REXIS Detector Packaging and Assembly

1. REXIS Detector Packaging and Assembly Harrison Bralower REXIS Engineering Peer Review 9/18/12

2. What is OSIRIS-REx? • The Origins, Spectral Interpretation, Resource Identification, (and) Security REgolith eXplorer is a NASA New Frontiers mission to asteroid 1999 RQ36 • Asteroid has a heliocentric orbit that intersects Earth’s orbit; it can usually be found between here and Mars • 1-in-1000 chance asteroid hits Earth in 2169 • OSIRIS-REx Mission objectives • Return at least 60g of asteroid regolith to Earth and characterize the sample in situ • Globally map the asteroid topology and mineralogy • Measure orbital perturbations due to Yarkovsky effect • Compare data to ground measurements of other nearby asteroids • REXIS is a Class-D student payload onboard OSIRIS-REx Regolith X-ray Imaging Spectrometer – REXIS – Detector Packaging Peer Review September 18, 2012

3. What is REXIS? Coded Mask • REgolith • We study asteroid dust • X-ray • We monitor X-rays fluoresced out of the asteroid by incident solar X-rays (which we also monitor) • Imaging • We use CCDs to capture images of the asteroid and create a global map • Spectrometer • We identify spectral lines in the collected X-rays to determine the chemical composition of the asteroid Radiation Cover Detector Array DASS Radiator Electronics Box Thermal Isolation Regolith X-ray Imaging Spectrometer – REXIS – Detector Packaging Peer Review September 18, 2012

4. REXIS Science Mission Sun • We want to identify the distribution of carbon on the surface of RQ36 with a spatial resolution of 50m or better • Problem: Our CCDs cannot detect carbon emission lines—too low energy • We can indirectly determine the carbon content by measuring distribution of other elements • O, Mg, Fe, Al, Si, S with SNR > 10 • These all emit X-rays between 0.5-10keV, which the REXIS CCDs can detect with high quantum efficiency and spectral resolution • Monitoring solar X-rays provides context for X-rays measured off the asteroid RQ36 Solar X-rays X-ray Fluorescence (XRF) Regolith X-ray Imaging Spectrometer – REXIS – Detector Packaging Peer Review September 18, 2012

5. Coded-Mask Imaging • Consider a mask pattern in front of the detectors • Photons from any direction cast a shifted shadow of the mask pattern onto the detectors • Intensity of shifted pattern corresponds to how bright that direction is in the field of view • Simple ray-tracing algorithms can determine what all possible shifted patterns look like • Spatial cross-correlation of data and mask pattern reveals the most statistically likely direction of incident X-rays Detector Plane Image Coded Aperture Mask Reconstructed Image

6. CCD Thermal Environment Asteroid QIR & QAlbedo QSolar QRadiation Sun Radiator Sun Shield Heat Strap (DASS to Radiator) QPower QParasitic EB CCDs OSIRIS-REx S/C Deck Regolith X-ray Imaging Spectrometer – REXIS – Detector Packaging Peer Review September 18, 2012

7. Thermal Requirements • Each CCD must dissipate ~61mW of heat (TBR) • Asteroid IR and albedo, intra-instrument radiative exchange, power to operate the CCDs, and parasitic conduction from electronics • Science requirements demand CCDs remain at -60°C or colder • Data thrown out at temperatures above -50°C • CCDs are passively cooled via conduction through DASS and thermal strap to radiator, which radiates to deep space • Instrument position on spacecraft deck has been an issue • We believe the radiator can reach -70°C, which leaves 10°C of conduction losses, contact resistances, and margin

8. Lincoln Labs CCID-41 • Heritage dating to ACIS (CCID-17) and Astro-E2 (CCID-41) • Designed for X-ray applications • 1024 x 1024 pixels • 24µm x 24µm pixel size • Consistent with L3 specs • Split frame-store • Back-illuminated • Covered with 220nm aluminum for optical blocking • Supports charge injection • REXIS uses a 2 x 2 array of these CCDs Regolith X-ray Imaging Spectrometer – REXIS – Detector Packaging Peer Review September 18, 2012

