generalon Surface currents High field regions ConvenLonally solved by guard rings Slim edge AcLve edge Rossi, Pixel Detectors 4
Simulations: transient signals at edges ] -1 Weighting potential [cm 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 Edge pixel Centre pixel 0 0 100 200 300 400 500 600 700 800 900 1000 Depth [µm] Total current [na] 12 10 8 6 Edge pixel Centre pixel 4 2 0 0 5 10 15 20 25 30 35 40 45 50 Time [ns] 5
Detector: 2 slim edge CdTe pieces Detector readout: Medipix-MXR Sensor specs (Acrorad): Quasi-Ohmic (Pt electrodes) 4.05 x 4.05 x 1 mm 3 36 x 36 pixels of (110 μm) 2 65 μm pixel-to-edge distance 6
Measurements Comparison between centre and edge pixels Leakage current Laser setup: Pixel response function Charge collection efficiency X-ray setup: Noise power spectrum 7
Dark current transients The leakage current as a funclon of Lme aver voltage stepping ( 200V 200V 200V). The current was monitored for one minute per voltage step of 40V Leakage current [A] 0 10-2 -4-6 -8-10 -12-14 -6-200 V 0 V - 40 V - 80 V - 120 V - 160 V - 200 V 200 400 600 800 1000 1200 1400 0 V - 40 V - 80 V - 120 V - 160 V Leakage current [A] 10 0.35 0.3 0.25 0.2 0.15 0.1 0.05-3 40 V 0 V 120 V 80 V 200 V 160 V 200 400 600 800 1000 1200 1400 Time [s] Time [s] Possibly caused by deep level defects 8
Dark current transients Flood field image ( 400V bias) Voltage stepping from 0V to 400V 9
Bare sensors: I V single pixels At edge and corner pixels higher leakage current. Probably caused by: More surface current More generalon current due to edge imperfeclons Leakage current [A] 0.8 0.6 0.4 0.2 0-0.2-0.4-0.6-0.8 1!10-9 Corner pixel Edge pixel Centre pixel In general, more forward than reverse current -1-200 -150-100 -50 0 50 100 150 200 Voltage [V] 10
Energy calibration Count rate 4 10 Tube voltage 15 kv 20 kv 25 kv Micro focus X ray tube, 2.5 mm. Al filter. Used tail end of spectrum as a reference. 3 10 2 10 30 kv 35 kv 40 kv 45 kv 50 kv LeV: Right: E = 0.1915 * THL 79.694 E = 0.1904 * THL 79.333 450 500 550 600 650 700 750 Lower threshold DAC value LeV sensor Right sensor Lower threshold DAC value 680 660 640 620 600 580 560 540 Lower threshold DAC value 680 660 640 620 600 580 560 540 520 500 480 p0 414.2 ± 2.23 p1 5.221 ± 0.06473 15 20 25 30 35 40 45 50 520 500 480 p0 413.3 ± 2.645 p1 5.252 ± 0.07676 15 20 25 30 35 40 45 50 Energy [kev] Energy [kev] 11
Laser data: pixel response function Threshold at half the energy equivalent intensity ensures minimal charge sharing Min. 1 μm steps Sensor s physical edge Count rate [10 Hz] 1000 800 600-140V 400-170V - 200V 200-400V - 500V 0 5 10 15 20 25 30 Count rate [10 Hz] 1000 800 600 400 200 Laser position [µm] 0 50 100 150 200 250 300 350 Laser position [µm] 12
Laser data: charge collection efficiency Fit function described by Hecht relation: Q(U) = Q 0 µτ/l 2 (U U 0 ) [1 exp(dl/µτ(u U 0 )] Charge collection efficiency 0.85 0.8 0.75 0.7 0.65 0.6 0.55 0.5 0.45 Centre pixel: (μτ) e = 1.25 E 4 Edge pixel: (μτ) e = 1.15 E 4 Centre pixel Edge pixel 100 200 300 400 500 Bias voltage [V] 13
X ray data: noise power spectrum NPS tends to be flaiened due to under sampling (110 μm pixels) Low pass filtering at low threshold levels More noise at close to photopeak threshold levels Differential count rate 2000 1800 1600 1400 1200 1000 800 600 400 200 Noise power [a.u.] Centre pixels Lower threshold level 8 kev 12.5 kev 19 kev 8 kev averaged 12.5 kev averaged 19 kev averaged Noise power [a.u.] 0 40 39 38 37 36 35 8 10 12 14 16 18 20 22 24 26 28 Edge pixels Energy [kev] Lower threshold level 8 kev 12.5 kev 19 kev 8 kev averaged 12.5 kev averaged 19 kev averaged 34 33 32 1 31 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 30 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 Frequency [lp/mm] Frequency [lp/mm] 14
Summary and outlook Summary: Dark current transients after voltage stepping Leakage current at edge and corner pixels larger than that of centre pixels Bias dependence of edge-pixel response function μτ-products of edge and centre pixels comparable Noise power at edge higher and low-frequency dominated Outlook: Laser-induced transient analysis Energy resolution measurements Determine DQE at edge (MTF from pixel response function) 15
Thank you High energy nuclear interaclons with Schoiky CdTe. MulLpixel spread splashes, possibly due to polarisalon or a cascade of fluorescence effects (?) Bias voltage and interaclon depth dependence under study. 16
BACK UP slides 17
Detector: I t characteristics (GaAs) The leakage current as a funclon of Lme aver voltage stepping ( 200V 200V 200V). The current was monitored for one minute per voltage step of 40V Leakage current [A] 10 8 6 4 2 0-2 -4-6 -8-6 10-200 V - 160 V - 80 V - 120 V 0 V - 40 V 40 V 80 V 120 V -10 0 200 400 600 800 1000 1200 Time [s] 18
X ray data: energy resolution Sekngs: 90 kv, 89 μa, 2.5 mm Al filtering, 45 cm distance, 400V sensor bias, 10 Lmes 1s acquisilon Lme per THL step Equal peak posilon, FWHM and peak to background ralo. K edges slightly less pronounced at edge pixels probably due to limited stalslcs (less pixels). Centre pixels Edge pixels Differential count rate 50000 Left sensor 40000 30000 20000 10000 Filter Charge sharing Cd K edge Te K edge Right sensor Differential count rate 5000 Left sensor Right sensor 4000 3000 2000 1000 Cd K edge Te K edge 0 10 20 30 40 50 60 70 80 90 Energy [kev] 10 20 30 40 50 60 70 80 90 Energy [kev] 19 0