Synopsis:Ten years after the plan of the Large-scale Cryogenic Gravitational wave Telescope (LCGT) project, its construction started in June, 2010. The scientific objective of the project is to detect gravitational waves that are produced by astrophysical events in the Universe. Since the events are so rare that the detection capability of the detector should be enhanced to the extent where its sensitivity is limited not by technical reasons but by ultimate physics of instrument such as thermal noise, quantum noise, and so on. LCGT adopts cryogenic mirror to reduce thermal noise, which is meaningful only after ultimate sensitivity is technically attained. Cryogenic mirror is still adventurous technique after ten years of development. I present here the overview of LCGT project.

Synopsis: A very small displacement (∼ 10−20 m/ √ Hz at 100 Hz) needs to be measured to directly detect gravitational waves (GW), which have been predicted by Einstein's theory of general relativity. It is possible to detect such small displacement in the mirrors of the Large-scale Cryogenic Gravitational wave Telescope (LCGT) using interferometry. To reduce the noise level caused by thermal oscillation of the mirrors, LCGT mirrors will be cooled down to cryogenic temperature (∼20 K). For that purpose, the thermal radiation generated at room temperature has to be reduced, and this can be done using metal shields with low emissivity. To study the emissivity of some metals at low temperature, reflectivity at cryogenic temperatures has been measured at a wavelength of 10 m, where black body radiation of 300 K has the largest intensity. As a result, the three kinds of samples measured satisfied the requirements for the LCGT with a safety factor of more than 2. In addition, the incident heat through the duct shields of the LCGT was calculated using the results of these measurements, and it was concluded that 5 baffles in the duct shield can reduce the incident heat to a sufficient level.Keywords: gravitational wave, cryogenic, radiation, emissivity, reflectivity 1.Received