Groningen, The Netherlands.

We study the behavior of NbN SIS junctions as radiation detectors with emphasis on the shotn.oise generated at voltages below the gap voltage. The intrinsically large subgap current of NbN junctions is carried by pinholes with a conduction attributed to multiple Andreev reflection, leading to transported charges q>> e. Using this charge enhancement mechanism we explain the junction shotnoise characteristics in the unpumped case as well as in the pumped case. The measured mixer noise tempe-rature in the pumped case is more than twice that calculated with standard Tucker theory. Measured double side band noise temperatures are 1450 K and 3300 K at 600 and 850 GHz respectively. Main limitations to the receiver sensitivity are the loss in the aluminum circuit and the subgap current induced shotnoise