Submillimeter spectrum of the cosmic background radiation

Absolute measurement of the sky brightness is necessary in order to obtain an accurate cosmic backround radiation (CBR) spectrum. The report briefly describes the results of rocket observation with cold optics which was carried out to collect accurate data below the millimeter region. The sounding rocket was launched in February 1987. The submillimeter radiometer used consists of a horn antenna and photometer both of which are cooled down to 1.0 K. Radiation is collected by the horn antenna with a throughput of 0.16 cm2sr and a 7.6deg FWHM field of view. The radiation is modulated at 6.5 Hz by a cold chopper at the entrance of the photometer. The modulated beam is separated by a series of dichroic beam splitters, enabling simultaneous observation in six passbands. The characteristic features of this radiometer are that the instrumental thermal background is negligible compared with 3 K CBR and that the passbands of filters have a sharp cutoff. In the Wien part of the CBR spectrum, a distortion of the spectrum is expected from several theories. The most familiar one of them is inverse Compton scattering due to high energy electrons. An effort is made to fit the obtained data to this model so that the model gives the highest possible Rayleigh-Jeans temperature. An excess radiation energy density is found to exist which amounts to ∼10 % of CBR. (N.K.)