9. CCD Packaging Options • Lincoln can provide different packaging options depending on the application • Invar pedestal • Used for tight-tolerance applications • Invar can be heat-treated to match the CTE of silicon • Kovar/alumina ZIF socket • Used primarily for development applications in a lab setting • Ceramic package • Used for looser-tolerance applications • Cheapest and easiest to produce • Custom packaging(e.g. integrating mechanical and electrical interfaces in one structure for PanSTARRS) Regolith X-ray Imaging Spectrometer – REXIS – Detector Packaging Peer Review September 18, 2012

10. REXIS Packaging Trade • Invar package is tighter tolerance than we need • Coded-mask imaging is very robust against misalignment and other error sources • Invar is heavy and has poor thermal conductivity • Kovar/alumina ZIF package isn’t really intended for flight • Kovar is heavy • Would require custom ZIF socket or other connector • Thermal management would be very difficult • CCDs not replaceable • Package not rated below -40C; REXIS operates at -60C • CCDs already ship in alignment • Ceramic package is (relatively) simple • Low mass (4 packaged ACIS CCDs have a total mass of ~200g) • Easy to connect to electronics since we pick the connectors • CCDs are individually replaceable • Thermal management much simpler • Rated to -80C • Harder to align—each CCD must be in alignment individually

11. REXIS Packaging Design • What does REXIS need in a detector package? • Low mass • Good CTE match to silicon • Easy alignment • Replaceable CCDs • Low thermal resistance • Launch survivability • ACIS heritage package meets most of these requirements Regolith X-ray Imaging Spectrometer – REXIS – Detector Packaging Peer Review September 18, 2012

12. REXIS Packaging Design • Design is almost exactly the same as ACIS package • Some design elements subjected to trades or redesign • Ceramic substrate material • Metal parts material • Flexprint design Regolith X-ray Imaging Spectrometer – REXIS – Detector Packaging Peer Review September 18, 2012

13. Detector Assembly Requirements • What do we need in a detector assembly? • Abutment gap of an integer multiple of 384µm • Low mass • Low thermal resistance • Keep CCDs aligned with respect to each other and the mask • Protect CCDs from radiation damage • Shield frame-store from incident photons • Minimize stray light from striking CCDs • Provide means of checking CCD degradation in flight • Fit within existing REXIS footprint (if possible) • Provide breakout of CCD electrical connections to electronics • Must not fluoresce in detectable range • All aluminum parts in field of view will be gold-coated

14. Detector Assembly Requirements • Distance between active imaging areas of CCDs an integer multiple of 384µm • REXIS bins single 24µm x 24µm pixels into 16 x 16 superpixels • Gaps in data should be an integer multiple of superpixel size • Actual gap between packages is smaller due to inactive silicon on die edges • Maximum misalignment requirements • ΔX/ΔY translational shift (in detector plane): ±0.0384cm • ΔZ translational shift (normal to detector plane): +6.67/-4.00cm • Rotational shift about X and Y: ±1.54° • Rotational shift about Z: ±0.40° • All mechanical parts must have first natural frequency >90Hz • All mechanical parts must survive launch • Will perform launch load and vibration analysis before PDR

15. REXIS Detector Assembly Flexprint Attachment Points Fe-55 Check Source Tee (bonded to CCD package) Active Imaging Area “Fun Part” Flexprint Slot Radiation Shield Alignment Pin Regolith X-ray Imaging Spectrometer – REXIS – Detector Packaging Peer Review September 18, 2012

16. Thermal Model of Assembly Regolith X-ray Imaging Spectrometer – REXIS – Detector Packaging Peer Review September 18, 2012

