Photoconductor Detector Arrays for PAC S IIDR - ESTEC

1. Photoconductor Detector Arrays for PACS • IIDR - ESTEC • Stefan Kraft • ANTEC-GmbH • GermanyGünter Bollmann, Peter Dinges, Otto Frenzl, Marco Jasinski, Heidrun Köppen, Heribert Krüger, Claudia Popp Photoconductor Detector Arrays

2. 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 Photoconductor Detector Arrays

3. Impacts of Specifications and Requirements to Detector Array Design Photoconductor Detector Arrays

4. Design Detector Housing Front view Side view • 5x5 linear arrays arranged according to needs of acquisition mode for spectroscopy • Light metal design (Al) - total weight ~ 6.6 kg • Thermally isolated Photoconductor Detector Arrays

5. Design Detector Housing Red array • 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 Blue array Photoconductor Detector Arrays

6. Design Detector Arrays • Light weight • FEE thermally isolated from array • Thin wire harness (Nano/micro connectors) • High stability • Proper stressing mechanism • Low cross-talk (<0.1%) • High collection efficiency • High quantum efficiency • Proper wiring concept (CDet < 2pF, dark current <5·104 e-/s) • Low EMC impacts • High degree of light tightness • Good uniformity of responsivities • Uniform cutoff wavelengths (CWs) • Low variation of CWs Photoconductor Detector Arrays

7. Design - Stress mechanism Low stress module High stress module • Maximum force: • 800 N highly stressed • 200 N low stressed • Spring travel: ~2 mm both types of modules • 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 Photoconductor Detector Arrays

8. Low Stress Array: 50 g High Stress Array: 57 g Mass Budget 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 kgin Total Photoconductor Detector Arrays

9. Thermal Budget Photoconductor Detector Arrays

10. Vibration Load • Static Load Test: 420 g @ RT on 2 posts  ~ 200 g Photoconductor Detector Arrays

11. Design - FEM analysis • Addition of cushion pads between detector and pistonsreduces the pressure gradient considerably • High centring accuracy necessary Photoconductor Detector Arrays

12. Instrument Description Schematic view of the linear photoconductor array design Photographs • Mounting accuracy ~10 µm • Slit size 30 to 70 µm • Rotational mounting accuracy <5° Photoconductor Detector Arrays

13. 1 mm ”Detector – metal contact – insulator – ball joint – metal contact”Block Design: Purposes • 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 Photoconductor Detector Arrays

14. Design - Fore optics • 16 linear light cones • Optical cavities with small apertures • Radial orientation topupil at 240 mm distance • Design optimised by optical calculations • 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 Photoconductor Detector Arrays

15. y Performance Aspects - Photon Losses • Design impacts / biasing concept: • Slits unavoidable • Polarisation dependence, glancing angle of impinging photon: Effective slit size is small •  Experimentally verified by spectral responsivity • 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 Photoconductor Detector Arrays

16. Performance Aspects - Detector Efficiency • h = Pabs/(Pabs + Ploss) • Ploss : Loss area inside cavity = entrance hole + slits (25 µm, 50 µm) + wires • Pabs = 2(a+b)·h·(1-R) ·h’ : Absorbing area • Abs. eff.: h’ = a · L, Abs. coeff.: a = 2.4 cm-1, Abs. length: L = 2.1 mmLength: a = width: b = 1 mm, height: h = 1.5 mm, Reflectivity: R = 0.4 Photoconductor Detector Arrays

17. QM 13 - Low Stress 10 % level Measured Relative Responsivities QM 2 - High Stress (re-stressed) Good uniformity, close to expectation R  20% @ 205 µm Photoconductor Detector Arrays

18. Cutoff wavelengths and variations highly stressed QM arrays Uniform mean cutoff wavelengths Relation between RRT and CW (pressure) Low variations within one array Photoconductor Detector Arrays

19. Cutoff wavelengths and variations lowstressed QM arrays Uniform mean cutoff wavelengths Low variations within one array Photoconductor Detector Arrays

20. Specification limit FM: lCW > 200 µm @ 40 mV Initial specification limits Status Min-Max Cutoff Wavelengths QM arrays Photoconductor Detector Arrays

21. Bias dependence QM 10 - Pixel 2 Higher Bias  Higher CW Higher stress  Higher CW  Lower Bias Break Through Voltage Lower stress means less risk of detector breakage Specification close to optimum Photoconductor Detector Arrays

22. Relative spectral responsivity - absolute uniformity EM 6 TIA measurement ANTEC T = 1.9 K High stress (700 N) Ubias = 30 mV EM 5 T = 2.5 K Low stress (62 N) Ubias = 100 mV Photoconductor Detector Arrays

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

24. Summary: Specifications fulfilled Photoconductor Detector Arrays