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

614平成23年 3 月 9 日 第58回応用物理学関係連合講演会で発表  1 東京大学宇宙線研究所（〒277 8582 千葉県柏市柏の葉 5 1 5）  2 産業技術総合研究所（〒305 8561 茨城県つくば市東 1 1 1）  3 東京大学宇宙線研究所神岡宇宙素粒子研究施設（〒506 1205 岐阜県飛騨市神岡町東茂住456） Fig. 1 The Cryogenic Laser Interferometer Observatory (CLIO) is a laser interferometric gravitational wave (GW) detector. CLIO is a prototype of the Large-Scale Cryogenic Gravitational Wave Telescope (LCGT). Both CLIO and LCGT use the same cryogenic mirror technique as well as the underground site of Kamioka mine. The cryogenic mirror technique is a method to reduce thermal noises that are fundamental in GW detectors. The most important purpose of CLIO is to demonstrate sensitivity improvement by using the cryogenic mirror technique. During the operation of CLIO with cryogenic mirrors, several unique behaviors in the interferometer systems were found. One of the serious issues that we found was low eŠectiveness of the magnet damping system for a mirror suspension system at cryogenic temperatures. Thus, we have developed a new damping system, the Local Suspension Point Interferometer (LSPI). In this article, we give a brief introduction to CLIO and recent results of the LSPI experiments in CLIO.