17. Ceramic Substrate Trade • Two metrics: CTE match to silicon and thermal conductivity • Cost not an issue for this part • Substrate thickness still open trade • Nominally 1/8” from ACIS heritage • Need to do shock/vibration analysis to find minimum substrate thickness Regolith X-ray Imaging Spectrometer – REXIS – Detector Packaging Peer Review September 18, 2012

18. Ceramic Substrate Trade

19. Metal Parts Trade • Detector assembly has three metal parts (not including radiation shielding) • Tee (bonded to package) • Array structure (holds CCDs and is screwed into the DASS) • “Fun Part” (screwed into array structure and bonded to CCD package with RTV epoxy) • Considered two materials: Beryllium and aluminum • Plot on previous slide includes only aluminum parts in model • ΔT across DAM lower than expected • Beryllium provides lower mass, higher thermal conductivity, and better CTE match to other parts at significant risk to project • No justification for using beryllium in design

20. Fe-55 Check Source • CCDs degrade with exposure to space environment • Spectral lines shift in mean energy and become broader with time • Installing a known radiation source in the assembly can be used for in-flight calibration and monitoring of CCD health • Fe-55 is a very common laboratory radiation source • Fe-55 has peaks at 5.9keV and 6.4keV, which can be used to check CCD performance against requirements set at 6keV • Requirements for check source • Must not radiate towards other instruments on OSIRIS • Must provide enough counts (3kBq source chosen) • Must only illuminate 8-16 pixels on CCD edge so as not to contaminate asteroid data

21. REXIS Detector Assembly Radiation Shield Collimator Abutment Gap Fe-55 Source Pockets for Mass Reduction Shim Regolith X-ray Imaging Spectrometer – REXIS – Detector Packaging Peer Review September 18, 2012

22. Radiation Cover Hinge Frangibolt Aluminum Cover Weak Point Titanium Bolt Regolith X-ray Imaging Spectrometer – REXIS – Detector Packaging Peer Review September 18, 2012

23. Radiation Damage • All electronics in flight require shielding from ionizing and non-ionizing radiation • Most chips will latch up or fail • CCDs degrade instead • Non-ionizing radiation will displace silicon atoms from crystal lattice and create high-energy electron traps • Decreases charge transfer efficiency (CTE) in CCDs, which shows up as a gain decrease • To first order, CCDs will record sum of actual and gain-shifted peaks • As gain shift increases with damage, spectral lines get broader Regolith X-ray Imaging Spectrometer – REXIS – Detector Packaging Peer Review September 18, 2012

24. Radiation Damage • Spectral lines widen until science requirements are violated • Charge injection will delay this • Data from Astro-E2 relates change in CTE to radiation dose for REXIS CCDs • By finding lowest tolerable CTE we can find the maximum acceptable dose and size the shield • Current depth: 5.5mm (TBR) Regolith X-ray Imaging Spectrometer – REXIS – Detector Packaging Peer Review September 18, 2012

25. Flexprint Cable Requirements • Low mass • High thermal resistance • Circuit remains out of CCD FoV • Mounts to detector assembly • Must survive launch • Must provide strain relief on flexprint • No vias on CCD end • Kavli has shownvia failure under thermal cycling • Shielded video output/frame-store lines • May not appear in Gerber files • Glenair 890-011 37-pin Nanominiature connector • .725”(L) x .125” (W) x .255”(D) Regolith X-ray Imaging Spectrometer – REXIS – Detector Packaging Peer Review September 18, 2012

26. Flexprint Cable Design • This circuit delivers analog video through output JFETs to the detector electronics • Also contains reverse-protection diodes and filter capacitors, plus an RTD for monitoring CCD temperature • Four layers • Smaller and more compact than Astro-E2 flexprint • Output connects to electronics box via connector saver • Routed either through DASS or out truss side panels—trade still open 4-Wire RTD 2-56 Through Holes Output Protection Diodes Connector Regolith X-ray Imaging Spectrometer – REXIS – Detector Packaging Peer Review September 18, 2012

27. Questions?