2. PACS IIDR 01/02 Mar 2001 Photoconductor Detector Arrays1 Photoconductor Detector Arrays for PAC S IIDR - ESTEC Stefan Kraft ANTEC-GmbH Germany Günter Bollmann, Peter Dinges, Otto Frenzl, Marco Jasinski, Heidrun Köppen, Heribert Krüger, Claudia Popp

3. PACS IIDR 01/02 Mar 2001 Photoconductor Detector Arrays2 Overview Requirements & Specifications Design Implications on Arrays –Mass Budget –Thermal Budget –Vibration Load –Stress Mechanism / FEM analysis –Fore Optics / Optical Design –Detection efficiency Achieved Performance versa Spec –Detector Responsivity –Cutoff Wavelengths –Stress Uniformity / Variations in CW –Bias dependency –Uniformity of abs. Responsivity Summary

4. PACS IIDR 01/02 Mar 2001 Photoconductor Detector Arrays3 Impacts of Specifications and Requirements to Detector Array Design

5. PACS IIDR 01/02 Mar 2001 Photoconductor Detector Arrays4 Design Detector Housing  5x5 linear arrays arranged according to needs of acquisition mode for spectroscopy  Light metal design (Al) - total weight ~ 6.6 kg  Thermally isolated Front viewSide view

6. PACS IIDR 01/02 Mar 2001 Photoconductor Detector Arrays5 Red array Blue array Design Detector Housing  Optical path requires different arrangements of blue and red array  Red array rotated by 90°  Detectors are optically shielded from environment by light tight envelopes  Shielding structures coated by black paint

7. PACS IIDR 01/02 Mar 2001 Photoconductor Detector Arrays6  Light weight  FEE thermally isolated from array  Thin wire harness (Nano/micro connectors)  High stability  Proper wiring concept (C Det < 2pF, dark current <5·10 4 e - /s)  Low EMC impacts  High degree of light tightness  Good uniformity of responsivities  Uniform cutoff wavelengths (CWs)  Low variation of CWs Design Detector Arrays  Proper stressing mechanism  Low cross-talk (<0.1%)  High collection efficiency  High quantum efficiency

8. PACS IIDR 01/02 Mar 2001 Photoconductor Detector Arrays7 Design - Stress mechanism Maximum force:  800 N highly stressed  200 N low stressed  Spring travel: ~2 mm both types of modules High stress module Low stress module Al alloy with strength of steel Design verified by FEM analysis Detector cavity remains stress free Controlled adjustment of stress possible Stress is predictable even after cool down

9. PACS IIDR 01/02 Mar 2001 Photoconductor Detector Arrays8 Mass Budget Low Stress Array: 50 g High Stress Array: 57 g Harness: 3g Fore Optics: 9g FEE: 2g 25 Low Stress Arrays 1.24 kg 25 Low Stress Arrays 1.4 kg 50 Arrays 2.64 kg in Total

10. PACS IIDR 01/02 Mar 2001 Photoconductor Detector Arrays9 Thermal Budget

11. PACS IIDR 01/02 Mar 2001 Photoconductor Detector Arrays10 Vibration Load Static Load Test: 420 g @ RT on 2 posts  ~ 200 g

12. PACS IIDR 01/02 Mar 2001 Photoconductor Detector Arrays11 Design - FEM analysis Addition of cushion pads between detector and pistons reduces the pressure gradient considerably High centring accuracy necessary

13. PACS IIDR 01/02 Mar 2001 Photoconductor Detector Arrays12 Photographs Instrument Description Schematic view of the linear photoconductor array design Mounting accuracy ~10 µm Slit size 30 to 70 µm Rotational mounting accuracy <5°

14. PACS IIDR 01/02 Mar 2001 Photoconductor Detector Arrays13 Force transmittance from detector to detector Equalisation of non-parallel surfaces Minimisation of stress non- uniformity within detectors Optical shielding between the cavities in the detector channel via the metal contacts Electrical insulation of the detector contacts from the housing and each other Electrical contacts made by 70 µm Cu wires and 25 µm Au wires in cavity ”Detector – metal contact – insulator – ball joint – metal contact” Block Design: Purposes 1 mm

