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

LEO

Peter Wass, Henrique Araújo, Tim SumnerImperial College London, UK

Mokhtar Chmiessani, Alberto Lobo, IFAE & IEEC, Barcelona, Spain

Lenny Sapronov, Sasha BuchmanStanford University, California, USA

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

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

Test mass charging• 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 Solar particles (CME)

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

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.

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.

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 noise Noise + threshold

SEP + alphas

Isotropic 19.1 18.8

Coincident 0.97 0.95

GCR + alphas

Isotropic 7.4 7.2

Conincident 0.38 0.37

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.

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

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)

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 modelFeature Material Thickness

(cm) g/cm2 Approx. Geometry

Outer vacuum shell Al 0.25 0.68 Sphere =200cmInsulation/Silk Mesh MLI 0.27+0.1 0.52 Sphere =170cmRadiation shields Al 0.20 0.54 Sphere =160cmMain Tank Al 0.23 0.62 Sphere =155cmProton Shield Al 3.71 10.02 Sphere =32cmCryoperm shield Fe 0.10 0.87 Sphere =27.1cmProbe vacuum shell Al 0.53 1.43 Sphere =26cmLead bag Pb 0.01 0.11 Sphere =25cmQuartz block SiO2

(quartz)

2.50 5.50 Cylinder =6.1cm h=16cm

Niobium shield Nb 0.05 0.43 Cylinder =6 cm h=16cm

Gyroscope housing SiO2 (quartz)

1.00 2.20 Sphere =4cm

Gyroscope SiO2 (quartz)

Solid 8.36 Sphere =3.8cm

Total 31.3

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

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

Simulation and data comparison• Simulate average measured spectrum & compare

with measurements from GP-B• Higher resolution data available for more detailed

analysis

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?