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

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 (lt10-14ms-2Hz-1/2 at 1mHz)
• Spurious non-gravitational forces arise if there
  is excess charge on the test mass caused by

• Galactic Cosmic Rays

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
  Elt70-90MeV
• 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
• gt90 of particles with Egt120 MeV are detected.
• GCR (min) count rate of 7 counts/s from both
  diodes

No noise Noise threshold
SEP alphas Isotropic 19.1 18.8
SEP alphas Coincident 0.97 0.95
GCR alphas Isotropic 7.4 7.2
GCR alphas Conincident 0.38 0.37
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 lt120 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

Feature Material Thickness (cm) g/cm2 Approx. Geometry
Outer vacuum shell Al 0.25 0.68 Sphere 200cm
Insulation/Silk Mesh MLI 0.270.1 0.52 Sphere 170cm
Radiation shields Al 0.20 0.54 Sphere 160cm
Main Tank Al 0.23 0.62 Sphere 155cm
Proton Shield Al 3.71 10.02 Sphere 32cm
Cryoperm shield Fe 0.10 0.87 Sphere 27.1cm
Probe vacuum shell Al 0.53 1.43 Sphere 26cm
Lead bag Pb 0.01 0.11 Sphere 25cm
Quartz block SiO2 (quartz) 2.50 5.50 Cylinder ?6.1cm h16cm
Niobium shield Nb 0.05 0.43 Cylinder ?6 cm h16cm
Gyroscope housing SiO2 (quartz) 1.00 2.20 Sphere ?4cm
Gyroscope SiO2 (quartz) Solid 8.36 Sphere ?3.8cm
Total 31.3
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

• 414mm 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?