No abstract

The next generation of gravitational experiments in space is likely to use completely isolated proof-masses. For example, LISA uses proof-masses as mirrors in interferometers for gravitational wave astronomy (Bender et al 1998 Pre-phase A report MPG-233 pp 1-191) and STEP uses proof-masses in Earth orbit for an equivalence principle test (Sumner et al 2003 at press). Nongravitational forces will act on these proof-masses if they become charged, through the action of cosmic rays and solar flare particles for example. This paper examines the consequences of proof-mass charging for LISA, and presents results from using GEANT4 to assess the charging processes. Finally, there is a brief discussion of a means of controlling the charge down to an acceptable level.

This paper concerns the effects of the build-up of electrical charge on the LISA test masses. Charge accumulates on the isolated test masses due to the bombardment of the spacecraft by galactic cosmic rays and solar particles. This will result in forces on the test masses, due to Coulomb and Lorentz interactions, which will disturb their geodesic motion. The three main disturbances associated with this charge are an increase in the test mass acceleration noise, coupling between the test mass and the spacecraft and the appearance of coherent Fourier components in the measurement bandwidth. These disturbances are estimated using the latest charging rate and noise predictions from GEANT4 for both the LISA mission and the technology demonstration mission, LISA Pathfinder, at different times in the solar cycle. The Coulomb disturbances are evaluated based on a detailed 3D, electrostatic, finite element model and submodels of the LTP sensor. These results are compared with those derived using the customary parallel plate approximation to calculate capacitances, and the accuracy of these approximations is assessed for typical parameter settings. The variation of the magnitude of charging disturbances as different parameters are changed, and the management of such disturbances are discussed.

Electrical charges will build up on the LISA test masses due to particle radiation from cosmic rays and from the Sun. This charging can lead to spurious forces both through Lorentz and Coulomb interactions. These effects can give rise not only to acceleration noise, but also to unwanted coherent signals in the LISA bandwidth, due to the time dependence of the amount of charge accumulated on a test mass. The amplitude of these unwanted coherent signals depends on several factors, including the charging rate, the maximum allowed charge, the discharge procedure and a number of design parameters of the sensor itself. This paper estimates the magnitude of such unwanted charging signals based on the current, nominal LTP sensor dimensions and discusses approaches that may be taken to substantially reduce them for LISA.

Scintillation produced in liquid xenon by alpha particles and gamma rays has been studied as a function of applied electric field. For back scattered gamma rays with energy of about 200 keV, the number of scintillation photons was found to decrease by 64±2% with increasing field strength. Consequently, the pulse shape discrimination power between alpha particles and gamma rays is found to reduce with increasing field, but remaining non-zero at higher fields.