Cooling down MiniGRAIL to milli-Kelvin temperatures

The latest developments in the construction of the ultra-cryogenic spherical detector MiniGRAIL are presented. The room temperature part of the vibration isolation system was improved and provided with an attenuation of about 60 dB around 3 kHz. The transfer function of the cryogenic stages gave about 20 dB per stage, at the resonant frequency of the sphere. The latest results of three cryogenic tests at ultra-low temperature of the spherical detector MiniGRAIL, using several thermal anchorings, are presented. Minimum temperatures of 20 mK on the mixing chamber of the dilution refrigerator and 79 mK on the surface of the sphere were reached. During the last cool down, two capacitive transducers were mounted on the sphere. The first was coupled to a room temperature FET amplifier and the second to a transformer and a double stage SQUID amplifier. Unfortunately the SQUID did not work, so only the first resonator could be used. An equivalent temperature of about 20 K was measured during an acquisition run of 7 h, using the first transducer corresponding to the FET white noise.