A vertical accelerometer for cryogenics implementation in third-generation gravitational-wave detectors

The design of third-generation gravitational-wave detectors requires dedicated sensors to perform very accurate measurements of the residual motion of mechanical components cooled down at cryogenic temperatures and accommodated close to the test masses. For this reason, we developed a vertical accelerometer prototype derived by the classical scheme widely used in Virgo seismic suspension control. Thermal contractions are the main concern when cooling down such a device and the calibration check at low temperature, in the absence of commercial sensors working in parallel, plays a crucial role. The accelerometer was conceived to be used at low frequencies (0.3–3 Hz) in a quite specific environment, where the noise produced by cryocoolers has to be suppressed. However, it can be easily operated over a wider frequency band, up to ~100 Hz. The achieved sensitivity is ~10−8 m s−2 below 3 Hz. During 2013, the device was successfully installed in the KAGRA cryostat, where it was tested at low temperatures down to 8 K and provided the measurement of vertical vibrational modes of the inner thermal shield.