C.8.8 Electric field sensitive systems

Internal charging also affects systems where electric fields are being measured sensitively but where components are insulators, semiconductors or electrically isolated conductors.

Triaxial accelerometers and gravitometers represent a uniquely sensitive case of an isolated conductor. In these instruments, acceleration or gravitational pull is usually measured by the force used to maintain a metallic test mass stationary between electrodes e.g. as implemented in the ASTRE accelerometer (see [35]). Alternatively, the test mass can be floating freely, with the spacecraft keeping station around it, as in the forthcoming LISA gravitational astronomy mission (see [37]). Any electrical charging of the mass, due to deposition of penetrating particles produces an additional electrostatic force that is indistinguishable from the acceleration being measured. Some accelerometers have used a fine wire for grounding but this can compromise accuracy. Alternatively, occasional controlled discharging by photoemission, using a UV source, can be conducted. Such an instrument is shielded from penetrating particles in order to minimize internal charging. It is an unusual feature of such systems that it is penetrating ions (>100 MeV) that represent the greatest hazard. Past missions have experienced enhanced charging in LEO when passing through the South Atlantic Anomaly (see [35]) while missions in GEO or interplanetary space can be subject to Solar energetic particle events.

In a similar way micro-electromechanical systems (MEMS) can be affected by radiation-induced electric fields. These systems have electronic and mechanical parts integrated on the same semi-conducting chip. These can include thin insulating layers of e.g. SiO₂ or Si₃N₄. Dose-dependent production of electron-hole pairs can lead to charge trapping in the dielectric. These layers are too thin for the electric fields in them to approach breakdown levels. Nevertheless, for some applications, smaller fields can have serious consequences on performance. In one study (see [36]) a certain 1-d MEMS accelerometer was shown to exhibit spurious measurements under proton irradiation of its mechanical part. A similar device, in which the dielectric layer was electrically shielded by a conductor did not experience the same problem.