Calibration System with Modulated Polarization Source for Superconducting Detectors at 0.1 K

We developed a calibration system with a modulated polarization source for superconducting detectors at the 0.1-K stage in a dilution cooler. Our target application for this system is detector calibration for observations of the cosmic microwave background polarization. For this application, the calibration system is required to generate a well-characterized polarization signal in a wide frequency range; e.g., 20–300 GHz. The calibration system is attached at the bottom of the 0.1-K stage. Radio absorbers, which are attached to the inner wall of a cylindrical metal shield, emit unpolarized black-body radiation (4.5 K). The radiation reflects off an aluminum mirror at 120 K, which induces a linearly polarized component because of the finite emissivity of the mirror; the magnitude of the polarization is 60 mK in this configuration. The axis of polarization can be varied by rotation of the mirror. Therefore, the detectors measure the modulated polarization; however, unpolarized radiation into the detector is maintained constant. We succeeded in cooling the system properly. The sample stage for setting the detector achieved a temperature below 0.1 K under the 5 K load condition (some of the radiation from the absorbers and the mirror emission, 0.5 K). High-frequency components of emission from the mirror are shielded by using two thermal filters: polytetrafluoroethylene and nylon 66.