Advanced Analog Design for Space Particle Detectors

1. Detectors and Analog Electronics Bill Crain The Aerospace Corporation 310-336-8530 Bill.crain@aero.org

2. Introduction • Design Overview • Requirements Flowdown • Detector Specification • Signals, Noise, and Processing • Board Descriptions • Interface Block Diagram • Power Consumption • Trade Studies • Summary

3. Detector Electronics Design Overview • Detector electronics comprised of two board designs • Detector Boards in Telescope assembly • Analog Processing Board (APB) in E-box • Heritage approach from Polar CEPPAD/IPS unchanged from proposal • Linear pulse processing system utilizing Amptek hybrids • Circuits re-designed for CRaTER requirements • Functional requirements summary • Measure LET of high LET particles in thin detectors • Measure LET of low LET particles in thick detectors • Provide good resolution for TEP effects • No fast timing requirements • Robust to temperature drift and environments

4. Analog Signal Flow Diagram • Single fixed gain, linear transfer function • All detector channels use same topology • Differences in preamp input transistor, detector biasing, and gain settings are made to optimize thin and thick channels

5. Requirements Flowdown Electronics reqs. derived from Level 2 reqs. and detector sims.

6. Detector Specification (1) • Micron Semiconductor Limited • Lancing Sussex, UK • 20 years experience in supplying detectors for space physics • CEPPAD, CRRES, WIND, CLUSTER, ACE, IMAGE, STEREO, and more… • Detector Type • Ion-implanted doping to form P+ junction on N-type silicon • Very stable technology • Advantages to science include good carrier lifetime, stable to environmental conditions, and thin entrance windows

7. 35 mm, diam. Guard P+ Contact Guard P+ window 0.1 um Active Volume (depletion region) 140 um thin; 1,000 um thick E-field N window 0.1 um N contact Detector Specification (2) • Circular detectors having active area of 9.6 cm2 • Two different detector thicknesses: thin and thick • note: state-of-the-art is 20um for thin and 2,000um for thick detectors • Guard ring on P-side to improve surface uniformity • Very thin dead layers (windows) reduce energy loss, lower series resistance, and reduce noise

8. Guard ring Al. contact plane Al. contact grid reduces surface resistivity Detector Specification (3) • Detector drawings (Micron)

9. Detector Specification (4) • ISO9001 • Full traceability and serialization • Travelers maintained • Verification and test prior to detector shipment • Random vibration test • Thermal cycling and thermal vacuum • Stability • Test criteria • Leakage current • I-V characteristic • Alpha resolution / pulser noise measurement (final test)

10. Nominal Threshold Nominal Threshold Proton Energy Deposition Simulations Reference: M. Looper GEANT4 Thin 150MeV incident E Thick 150MeV incident E Thin 1000MeV incident E Thick 1000MeV incident E

11. Iron Energy Deposition Simulation Reference: J.B. Blake

12. RFB CFB Ao Vpk = Qtot/CFB qμnNe(t)E qμpNh(t)E Cdet CFB (Ao) >> Cdet Signal Characteristics

13. Signal Processing (1) • Combined dynamic range of thin/thick pair is 5,000 • Thin threshold to provide overlap with thick range • Thin Detector Signal • Preamp input stage designed for 97% charge collection • High gain input jFET to raise dynamic input capacitance • 4% drift in operating point will result in 0.1% in output peak (< 1 bit) • Large feedback capacitance needed to handle Fe deposit • Preamp compensation to maintain closed-loop stability • Thick Detector Signal • Not as sensitive to detector capacitance • Design for low noise to maintain reliable 200 KeV low threshold and achieve < 1-bit resolution

14. + input cap. T=peaking time F=shaping factors Noise Model (1) Reference: Helmuth Spieler IFCA Instrumentation Course Notes 2001

15. Optimum Noise Model (2)

16. Signal Processing (2) • Noise dominated by detector leakage and input jFET • Shaping time for both thin and thick detectors set at thick optimum point • ~1 usec • Compatible with A/D signal acquisition timing • 3-pole gaussian shaping improves symmetry and 2-complex poles shortens tail • Shaping reduces noise but also impacts signal level • S/N at thick detector threshold for this design ~ 4 • Translates to a noise occupancy in the coincidence window of < 0.1% for time period not greater than shaping time

17. Signal Processing (3) • Other factors affecting noise performance • Bias resistor sized to minimize voltage drop (i.e., maintain stable operating point) • Detector shot noise doubles every 8 C • Beneficial to operate cold; preferably below 20 C

18. Signal Processing (4) • Pileup is rare due to low event rate and relatively short shaping time • Exception: occasional periods of high ESP flux • Leading edge trigger technique causes timing uncertainty but coincidence window is large by comparison • Amplified low-level discriminator available to reduce walk • Ballistic deficit is not an issue due to short collection times relative to peaking time of shaper • Output voltage scaled for A/D input specifications

19. Detector Board • Thin/thick detector pair use same design topology • Signal collected on P-contact • Negative bias • Guard signal shunted to ground • No guard leakage noise • AC coupling to isolate DC detector leakage current • Low noise / high gain JFET input stage

20. Analog Processing Board • Single board in E-box contains 3 thin and 3 thick detector processing channels • Flight proven pulse processing components • Amptek A250 preamp hybrid utilizing external jFET on detector board • Shaping amps use Amptek A275 for active filtering • Baseline restorer, Amptek BLR1, compensates for baseline shifts on interface to A/D • Pole-zero cancellation circuit for correcting preamp pulse decay to eliminate undershoot • Test pulser injects ΔQ at preamp input jFET

21. Analog Interface Block Diagram

22. Power Estimate Total estimated power dissipation is < 1 Watt

23. Trade Studies • Determine if one detector board can be implemented instead of three • Determine if A250 device should be located on detector board • Determine sensitivity of APB to detector performance • Want to avoid rework of APB when selecting/changing detectors during development • Determine how best to isolate chassis noise from detectors and preamp

24. Summary • Detectors are well-established technology from experienced supplier • Detector electronics design meets requirements of Level 2 mission and satisfies energy deposition levels determined by detector/TEP simulations • Trade studies in progress