15. PACS IIDR 01/02 Mar 2001 Photoconductor Detector Arrays14 Design - Fore optics Low surface roughness (<0.3 µm) obtained by electric discharge machining (EDM) for high reflectivity Coating with a 10  µm thick Ni-Au layer ensures high reflectivity close to 1 as proven by measurements on flat samples 16 linear light cones Optical cavities with small apertures Radial orientation to pupil at 240 mm distance Design optimised by optical calculations

16. PACS IIDR 01/02 Mar 2001 Photoconductor Detector Arrays15 Design impacts / biasing concept: Slits unavoidable Polarisation dependence, glancing angle of impinging photon: Effective slit size is small  Experimentally verified by spectral responsivity Performance Aspects - Photon Losses y  Supported by ray tracing Photon starting point: 240 mm from focus with 15 mm diameter (conditions of the optics in the instrument PACS)  High detection efficiency

17. PACS IIDR 01/02 Mar 2001 Photoconductor Detector Arrays16  = P abs /(P abs + P loss ) P loss : Loss area inside cavity = entrance hole + slits (25 µm, 50 µm) + wires P abs = 2(a+b)·h·(1-R) ·  ’ : Absorbing area Abs. eff.:  ’ =  · L, Abs. coeff.:  = 2.4 cm -1, Abs. length: L = 2.1 mm Length: a = width: b = 1 mm, height: h = 1.5 mm, Reflectivity: R = 0.4 Performance Aspects - Detector Efficiency

18. PACS IIDR 01/02 Mar 2001 Photoconductor Detector Arrays17 Measured Relative Responsivities Good uniformity, close to expectation R  20% @ 205 µm QM 13 - Low Stress QM 2 - High Stress (re-stressed) 10 % level

19. PACS IIDR 01/02 Mar 2001 Photoconductor Detector Arrays18 Cutoff wavelengths and variations highly stressed QM arrays Uniform mean cutoff wavelengths Relation between R RT and CW (pressure) Low variations within one array

20. PACS IIDR 01/02 Mar 2001 Photoconductor Detector Arrays19 Cutoff wavelengths and variations low stressed QM arrays Uniform mean cutoff wavelengthsLow variations within one array

21. PACS IIDR 01/02 Mar 2001 Photoconductor Detector Arrays20 Status Min-Max Cutoff Wavelengths QM arrays Initial specification limits Specification limit FM: CW > 200 µm @ 40 mV

22. PACS IIDR 01/02 Mar 2001 Photoconductor Detector Arrays21 Bias dependence QM 10 - Pixel 2 Higher stress  Higher CW  Lower Bias Break Through Voltage Higher Bias  Higher CW Lower stress means less risk of detector breakage Specification close to optimum

23. PACS IIDR 01/02 Mar 2001 Photoconductor Detector Arrays22 Relative spectral responsivity - absolute uniformity EM 6 TIA measurement ANTEC T = 1.9 K High stress (700 N) U bias = 30 mV EM 5 T = 2.5 K Low stress (62 N) U bias = 100 mV

24. PACS IIDR 01/02 Mar 2001 Photoconductor Detector Arrays23 Dependence of the DC signal (responsivity) on RT resistance Measured detector output signal U DC during operation as a function of the RT resistance of the Ge:Ga crystals - no stress applied U DC ~ Current sensitivity 1/R ~ Doping density FM LS FM HS  A linear fit derived from the data points gives a slope of -0.0339  V/   Sensitivity increases with decreasing resistance  Selection of crystals with variation of less than 15  (35  ) should give less than  10% (25%) variation within one array (whole array)

25. PACS IIDR 01/02 Mar 2001 Photoconductor Detector Arrays24 Summary: Specifications fulfilled