Portable Submillmeter-wave SIS Receiver

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We have developed a portable submillmeter-wave receiver with a low-power consumption cryocooler. Because of lightweight(30kg), it is easy to transport the SIS receiver to high altitude sites such as Pampa la Bola in northern Chile. A careful thernal design include a new 250 K shield gives an operational temperature 3.5 K at the 2^<nd> stage. A simple optical design allows high aperture efficiency in submillimeter-wave frequencies above 400 GHz. With a new low noise SIS receiver, we have surveyed a giant molecular cloud, M17 in the neutral carbon atom(CI : 492GHz) and CO J=3-2(345 GHz)lines. The integrated intensity ratio of CI to CO J=3-2([CI]/[CO(3-2)])in M17 shows a spatial gradient that it continuously increases from 0.2 in the NE part(M17A)near OB clusters to 0.8 in the SW part(M17D). The [CI]/[CO(3-2)]of 0.5 in total is close to the Galactic value of 0.57 obtaied by COBE FIRAS.Thus, the giant molecular clouds without strong UV sources such as M17D largely contribute to the CI emission in the Galactic plane. Our developments on the small, low-power consumption and low-noise SIS receiver are useful for molecular spectroscopy and atmospheric research as well we the Atacama Large Millimeter Submillimeter Array(ALMA).