Reflectivity Measurements of Metals for LCGT Thermal Radiation Shields at Cryogenic Temperature and Wavelength of 10 μm

Abstract

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 μm, 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.