MiniGrail

MiniGRAIL is an instrument that is designed to detect gravitational waves. The MiniGRAIL is the first such detector to use a spherical design. It is located at Leiden University in the Netherlands. The project is being managed by the Kamerlingh Onnes Laboratory.^[1] A team from the Department of Theoretical Physics of the University of Geneva, Switzerland, is also heavily involved.

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 expected 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.^[2]

A spherical design has the benefit of being able to detect gravitational waves arriving from any direction, and it is sensitive to polarization.^[3] When gravitation waves with frequencies around 3,000 Hz pass through the MiniGRAIL ball, it will vibrate with displacements on the order of 10^-20 m.^[4] For comparison, the cross-section of a single proton (the nucleus of a hydrogen atom), is 10^-15 m (1 fm).^[5]

To improve sensitivity, the detector was intended to operate at a temperature of 20 mK.^[1] The original antenna for the MiniGRAIL detector was a 68 cm diameter sphere made of an alloy of copper with 6% aluminum. This sphere had a mass of 1,150 kg and resonated at a frequency of 3,250 Hz. It was isolated from vibration by seven 140 kg masses. The bandwidth of the detector was expected to be ±230 Hz.^[2]

During the casting of the sphere, a crack appeared that reduced the quality to unacceptable levels. It was replaced by a 68 cm sphere with a mass of 1,300 kg. This was manufactured by ItalBronze in Brazil. The larger mass lowered the resonant frequencies by about 200 Hz.^[6] 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.^[7]

Tests at temperatures of 5 K showed the detector to have a peak strain sensitivity of 1.5 × 10^-20 Hz^-½ at a frequency of 2942.9 Hz. Over a bandwidth of 30 Hz, the strain sensitivity was more than 5 × 10^-20 Hz^-½. This sensitivity is expected to improve by an order of magnitude when the instrument is operating at 50 mK.^[3]

A similar detector named "Mario Schenberg" is being built in São Paulo, which will strongly increase the chances of detection by looking at coincidences.^[8]

References

1. ^ ^{a b} de Waard, A,; et al. (2003). "MiniGRAIL, the first spherical detector". Classical and Quantum Gravity 20: S143–S151. Bibcode 2003CQGra..20S.143D. doi:10.1088/0264-9381/20/10/317. 
2. ^ ^{a b} Van Houwelingen, Jeroen (2002-06-24). "Development of a superconducting thin-film Nb-coil for use in the MiniGRAIL transducers". Leiden University. pp. 1–17. http://www.minigrail.nl/Student/Jeroen-report.pdf. Retrieved 2009-09-16. 
3. ^ ^{a b} Gottardi, L.; De Waard, A.; Usenko, O.; Frossati, G.; Podt, M.; Flokstra, J.; Bassan, M.; Fafone, V. et al. (November 2007). "Sensitivity of the spherical gravitational wave detector MiniGRAIL operating at 5K". Physical Review D 76 (10): 102005.1–102005.10. Bibcode 2007PhRvD..76j2005G. doi:10.1103/PhysRevD.76.102005. 
4. ^ Bruins, Eppo (2004-11-26). "‘Listen, two black holes are clashing!’". innovations-report. http://www.innovations-report.com/html/reports/physics_astronomy/report-36884.html. Retrieved 2009-09-16. 
5. ^ Ford, Kenneth William (2005). The quantum world: quantum physics for everyone. Harvard University Press. p. 11. ISBN 067401832X. 
6. ^ de Waard, A.; et al. (2005). "MiniGRAIL progress report 2004". Classical and Quantum Gravity 22: S215–S219. Bibcode 2005CQGra..22S.215D. doi:10.1088/0264-9381/22/10/012. 
7. ^ de Waard, A.; et al. (March 2004). "Cooling down MiniGRAIL to milli-Kelvin temperatures". Classical and Quantum Gravity 21 (5): S465–S471. Bibcode 2004CQGra..21S.465D. doi:10.1088/0264-9381/21/5/012. 
8. ^ Frajuca, Carlos; et al. (December 2005). "Resonant transducers for spherical gravitational wave detectors". Brazilian Journal of Physics 35 (4b): 1201–1203. Bibcode 2005BrJPh..35.1201F. doi:10.1590/S0103-97332005000700050. 

External links

• MiniGRAIL on the internet