BRAIN/MBI: a bolometric interferometer dedicated to the CMB polarization

BRAIN/MBI is a mm-wave ground-based experiment dedicated to the study of the B-modes of CMB polarization. It is based on the bolometric interferometry (BI) technique, and it is the result of the merging of two different projects proposed in the last years: BRAIN (France, Italy, UK) and MBI 4 (US, UK). BI is a technique already validated in a laboratory environment using small scale demonstrators employing as detectors standard 4 K bolometers. In fact, both the MBI and BRAIN teams demonstrated that interference fringes are observed once the signals coming from the two antennas of a single baseline are phase-shifted, combined and then detected. Both optical and wave-guided combination schemes have been tested within the collaboration. From the hardware point of view, the simplest unity of BRAIN/MBI is constituted by a feed horn, an OMT, and two independent phase shifters for each polarization. After phase shifting, the two beams coming out from each unity, feed a beam combiner redirecting the signals towards the detectors. Some of the most critical components, that are OMTs, phase-shifters and possibly detectors, will be made using superconducting technologies and are now under development. Moreover, detailed calculations showed that the sensitivity of a bolometric interferometer, if operated with a proper phase modulation-demodulation scheme, can approach that one of an imager with a comparable number of detectors, while exploiting the systematics cleaness of compact interferometers. On one side, BRAIN/MBI will be an instrument with a sensitivity and a polarization purity suitable for the detection of B-modes. On the other side, it will be a benchmark for new and promisings technologies that may be qualified for the CMB science in perspective of a post-Planck space mission. Since BI is prone to different contaminations from systematics with respect to imagers, a cross-check of the results and a comparison of two conceptually orthogonal strategies will finally be possible