Ultra Low Background Cryogenic Test Facility for Far-Infrared Radiation Detectors

The next generation of far infrared radiation detectors is aimed to reach photon noise limited performance in space based observatories such as SPICA and BLISS. These detectors operate at loading powers of the astronomical signal of a few Attowatt (10(-18) W) or less, corresponding to a sensitivity expressed in noise equivalent power as low as NEP = 2 x 10(-20) W/root Hz. We have developed a cryogenic test setup for microwave Kinetic Inductance Detectors (MKIDs) that aims to reach these ultra-low background levels. Stray light is stopped by using a box in a box design with a sample holder inside another closed box. Microwave signals for the MKID readout enters the outer box through custom made coax cable filters. The stray light loading per pixel is estimated to be less than 60x10(-18) W during nominal operation, a number limited by the intrinsic sensitivity of the MKIDs used to validate the system.