Editing MiniGrail (section)

==Design==
A spherical design has the benefit of being able to detect gravitational waves arriving from any direction, and it is sensitive to polarization.<ref name="prd76">{{cite journal
  | last1=Gottardi | first1=L. |date=November 2007
  | title=Sensitivity of the spherical gravitational wave detector MiniGRAIL operating at 5K
  | journal=Physical Review D | volume=76 | issue=10
  | pages=102005.1–102005.10
  | doi=10.1103/PhysRevD.76.102005
  | last2=De Waard
  | first2=A.
  | last3=Usenko
  | first3=O.
  | last4=Frossati
  | first4=G.
  | last5=Podt
  | first5=M.
  | last6=Flokstra
  | first6=J.
  | last7=Bassan
  | first7=M.
  | last8=Fafone
  | first8=V.
  | last9=Minenkov
  | first9=Y. | last10=Rocchi | first10=A. |bibcode = 2007PhRvD..76j2005G |arxiv = 0705.0122 | s2cid=119261963 | display-authors=8 }}</ref>  When gravitation waves with frequencies around 3,000&nbsp;Hz pass through the MiniGRAIL ball, it will vibrate with displacements on the order of 10<sup>−20</sup> m.<ref>{{cite web
  | first=Eppo | last=Bruins
  | title=Listen, two black holes are clashing!
  | date=2004-11-26 | publisher=innovations-report
  | url=http://www.innovations-report.com/html/reports/physics_astronomy/report-36884.html
  | accessdate=2009-09-16 }}</ref> For comparison, the cross-section of a single [[proton]] (the nucleus of a [[hydrogen]] [[atom]]), is 10<sup>−15</sup> m (1&nbsp;fm).<ref>{{cite book | first=Kenneth William | last=Ford | date=2005 | title=The quantum world: quantum physics for everyone | page=[https://archive.org/details/quantumworldquan00kenn/page/11 11] | publisher=[[Harvard University Press]] | isbn=0-674-01832-X | url=https://archive.org/details/quantumworldquan00kenn/page/11 }}</ref>

To improve sensitivity, the detector was intended to operate at a temperature of 20&nbsp;mK.<ref name="cqg20" /> The original antenna for the MiniGRAIL detector was a 68&nbsp;cm diameter sphere made of an alloy of copper with 6% aluminium. This sphere had a mass of 1,150&nbsp;kg and resonated at a frequency of 3,250&nbsp;Hz. It was isolated from vibration by seven 140&nbsp;kg masses. The [[Bandwidth (signal processing)|bandwidth]] of the detector was expected to be ±230&nbsp;Hz.<ref name="Houwelingen02" />

During the casting of the sphere, a crack appeared that reduced the quality to unacceptable levels. It was replaced by a 68&nbsp;cm sphere with a mass of 1,300&nbsp;kg. This was manufactured by ItalBronze in Brazil. The larger mass lowered the resonant frequencies by about 200&nbsp;Hz.<ref>{{cite journal
  | author=de Waard, A.
  | display-authors=etal
  | title=MiniGRAIL progress report 2004
  | journal=Classical and Quantum Gravity | volume=22
  | issue=10
  | pages=S215–S219 | doi=10.1088/0264-9381/22/10/012
  | date=2005 |bibcode = 2005CQGra..22S.215D | s2cid=35852172
 | url=https://research.utwente.nl/en/publications/minigrail-progress-report-2004(e2ae42cb-9b18-4d10-84c4-de3c1c77d9dd).html
 | url-access=subscription}}</ref> 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]].<ref>{{cite journal
  | author=de Waard, A. |display-authors=etal
  | title=Cooling down MiniGRAIL to milli-Kelvin temperatures
  | journal=Classical and Quantum Gravity | volume=21
  | issue=5 | pages=S465–S471 |date=March 2004
  | doi=10.1088/0264-9381/21/5/012 |bibcode = 2004CQGra..21S.465D |s2cid=250811527
 |url=https://ris.utwente.nl/ws/files/6702784/Waard004cooling1.pdf
 }}</ref>

Tests at temperatures of 5 K showed that the detector had a peak strain sensitivity of {{nowrap|1.5 × 10<sup>−20</sup> Hz<sup>−{{frac|1|2}}</sup>}} at a frequency of 2942.9&nbsp;Hz. Over a bandwidth of 30&nbsp;Hz, the strain sensitivity was more than {{nowrap|5 × 10<sup>−20</sup> Hz<sup>−{{frac|1|2}}</sup>}}. This sensitivity is expected to improve by an order of magnitude when the instrument is operating at 50 mK.<ref name="prd76"/>

A similar detector named "[[Mario Schenberg (Gravitational Wave Detector)|Mario Schenberg]]" is located in [[São Paulo]]. The co-operation of the detectors strongly increase the chances of detection by looking at coincidences.<ref>{{cite journal
  | author=Frajuca, Carlos |display-authors=etal
  | title=Resonant transducers for spherical gravitational wave detectors |date=December 2005
  | journal=Brazilian Journal of Physics | volume=35
  | issue=4b | pages=1201–1203
  | doi=10.1590/S0103-97332005000700050 |bibcode = 2005BrJPh..35.1201F |url=http://www.scielo.br/pdf/bjp/v35n4b/a50v354b.pdf| doi-access=free }}</ref>