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A particle monitor for LISA Pathfinder and Gravity Probe-B gyroscope charging in LEO

A particle monitor for LISA Pathfinder and Gravity Probe-B gyroscope charging in LEO. Peter Wass, Henrique Ara ú jo, Tim Sumner Imperial College London, UK Mokhtar Chmiessani, Alberto Lobo, IFAE & IEEC, Barcelona, Spain Lenny Sapronov, Sasha Buchman Stanford University, California, USA.

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A particle monitor for LISA Pathfinder and Gravity Probe-B gyroscope charging in LEO

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  1. A particle monitor for LISA Pathfinder and Gravity Probe-B gyroscope charging in LEO Peter Wass, Henrique Araújo, Tim Sumner Imperial College London, UK Mokhtar Chmiessani, Alberto Lobo, IFAE & IEEC, Barcelona, Spain Lenny Sapronov, Sasha Buchman Stanford University, California, USA

  2. Talk outline • LISA and LISA Pathfinder • Previous GEANT work • LISA Pathfinder radiation monitor definition • Radiation monitor simulations • Conclusions • Gravity Probe B • Gyroscope charging simulations and data • Proton monitor simulations and data • Conclusions

  3. Laser interferometer space antenna for detecting gravitational waves in space 3 spacecraft each with 2 free-floating test masses 5 million km arm-length 1 AU orbit Launch 2014 LISA Pathfinder Drag-free technology demonstrator for LISA 1 spacecraft 2 test masses 30 cm baseline interferometer L1 Lagrange point orbit Launch 2008 LISA and LISA Pathfinder

  4. Science goals require almost perfect free falling test masses (<10-14ms-2Hz-1/2 at ~1mHz) Spurious non-gravitational forces arise if there is excess charge on the test mass caused by: Galactic Cosmic Rays Test mass charging Solar particles (CME)

  5. Calculating TM charging • Complex model of spacecraft • Track all charged particles entering/leaving test masses • Average charging rate & stochastic charging noise • Charging sensitivity to primary energy

  6. LISA Pathfinder radiation monitor • Variations in charging can compromise science goals of the mission • Want to measure the flux responsible for charging • A particle monitor is proposed based on a telescopic arrangement of PIN diodes. • 5-10 g/cm2 of shielding stops particles E<70-90MeV • Count rates sufficient to detect small fluctuations in flux • Energy resolution to distinguish GCR and SEP spectra.

  7. Simulations • Simulate performance of the monitor using GEANT4 • Predict the count rates due to GCR flux and during SEP events • Record deposited energy spectrum measured from coincident hits in the PIN diodes.

  8. Results • Particles with energy below 72 MeV can not penetrate shielding • >90% of particles with E>120 MeV are detected. • GCR (min) count rate of ~7 counts/s from both diodes

  9. Results • The energy spectrum deposited in the diodes during small SEP events can be distinguished in measurement periods shorter than 1hr. • The average angular acceptance of the telescopic configuration of diodes is 30 deg FWHM. • For particles with energies <120 MeV the acceptance is ~15 deg.

  10. Conclusions and Future work • According to simulations, the monitor fulfils all requirements • 28 October 2005 - Radiation monitor testing at PSI • Using 50-250MeV protons, measure: • Shielding cut-off • Max count rates • Angular dependence • Diode degradation

  11. Gravity Probe B • Aims to detect geodetic and frame-dragging effects on free-falling gyroscopes in low earth orbit • 600km polar orbit • Gyroscopes accumulate charge from SAA • GP-B payload also includes a high energy proton monitor (30-500MeV)

  12. Simulations • Use simulation code adapted from LISA/LISA Pathfinder work • Simplified model of GP-B spacecraft – concentric shielding • Use orbit averaged proton spectra to calculate charging rate • AP-8 solar maximum model

  13. Results and data comparison • The average charging rate, calculated from simulations is +12.5e/s • Charging rate measured on orbit is +0.11mV/day or +8.0e/s

  14. GP-B proton monitor • 4×14mm diameter silicon detectors 150µm-150µm-700µm -150µm • 2mm Tantalum shielding restricts angular acceptance • 3mm aluminium window – 45 deg view angle • Energy determination from 700µm detectorrange 30-500MeV • GEANT model to simulate response of detector • Compare with data to check flux model

  15. Simulation and data comparison • Simulate average measured spectrum & compare with measurements from GP-B • Higher resolution data available for more detailed analysis

  16. Conclusions and Future work • Early results seem in good agreement • Test other radiation models • Charging/proton counts during solar particle event • Difference between gyros? • Simulate more complex geometry? • Dedicated post-science phase measurements?

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