Gravitational waves are a type of radiation that is emitted by objects that have mass and are undergoing acceleration. The strongest sources of gravitational waves are suspected to be compact objects such as neutron stars and black holes. This detector may be able to detect certain types of instabilities in rotating single and binary neutron stars, and the merger of small black holes or neutron stars.[3]
Design
A spherical design has the benefit of being able to detect gravitational waves arriving from any direction, and it is sensitive to polarization.[4] When gravitation waves with frequencies around 3,000Hz pass through the MiniGRAIL ball, it will vibrate with displacements on the order of 10−20 m.[5] For comparison, the cross-section of a single proton (the nucleus of a hydrogenatom), is 10−15 m (1fm).[6]
To improve sensitivity, the detector was intended to operate at a temperature of 20mK.[2] The original antenna for the MiniGRAIL detector was a 68cm diameter sphere made of an alloy of copper with 6% aluminium. This sphere had a mass of 1,150kg and resonated at a frequency of 3,250Hz. It was isolated from vibration by seven 140kg masses. The bandwidth of the detector was expected to be ±230Hz.[3]
During the casting of the sphere, a crack appeared that reduced the quality to unacceptable levels. It was replaced by a 68cm sphere with a mass of 1,300kg. This was manufactured by ItalBronze in Brazil. The larger mass lowered the resonant frequencies by about 200Hz.[7] The sphere is suspended from stainless steel cables to which springs and masses are attached to dampen vibrations. Cooling is accomplished using a dilution refrigerator.[8]
Tests at temperatures of 5 K showed that the detector had a peak strain sensitivity of 1.5 × 10−20 Hz−1⁄2 at a frequency of 2942.9Hz. Over a bandwidth of 30Hz, the strain sensitivity was more than 5 × 10−20 Hz−1⁄2. This sensitivity is expected to improve by an order of magnitude when the instrument is operating at 50 mK.[4]
A similar detector named "Mario Schenberg" is located in São Paulo. The co-operation of the detectors strongly increase the chances of detection by looking at coincidences.[9]
