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. LISA and LISA Pathfinder 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

4. Test mass charging Science goals require almost perfect free falling test masses (<10 -14 ms -2 Hz -1/2 at ~1mHz) Spurious non-gravitational forces arise if there is excess charge on the test mass caused by: Galactic Cosmic RaysSolar 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/cm 2 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 No noiseNoise + threshold SEP + alphas Isotropic19.118.8 Coincident0.970.95 GCR + alphas Isotropic7.47.2 Conincident0.380.37

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 FeatureMaterialThickness (cm)g/cm 2 Approx. Geometry Outer vacuum shellAl0.250.68Sphere =200cm Insulation/Silk MeshMLI0.27+0.10.52Sphere =170cm Radiation shieldsAl0.200.54Sphere =160cm Main TankAl0.230.62Sphere =155cm Proton ShieldAl3.7110.02Sphere =32cm Cryoperm shieldFe0.100.87Sphere =27.1cm Probe vacuum shellAl0.531.43Sphere =26cm Lead bagPb0.010.11Sphere =25cm Quartz blockSiO 2 (quartz)2.505.50 Cylinder  =6.1cm h=16cm Niobium shieldNb0.050.43 Cylinder  =6 cm h=16cm Gyroscope housingSiO 2 (quartz)1.002.20 Sphere  =4cm GyroscopeSiO 2 (quartz)Solid8.36 Sphere  =3.8cm Total31.3

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 detector range 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?