SWIPE multi-mode pixel assembly design and beam pattern measurements at cryogenic temperature

In this paper, we present beam pattern tests performed on the SWIPE multi-mode bolometric detector pixel assembly. A 20-mm-aperture horn is coupled to a large detector absorber (10 mm diameter) with a TES sensor located on the side. The pixel assembly has been tested at the bolometer base temperature of 340 mK, inside a custom cryogenic test bed, looking at a Gunn oscillator (128 GHz) located in the far field. We developed a custom cryogenic neoprene absorber, in addition to the stack of standard metal meshes low-pass filters, to reduce the background on the detector at a level similar to the one expected in flight, allowing to measure the main beam of the pixel assembly. The measured FWHM is 21°, slightly narrower than the expected one (24°), due to vignetting produced by the filters stack.